Marty's band, the Pinheads, are rejected mere seconds into their audition for being "too darn loud". Marty confides in Jennifer that he doubts he has a future in music, and doesn't want to send his demo tape to a studio out of fear of rejection. Jennifer reminds him of Doc's advice to him: "If you put your mind to it, you can accomplish anything." As they talk, Marty admires a brand new Toyota pickup truck being delivered to Statler's Toyota dealership on the Courthouse Square. As he and Jennifer are about to kiss, they are interrupted by a lady soliciting donations for the Hill Valley Preservation Society, who oppose Mayor Goldie Wilson's plans to restore the courthouse clock tower, which was struck by lightning at 10:04pm on November 12, 1955. They take a flyer to get the woman to leave them alone, but then Jennifer's father pulls up, and before she leaves, she uses the flyer to write down her phone number as she is going to be at her grandma's house.
When Marty gets home, he finds that the family car, a 1979 Chevrolet Nova, has been wrecked by his father George's supervisor, Biff Tannen, who is berating George for not telling him about the car's 'blind spot' (the fact that Biff was drinking beer while driving may have been the real cause of the accident). He bullies George into doing his reports for him and dismisses Marty as a 'butthead' before leaving. At night the family are having dinner, and we learn that Marty's older brother Dave works at a Burger King (and has to ride the bus to work), while his sister Linda has no love life because their alcoholic mother Lorraine disapproves of girls chasing boys. Lorraine remarks that fate brought her and George together when her father hit George with his car after he mysteriously fell from a tree in front of her house. Her tone as she tells the story makes it clear the romance has long since gone out of their marriage; Marty, tired of hearing the same story regularly for years, abruptly leaves the table.
Marty dozes off, but is woken up by a phone call from Doc around 12:30am. Doc asks him to bring his video camera to Twin Pines Mall. They meet up and while Marty starts to record his film, he learns that Doc has created a time machine out of a DeLorean DMC-12 and watches him perform a test run on his dog, Einstein, by sending him one minute into the future, a journey triggered when the car reaches 88 miles per hour. Overjoyed by this success, Doc demonstrates how the time machine works by entering several significant dates into the keypad. Doc enters the date November 5, 1955 and explains to Marty that on that day, he was hanging a clock over his toilet, slipped, and knocked himself out on the sink; when he came to, he had the idea for the flux capacitor, the device that makes time travel possible. Marty is more shocked that the DeLorean is nuclear-powered, as the only way Doc could find to generate the 1.21 gigawatts of electricity that powers the motor is to use nuclear fission. He admits to Marty that he got the plutonium by ripping off a group of Libyan terrorists who paid him to make a dirty bomb for them; he gave them an empty casing filled with pinball machine parts. But as Doc re-fuels the DeLorean and prepares to go into the future, the terrorists catch up with him and spray him with machine gun fire. Marty is able to escape in the DeLorean, but he accidentally activates the time circuits, and when he accelerates to 88 miles per hour in an attempt to outrun the terrorists, he inadvertently sends himself to the last date Doc entered into the keypad: November 5, 1955.
Twin Pines Mall was Otis Peabody's family farm in 1955, and Peabody and his family mistake the DeLorean for a crashed alien spacecraft and the radiation suit-wearing Marty for an extraterrestrial. As Marty flees the irate farmer's rifle fire in the car, he knocks over one of his twin pine trees. Marty tries to convince himself that he is dreaming, but as he returns to Lyon Estates, the subdivision where he lives with his family, he discovers that they have only just broken ground on the first houses. The DeLorean is out of power, so he hides it behind the large billboard advertising the glorious future awaiting families who live on Lyon Estates, then walks to the Courthouse Square, which is much cleaner and less run-down than it is in 1985. He goes to Lou's Cafe (occupied by an aerobics studio in 1985) to look up Doc Brown in the phone book, but when he calls the number, there is no answer, so Marty makes a note of Doc's address. He accidentally meets his father George, who is having breakfast at Lou's, when Biff and his friends bully him to copy his homework. After recognizing the janitor as future mayor Goldie Wilson (his offhand remark that Wilson will one day be mayor actually inspires him to pursue a career in local politics despite the racial barriers of the era), he follows George only to discover, to his horror, that his father is a peeping tom, hiding in a tree and using binoculars to spy on a girl undressing in a nearby house. Suddenly George loses his balance and falls into the path of an oncoming car; Marty instinctively shoves him out of the way and gets hit by the car instead while George flees.
Marty wakes up, convinced he's just had a nightmare about going back in time, but he bolts upright in horror when the teenage girl watching over him tells him he's back in "good old 1955", and is even more horrified to discover that the girl is his mother, Lorraine Baines. She assumes that his name is Calvin Klein since that is the name on his underwear, and she is clearly more than a little smitten by the mysterious boy who fell in front of her father's car. Marty joins Lorraine's family for dinner, meeting his then-infant uncle Joey, who cries whenever he is taken out of his crib (foreshadowing his future as a career criminal and jailbird), and confusing his uncle Milton when he claims to have seen the episode of The Honeymooners ("The Man from Space") that they are watching over dinner even though it is airing for the first time. Lorraine's mother Stella says Marty looks familiar, and asks if she knows his mother; Marty glances at Lorraine and says she probably does. When Lorraine begins aggressively flirting with him, he quickly makes excuses to leave in search of Doc Brown; Lorraine's unimpressed father Sam says that Marty is an idiot who must have idiot parents, and that if Lorraine has a child like that, he'll disown her.
Marty shows up at Doc's mansion (which had been blown up by a lab accident by 1985), but 1955 Doc refuses to believe that he is from the future; he says that Marty's picture of himself and his siblings (in which Linda is wearing a "Class of 1984" sweatshirt) is shoddy trick photography, as the top of his brother Dave's hair has been cut off, and finds the idea that B-movie actor Ronald Reagan is US President in 1985 to be ludicrous. But when Marty tells him that he knows how Doc got the bruise on his forehead - from hitting his head against the bathroom sink and conceiving the flux capacitor, the device that makes time travel possible - he finally convinces him. When Marty shows Doc the DeLorean and the flux capacitor, which looks just like Doc's drawing, Doc is overjoyed to discover that he has finally invented something that works. He watches the video Marty recorded in 1985, but is horrified to discover that the DeLorean needs 1.21 gigawatts of electricity to work, and he remarks that while he is sure plutonium is easy to find in 1985, it's far rarer in 1955, and the only power source they can use is a bolt of lightning - but there's no way to know where or when one will strike. Marty remembers the flyer about the clock tower and shows Doc that lightning will strike the clock tower the following Saturday at 10:04pm. Doc is delighted again, but hurriedly shoots down Marty's suggestion that he can show him around Hill Valley, as anything he does while in 1955 might irreversibly and disastrously alter the future. Marty awkwardly admits that he has already run into his parents, and Doc asks to see the picture of the McFly siblings again - and discovers that now Dave's entire head has vanished, erased from existence.
Doc takes Marty to Hill Valley High (which is much cleaner than it is in 1985, although Mr. Strickland is still the school disciplinarian and was bald even then), explaining that he needs to make his parents fall in love before he can go back to 1985. Marty admits that his parents' relationship has never made sense to him, as they have nothing in common. He sees George in the hallway between classes and tries to introduce him to Lorraine; she, however, is too smitten by Marty to even notice George. At lunch, Marty is surprised to discover that George writes science fiction short stories, but has never let anyone read them or submitted them to publishers for fear of rejection. Marty tries to persuade George to ask Lorraine to the Enchantment Under the Sea dance, but George refuses for further fear of rejection and because she's probably already going with Biff, who is aggressively hitting on her. Marty pulls Biff off Lorraine, and only the timely appearance of Mr. Strickland stops Biff from beating him up in retaliation. After school, Marty chases after George, who is adamant that he is not going to the dance as he doesn't want to miss Science Fiction Theater, and says "no one on the planet" can persuade him, which gives Marty an idea. That night, he dresses in his radiation suit, wakes George by putting headphones on his ears and blasting guitar music through them, then introduces himself as "Darth Vader... from the planet Vulcan" and threatens to fry George's brains if he doesn't ask Lorraine to the dance.
The next morning, George finds Marty and asks for his help in wooing Lorraine. Marty agrees, but suggests he keep quiet about "Darth Vader" as no-one will believe him. In Lou's Cafe, George attempts to flirt with Lorraine and, though fumbling the words due to nerves, he seems to be getting somewhere when Biff shows up and prepares to throw him out. Marty trips Biff before he can get to George, then distracts him for long enough to land the first punch before fleeing to the Courthouse Square, where he grabs a young boy's box trolley and snaps off the top to create a makeshift skateboard. Biff and his gang drive after him in their Ford and eventually pin him to the front of the car, but he outsmarts them by climbing over the car and hopping onto his skateboard as it appears from behind it; the bullies are so stunned, they do not notice they are about to crash into the back of a manure truck until it is too late. Unfortunately, this display of heroism makes Lorraine more attracted to Marty than ever.
Marty watches Doc demonstrate how he plans to get him back to 1985 using a model of the Courthouse Square, but his fascination gives way to alarm when he discovers that Lorraine has followed him back to Doc's mansion to ask him to the Enchantment Under the Sea dance. Since Lorraine tells Marty that she is attracted to strong people who protect those they love, he decides to set up a scenario that will make it seem as though he is trying to take advantage of Lorraine before George comes to her rescue. George warms to the idea and tells Marty he wishes he had his courage. Remembering what Jennifer told him about his music career, Marty tells George that he just needs self-confidence: "If you put your mind to it, you can accomplish anything." Marty borrows Doc's car for the date, and as Doc makes the final preparations to send Marty back to 1985, he says he'll miss him after he goes back, as he feels his life is better for having met Marty in 1955, especially now that he knows he has the experience of time travel to look forward to. Distraught over the thought that Doc will not live to use his greatest invention, Marty tries to warn Doc about the terrorists, but Doc refuses to learn anything about his future for fear of disturbing the space-time continuum. Marty writes Doc a letter urging him to take any precautions necessary to prevent his death at the hands of the terrorists and slips it into his jacket pocket.
On the night of the dance, Marty is stunned to discover that Lorraine has swiped liquor from her mother's drinks cabinet and smokes cigarettes, and has previous experience fooling around with boys in parked cars. However, when she aggressively kisses Marty, she withdraws, saying it's like kissing her brother, a reaction Marty finds more relieving than surprising. Just then, the car door opens - but Marty finds not George, but Biff, who is drunk and furious with Marty over the manure incident. He shuts himself in the car with Lorraine and tells his gang members to take care of Marty; they lock him in the trunk of a car which turns out to belong to the band for the dance, Marvin Berry and the Starlighters, who are outside for a smoke break and chase Biff's gang away. Unfortunately, the car keys are locked in the trunk with Marty.
George shows up at Doc's car on cue and tries to follow the script of his plan with Marty, but is alarmed to discover Biff in the car instead of Marty. Biff tells him to leave, but George refuses, so Biff twists his arm behind his back. When Lorraine tries to intervene, Biff shoves her to the ground and laughs. The enraged George finally discovers his self-confidence and knocks Biff out with a single punch to the jaw. Marvin finally gets Marty out of the trunk of his car by jimmying the lock with a knife, but slices his hand in the process; Marty arrives just in time to see George and Lorraine head into the dance together, but when he checks the photograph, Dave is completely gone and Linda is rapidly disappearing too. Marvin's injury means the band cannot play on, and so George and Lorraine's first kiss cannot happen - unless Marty knows someone else who can play guitar...
Marty joins the Starlighters on stage both to play guitar and to keep an eye on George and Lorraine, but during the first number, "Earth Angel", Mark Dixon, a student, cuts in between George and Lorraine. Lorraine tries to get George to intervene, but he has lost his confidence again, and Marty collapses on stage as he begins fading out of the photograph and out of existence, his hands becoming semi-transparent. Fortunately, George's crisis of confidence proves short-lived as he shoves Dixon to the floor and kisses Lorraine passionately. Marty springs to his feet and sees himself, Linda, and Dave re-appear in the photograph; overjoyed, he begins to play the guitar with renewed vigor. At the band's request, Marty plays one more song, "Johnny B. Goode." Marvin Berry calls his cousin, Chuck, and tells that he found the "new sound" Chuck was looking for. Marty does Chuck Berry's trademark duck walk, and then gets carried away imitating other guitar heroes: windmilling his arm and kicking over his amplifier in imitation of Pete Townshend, lying on the stage kicking his legs in imitation of Angus Young, playing behind his head like Jimi Hendrix, and tapping in the style of Eddie Van Halen. In the face of uncomprehending stares from the audience, Marty says, "I guess you guys aren't ready for that yet. But your kids are gonna love it."
As Marty leaves the dance, he runs into Lorraine and George one last time. Lorraine tells Marty that George has asked to walk her home; far from being upset, Marty is delighted, telling them he has a good feeling about their future together. When they ask if they'll ever see him again, he tells them, "I guarantee it." Before he leaves, he tells them that if they have a child who, when he is eight years old, accidentally sets fire to the living room rug, "Go easy on him."
He rejoins Doc Brown, who has suspended a cable from the top of the clock tower to channel the lightning into the DeLorean; Doc seems uncertain how to react to the news of George's bravery, which even Marty admits was completely unlike him. Just as Marty is getting ready to take the DeLorean to the starting line Doc drew, Doc discovers Marty's letter in his jacket pocket and, refusing to learn anything about his future for fear of disturbing the space-time continuum, begins tearing it up, but before he can throw the pieces away, a tree branch falls onto the cable, disconnecting it. Doc absently puts the pieces back in his pocket and goes to the top of the clock tower to re-connect the cable; Marty tries to yell his warning about the terrorists, but is drowned out by the clock striking ten. With only four minutes to go before the lightning strikes, Marty has to rush to the DeLorean, and decides to simply go back to eleven minutes before he left so that he can get to the mall in time to warn Doc of his fate - but when he tries to start the car, it goes dead. Meanwhile, Doc is poised to plug in the cable at the clock tower end, but it is pulled so tight by the tree branch that it unplugs at the street level end. Just as the DeLorean finally roars into life and Marty speeds down the road, Doc plugs the cable in at the clock tower end and slides, zipline style, down the cable and re-connects it at street level mere seconds before the lightning strikes and sends Marty and the DeLorean back to 1985. The ecstatic Doc runs down the fire trail left by the DeLorean, laughing with delight at his successful experiment before looking toward the clock tower.
Marty returns to 1985, but the DeLorean has stopped working again, and while Marty frantically tries to start it, he sees the terrorists' VW bus drive past on its way to Lone Pine Mall (as it is now known since Marty ran over one of the twin pines). Marty arrives just in time to see Doc get shot as his past self screams in grief before outdriving the Libyans and going back in time while they crash into a Fox Photo booth. Marty rushes down to Doc's body and turns away in tears. Suddenly, Doc wakes up and reveals to Marty that he has a bulletproof vest under his radiation suit. Marty asks how he knew, and Doc shows him the letter from 1955, yellowed with age and taped back together. When Marty jokingly asks about disturbing the space-time continuum, Doc tells him, "Well, I figured, what the hell." He drops Marty off at his house and drives off to 2015 with Einstein in the DeLorean.
Marty wakes up the next morning, convinced his time travel adventure was just a nightmare, but when he goes into the dining room for breakfast, he is amazed to discover the house is much more tastefully and expensively furnished, that Linda has so many prospective boyfriends that even she can't keep track of them, and that Dave has an office job. He is even more amazed to see George and Lorraine return from a tennis game looking fit and happy, and clearly still very much in love. Lorraine is now much more approving of Marty's relationship with Jennifer, but when Marty reminds her that the wrecked car has derailed his weekend plans with Jennifer, the family are shocked. George assures them that the car is just fine, and opens the door to reveal that not only is the McFly family car a brand new BMW, but Biff is outside waxing it in his capacity as an auto detailer. George even catches Biff in a fib about whether or not he has put on the two coats of wax for which George has paid him, revealing that now it is Biff who is afraid of George. Both George and Lorraine agree that for all his faults, Biff is the one who caused them to fall in love. Just then, Biff enters with a delivery box containing the proofs of George's first novel, a science-fiction story called A Match Made in Space. George proudly echoes Marty's advice to him in 1955: "If you put your mind to it, you can accomplish anything," advice he has taken to heart to become a successful writer. Biff also gives Marty his new keys, which he discovers are for the Toyota pickup he was admiring in the original timeline.
Jennifer arrives as Marty takes in the sight of his new truck, and they decide to go for a ride together. Just then, Doc reappears in the DeLorean, and urges Marty and Jennifer to return to 2015 with him, as something needs to be done about their kids. After Doc re-fuels the now fusion-powered DeLorean with some of the contents of the McFlys' garbage can, Marty and Jennifer enter the car and Doc pulls out of the driveway. Marty points out that there is not enough road to accelerate to 88 mph. Doc replies, "Roads? Where we're going, we don't need roads," and flies off in the now hover-converted DeLorean.
Disguised as his future son, Marty goes to Cafe 80's, where Marty Jr. is supposed to meet Griff. There, he meets an elderly Biff, and is confused when Biff rants about what a mess his father made of his life, as George became a successful writer - but Biff means Marty Sr., not George. Before Marty can learn more about how he supposedly flushed his life down the toilet, Griff arrives and orders his grandfather to get back to detailing his car, and tells Marty he's coming back for him. The plan is nearly derailed by the unexpected arrival of the real Marty Jr., a spineless coward who can neither fight nor stand up for himself. When Griff throws him over the counter, Marty Sr. manages to switch places with his future son and tells Griff that he won't take part in the robbery. Griff accuses Marty of being "chicken", which nearly provokes Marty into violence until he sees how obviously outmatched he is; he tries the same distraction tactic he used on Biff in 1955, but when he tries to hit Griff, he does more damage to himself. A Groin Attack is more successful, and Marty flees into the Courthouse Square (which has a fountain where the parking lot used to be in 1985). Borrowing a hoverboard, Marty manages to escape Griff and his gang, but is nearly caught when he lands in the middle of the fountain, unable to get the hoverboard to move. Just as Griff and his gang are about to charge into him, Marty jumps into the water, and the gang members crash into the courthouse and are arrested, preventing the robbery from ever occurring. The elderly Biff finds something very familiar about what he has just seen...
An offhand comment by a passer-by about going back to the beginning of baseball season to bet on the World Series-winning Chicago Cubs inspires Marty to buy a copy of Grays Sports Almanac 1950-2000 and go back in time to place winning bets on the sporting events recorded in it. He meets up with Doc, who is shocked by the chaotic events but is delighted when he and Marty notice that the newspaper now records Griff's arrest and not that of Marty Jr. As they prepare to return to 1985, Doc grabs the sports almanac and tells Marty it could cause devastating consequences to the timeline if it falls into the wrong hands - a comment Biff overhears. When he sees the real Marty Jr. leave the cafe, he realizes that the other Marty is from the past, and remembers seeing the DeLorean travel through time thirty years earlier. This gives him an idea.
Before Doc can throw away the sports almanac, he and Marty are shocked to see a still drugged Jennifer being collected by two policemen, who match her fingerprints to her 2015 self and take her to Hilldale, the shady subdivision where the McFly family now live (but which was seen as desireable in 1985, according to Marty). Doc tells Marty that if Jennifer sees her future self, she might simply faint from the shock, or she may cause a paradox that could destroy the entire space-time continuum. As they follow Jennifer and the cops, Doc throws the almanac away; he doesn't see Biff retrieve it from the trash moments later.
Jennifer wakes up in her future house and is shocked to discover that she and Marty got married in a cheap ceremony at a "Chapel O Love", and that her home and family life is nothing like she hoped it might be. She is forced to hide in various rooms and closets so as to avoid being seen by her family; her daughter Marlene is already home, the now elderly George and Lorraine (the former suspended upside-down in a contraption intended to treat his back problems) arrive soon after. Meanwhile, Doc and Marty fight heavy traffic to get to the McFly house to rescue Jennifer; Doc is confused by a taxi that appears to be following them, but dismisses it, unaware that Biff is its passenger. Jennifer overhears Lorraine tell Marlene about the problems Marty Sr. has caused for himself throughout his life because of his aversion to being called "chicken", and that his life went downhill after he took part in a drag race near Lone Pine Mall and crashed into a Rolls-Royce, breaking his hand and ruining his ability to play guitar. When the Rolls driver pressed charges, Marty fell into a spiral of guilt and depression. Intercut with Lorraine's story, the DeLorean arrives in Hilldale, followed by Biff's taxi. Doc tells Marty to wait in the car so as not to be seen by his future self, but Marty gets distracted by a passing robotic dog walker and leaves the car unattended for just log enough for Biff to climb in.
Marty Jr. and Marty Sr. return home, and the family sit down to a dinner of re-hydrated pizza. As Jennifer watches, Marty Sr. gets a video telephone call from his colleague Needles, who goads him into cooperating in a profitable, but illegal, scheme by calling him "chicken" for trying to refuse. However, their Japanese boss at CusCo, Ito T. Fujitsu ("The Jitz"), has been listening in. He appears suddenly on the video screen and Marty is summarily fired, receiving multiple copies of a fax bearing the words "YOU'RE FIRED!!!" (also superimposed over the video image in large letters filling the screen), one of which is taken by Jennifer. Doc finally finds Jennifer and tries to sneak her out of the house, but she runs into her future self, who is arriving home just as 1985 Jennifer is leaving. Fortunately for the space-time continuum, both Jennifers simply faint from the shock, and Doc and Marty carry 1985 Jennifer back to the DeLorean. They do not notice Biff exiting the car, breaking the head off his cane as he does so, and clutching his chest in obvious pain. He hides behind a nearby car and watches the three time travellers return to 1985, then collapses and fades out of existence.
Doc decides that after returning back to 1985, he is going to destroy the DeLorean, as the consequences of altering the timeline could be disastrous if the machine fell into the wrong hands, much as he would like to go back to the Old West. Back in 1985, Marty and Doc leave Jennifer on her porch swing, thinking little of the fact that her house now has security bars on the windows and the car parked in the driveway is a wreck. They also don't register the graffiti on the sign for Lyon Estates, the subdivision where Marty and his family live, or the stray dogs roaming the streets. However, Marty realizes there is something wrong when he is unable to open the back gate to get back into the house, and things get even worse when he enters what he thinks is his bedroom and finds a young black girl sleeping there. As she wakes up and screams "Rape!", Marty is chased from the house by the girl's enraged, baseball bat-wielding father. Rushing through the streets, he sees wrecked cars and chalk outlines of dead bodies, and hears the sound of gunfire and police sirens. He initially thinks he has gone back to the wrong year, but he finds a newspaper on a doorstep with the date October 26, 1985. Suddenly he is confronted by the house's owner, Mr. Strickland, who is wearing a bulletproof vest and pointing a gun at him. Mr. Strickland does not recognize Marty, and when Marty tries to remind him that he gave him detention last week, Strickland says the school burned down six years ago. A gang driving past his house opens fire on his plant pots; as Strickland returns fire, Marty flees.
As he arrives in Courthouse Square, Marty is shocked to find that Hill Valley is now a dilapidated, crime-infested, corrupt hell-on-earth. A biker gang circles the square, a tank rolls through the streets, and the buildings on the square include a toxic waste reclamation plant and a wide assortment of red-light district businesses including bars, sex shops, and live sex shows. The courthouse itself has been replaced with a casino hotel called Biff Tannen's Pleasure Paradise, with an attached Biff Tannen Museum. A video documents Biff's life, describing how he became a millionaire overnight after a trip to the racetrack on his 21st birthday, how subsequent successful bets on sports led to him being dubbed "the Luckiest Man on Earth," how he parlayed his lucky winning streak into a vast business empire called BiffCo, successfully legalized gambling in 1979, and in 1973 realized his lifelong dream by marrying his high school sweetheart, Lorraine Baines McFly. Marty wails in shock until Biff's goons appear and knock Marty out.
A dazed Marty comes to, hearing his mother's voice, and, just as he did in 1955, thinks he has had a nightmare, but when his mother assures him he's now back on the "good old 27th floor", he bolts upright and is shocked to see his mother showing signs of physical abuse and alcoholism, and sporting enormous breast implants. Just then, Marty's 'stepfather', Biff, bursts in and demands to know why he's not in Switzerland, accusing him of getting expelled from another overseas boarding school. Biff warns Lorraine that Marty is a butthead just like his father, and when the enraged Lorraine tells Biff he isn't half the man George was, Biff roughly pushes her to the floor. Marty rushes at Biff, but Biff's gang grab Marty, and Biff punches Marty in the stomach. Lorraine snaps and tells Biff she's leaving, but relents when Biff threatens to cut off her financial support, cancel Linda's credit cards, revoke Dave's probation, and have Marty put in jail, setting up a hellish family reunion with her criminal brother Joey. He leaves, warning Marty to get lost within an hour. Marty demands to know why Lorraine left George for Biff. Lorraine is initially confused, and thinks they must have hit Marty too hard this time, but finally explains that George is where he has been for the last twelve years: Oak Park Cemetery. A horrified Marty finds his father's gravestone, listing his death date as March 15, 1973, and begins sobbing with grief. Doc finds him, and says he knew Marty would be there when he heard about his father.
Doc takes Marty back to his lab and shows him bound collections of newspapers which he obtained by breaking into the boarded and closed down library. Marty tears out the page of the newspaper that reports his father was shot dead in an alleyway while on his way to receive a book award, with the headline "GEORGE MCFLY MURDERED". Using an old chalkboard, Doc theorizes that somewhere in the past, the timeline has diverged into an alternate reality. He then shows Marty the bag and receipt from when Marty purchased the almanac in 2015, as well as Biff's broken fist-shaped cane handle, which he had found in the DeLorean. The clues he has in the DeLorean reveal that 2015 Biff had given the book to himself sometime in the past, thus changing his future. He even shows proof that Biff is carrying an almanac in a photograph from a 1958 news story after he made his so-called first "lucky" win. Doc says this is how time travel can be misused and why the time machine must be destroyed... after they've corrected the timeline. Marty blames himself, as he thought of the almanac idea, but Doc tells him what's done is done, and they're going to fix this before destroying the DeLorean. Marty suggests they go back to the future to stop Biff from getting the almanac, but Doc tells him they can't as they would end up in the future of the current timeline, with Biff still very powerful, still married to Lorraine, and Doc himself in an asylum, as evidenced by a newspaper with the headline "EMMETT BROWN COMMITTED". They will have to return to the past to stop 2015 Biff from giving the almanac to his younger self, but Doc doesn't know which date to choose. Marty, however, has an idea.
Biff is in a jacuzzi with two topless women, watching the scene from A Fistful of Dollars in which the Man with No Name is revealed to be wearing a sheet of metal under his poncho as a makeshift bulletproof vest, when Marty comes in and turns off the TV with the remote, and confronts Biff about Grays Sports Almanac. Biff invites him to the office and reveals an old, faded copy of the book; Marty asks him where he got it. He tells him the date, saying he remembers that he had crashed his car in a drag race (Marty knows better; he crashed into the back of a manure truck), and when he picked it up from the garage, he met an old man who claimed to be a distant relative (Biff didn't see the resemblance) and gave him the book, claiming it would make him rich, no strings attached - but if a kid or a crazy scientist ever show up asking questions, he must kill them. Biff never expected Marty to be the kid, and draws a gun on him, but Marty manages to distract him and flees to the rooftop. Biff finds him there and waxes poetic about shooting two McFlys with the same gun, revealing himself as George's murderer - and as he owns the Hill Valley Police, he can't be prosecuted for the crime. Marty appears to fall over the edge of the roof, but as Biff walks over to see where Marty landed, he re-appears, standing on top of the DeLorean; Biff is then knocked out by the opening gullwing door.
Marty and Doc plan to head back to November 12, 1955; Doc tells Marty it's unbelievable that old Biff chose that date, and perhaps it has some sort of cosmic significance - or perhaps it's just a coincidence. Marty is worried about what will happen to Jennifer and Einstein, but Doc assures him that when they repair the timeline, Jennifer and Einstein will be exactly where they left them, unaware that the dystopian timeline ever existed.
Back in 1955, Doc advises Marty to wait until Biff of 2015 gives his younger self the almanac so that he can go back to the future with the time machine. He gives him a walkie-talkie so that they can stay in contact with each other. Marty buys some 1950s clothes as a disguise, and follows Biff to a mechanic's shop in the middle of town. He hides in the back of Biff's car and watches Biff argue with the mechanic over the cost of the repairs, then harass Lorraine, who insists she's going to the Enchantment Under the Sea dance with Calvin Klein (i.e., past Marty); she tells Biff she wouldn't go with him even if he had a million dollars, and Biff retorts that the two of them will be together one day. Biff is shocked to find his older self in the car; the old man tells him it's his lucky day and somehow knows both how to start Biff's car and where he lives. Back at Biff's house, Old Biff gives him the almanac saying it will make him rich. He proves that the almanac's results are correct by switching on a radio broadcast of a UCLA football game and predicting, correctly, that UCLA will snatch a last-second 19-17 victory. As Old Biff leaves, Marty is unable to grab the almanac and gets locked in the Tannen garage instead. He radios Doc for help, but is able to get out without him by hiding in the back of Biff's car again when he leaves for the dance. Doc arrives by bicycle moments after Biff leaves, wondering what happened to Marty.
Marty radios Doc again and tells him about his new plan to grab the almanac at the dance; Doc, who is now at the Courthouse Square and sees the tarpaulin-covered DeLorean that Marty originally brought back to 1955, tries to discourage him because of the risk of seeing his past self, only to run into his past self, setting up the apparatus that will send the original Marty back to the future. He studiously avoids making eye contact with his past self, although when he rides off on his bicycle, 1955 Doc does a double take as he gets a good look at the back of the head of the stranger who has just helped him.
At the dance, Marty sees Mr. Strickland confiscate the almanac from Biff, and he manages to sneak into Strickland's office and steal it back without being seen, only to discover Biff put the dust jacket of the almanac on an erotic magazine called Ooh La La. He then spots his father George knocking out Biff, like before, and finally manages to steal back the almanac - but not without being seen by other students, who think Marty has just stolen Biff's wallet. Biff's cronies mistake him for "Calvin Klein" and chase Marty backstage; unaware that there are two Martys, they prepare to attack the Marty currently on stage playing "Johnny B. Goode". Doc tells Marty that he must stop Biff's gang from attacking his past self, or he will miss being sent back to the future and create a time paradox. Doc then makes a shaky takeoff in the DeLorean, accidentally setting the malfunctioning time circuits for January 1, 1885.
Marty is able to knock out Biff's gang by dropping sandbags on them and radios Doc to pick him up, but he runs into Biff himself, who has been told where to find Marty by a witness to his "wallet theft", and though Marty tries to walk away from a fight, Biff provokes him by calling him "chicken". Before he can retaliate, past Marty knocks him out as he makes a hasty departure to meet 1955 Doc at the clock tower. Biff grabs the almanac back and kicks Marty for stealing it, for damaging his car, and for humiliating him, and then drives off in his newly cleaned car. Doc and Marty follow him in the DeLorean, and Doc lowers Marty to the ground on the hoverboard, allowing him to grab onto the back of Biff's car. He moves around to the side, but Biff sees him and tries to smash him against the wall of a tunnel. Marty dodges the attack, and, after a struggle, he finally grabs the almanac and is rescued by Doc from being run over by a furious Biff, who crashes into the back of a manure truck - again.
The storm that sent Marty back to 1985 in the original film is beginning to hit Hill Valley as Doc lowers Marty to the ground and tells him to burn the almanac. He lights it using a matchbook he picked up at Biff's Pleasure Paradise in the alternate 1985, and as the book burns, the matchbook cover changes to read "Biff's Auto Detailing" (making it one of the matchbooks Biff wanted to show Marty when he saw the DeLorean take off at the beginning of the film), while the headline "GEORGE MCFLY MURDERED" becomes "GEORGE MCFLY HONORED" and "EMMETT BROWN COMMITTED" becomes "EMMETT BROWN COMMENDED", the news stories now recording George and Doc receiving prestigious awards. Their delight at the repaired timeline is short-lived, as the DeLorean is struck by lightning and disappears. Marty panics, not just because Doc is gone but because he is now apparently stranded in 1955, but as the rain begins to fall, a Western Union delivery man appears with a letter. The letter was given to Western Union with the explicit instructions to deliver it to Marty "at this exact location, at this exact minute, November 12, 1955". Marty tears open the letter to find out it's from Doc, who is trapped in 1885 but tells him he is all right. Marty is overjoyed, and when the Western Union man asks him if he needs any help, Marty says there's only one man who can help him.
Cut to the Courthouse Square and the climax of the first film, with past Marty driving the DeLorean toward the contact point, and 1955 Doc sliding, zipline-style, down the cable from the clock tower to street level, where he re-attaches the cable seconds before the lightning strikes, sending the DeLorean back to 1985. He runs down the fire trail left by the departing car and laughs with delight at the success of his experiment... and is flabbergasted when Marty runs up behind him. He says he has just sent Marty back to the future, but Marty tells him he is now back from the future. Doc exclaims "Great Scott!" and faints. The words "TO BE CONCLUDED..." appear on the screen.
Doc and Marty take a photo of the tombstone and do some research, discovering that Buford 'Mad Dog' Tannen, Biff's great-grandfather, was a notorious gunman with a short temper and tendency to drool (hence his nickname) who may have killed a dozen men ('not including Indians or Chinamen'), but since he killed a newspaper editor in 1884 for printing an unflattering story about him, this claim remains unsubstantiated. They also discover that the McFly family had just immigrated to Hill Valley from Ireland in 1885. Marty suggests that perhaps there was another Emmett Brown in Hill Valley in 1885, but Doc's family did not arrive until 1908 (and Doc's father didn't change the family name from von Braun to Brown until ten years later), and when they find a picture of Doc posing at the dedication of the clock that would later go in the clock tower of Courthouse Square, they know there is no mistake: the grave is Doc's. Marty decides to defy the letter and go back to 1885 to rescue Doc.
At the Indian-themed Pohatchee Drive-In Theater, Doc finally restores the DeLorean, gives Marty a flashy cowboy outfit that fits the 1955 impression of the Old West, and thanks him again before Marty goes back to September 2, 1885 (the day after Doc wrote the Western Union letter) in the DeLorean. However, as he travels through time, the painted mural of Indians at the drive-in is replaced by an actual Indian charge, and Marty has to steer the DeLorean into a cave. The Indians do not pursue him, as they're already being pursued by the local Cavalry. Marty realizes that the fuel line has been ruptured by an Indian arrow, but before he can start to repair it (or put on the boots 1955 Doc gave him as part of his cowboy attire), he is chased out of the cave by a black bear. He falls down a hillside and collapses against a fencepost. As in the first two films, when he comes to, he thinks he has had a nightmare, and bolts upright when the woman taking care of him assures him that he's now safe and sound at "the McFly farm." The woman is Marty's great-great-grandmother, Maggie McFly, who lives on the farm with her husband Seamus and infant son William (the first McFly to be born in America and Marty's great-grandfather). Marty introduces himself as 'Clint Eastwood' and asks for help in finding his friend Doc. Seamus agrees.
The next day, Marty goes into Hill Valley, and goes into the local saloon to try and find out where Doc lives. The old-timers at the saloon are highly amused by Marty's flashy cowboy outfit (which, contrary to what 1955 Doc thought, looks nothing like actual cowboy clothing), but their mockery is cut off as Buford Tannen enters the saloon. Buford initially mistakes Marty for Seamus, saying he told him never to come into the saloon, then asks the bartender where he can find the blacksmith. Marty makes the mistake of identifying Buford as 'Mad Dog' Tannen; Buford is enraged, as he hates that name, and shoots at Marty's feet. Marty eventually improvises a Michael Jackson-inspired 'dance' to get out of the way of the bullets, until he accidentally trips on a loose board and launches the saloon's spittoon through the air and onto Buford, angering him further. Marty tries to run away but Buford and his gang capture him and intend to hang him from the partly-complete clock tower. Luckily, the noose is shot through by Doc with a custom-made rifle. We learn the source of the $80 dispute that leads to Doc's murder; Buford accuses the blacksmith of badly attaching a horseshoe to his horse, which caused him to buck and make Buford spill his bottle of Kentucky Red-eye. He demands $5 for the lost booze and another $75 for the horse. Doc refuses and asks Buford to bring the horse back to be re-shod, but Buford reveals he already shot the horse dead. Doc tells him there's nothing he can do, so Buford threatens that someday he's going to get a bullet in his back as his gang leaves. Although Doc is not happy about Marty defying his instructions and coming back to 1885, he's still happy to see him and decides to change him out of the 'ridiculous outfit' that his younger self chose for him.
At Doc's blacksmith shop, he is shocked to see the picture of his own tombstone and jokes that he should have paid Buford. He's curious as to who Clara is; Marty assumes she is his lover, but Doc jeers that he has no time for love. They get their answer when the Mayor visits to tell them that the new local school teacher, Clara Clayton, will be arriving by train the next day, and that Doc agreed to formally greet her on behalf of the residents of Hill Valley at the last town meeting. To avoid a potentially paradox-causing love affair, Doc decides not to collect Clara Clayton from the train station and chooses instead to head back to 1985 with Marty, but when Marty tells him the fuel line was broken in the Indian attack, he reveals that although the Mr. Fusion powers the flux capacitor, the engine runs on ordinary gasoline, so they now have no way to get the car to 88 mph.
As they would have to wait until the next century to find a gas station, they decide to find an alternative way to boost the DeLorean's speed. They try to use horses to pull it but it only reaches 24mph. They try to pour the strongest whiskey from the saloon inside the tank, but only succeed in blowing up the engine's fuel injector. Doc says it will take a month to fix it, but Marty reminds him he doesn't have that much time since he's going to get shot in a few days, which scuppers Doc's idea to wait until winter to get some decent ice to speed up the DeLorean. However, when they hear the train arriving at Hill Valley Station, they get an idea; talking to the conductor, they learn that, coincidentally, the next train arrives on Monday. Pretending to have made a wager with each other, they get the conductor to explain that the train might reach 90mph on a long enough section of straight track on a downhill grade, assuming it was pulling no carriages and had a fire hot enough to generate the necessary power without blowing up the boiler.
Doc hatches a plan to use the train locomotive to push the DeLorean across the bridge over Shonash Ravine (known as Clayton Ravine in their time), but when they check out the ravine, they realize the bridge isn't complete. However, Doc sticks to the idea as this would definitely work; when the DeLorean returns to 1985, the bridge will be complete. Just then, they spot a runaway carriage with a female passenger crying for help. Doc manages to save her just in time before her whole carriage drops down in the ravine. She introduces herself as Clara Clayton, and Doc quickly falls in love with her when he discovers that she too is interested in science. Doc and Marty leave her to unpack her belongings in her new home. On the way back, Doc is glad that they saved Miss Clayton from the ravine, but Marty realizes that the ravine in 1985 was called "Clayton Ravine" because she was killed in the original timeline; he and his classmates often muttered about wishing some of their own teachers would fall into Clayton Ravine. Horrified that he has altered the timeline again, Doc becomes more resolved to destroy the DeLorean when they get back to 1985. Doc demonstrates the plan to send him and Marty back to 1985 using a scale model, but his explanation is interrupted by the arrival of Clara, who asks if he can repair her telescope, which was damaged in the runaway carriage accident. She says she hopes she will see "Emmett" at the town festival that evening.
The festival also serves as the inauguration ceremony of the clock tower; acknowledging the pivotal role the clock tower has played in their lives, Marty and Doc get their photo taken in front of the clock before it is set in place. While Marty meets Seamus, Maggie, and William again, then decides to try his luck in the shooting gallery (impressing everyone with his shooting skills), Doc meets Clara again and they dance together. However, Buford and his gang arrive, and though Marshall Strickland makes them surrender their firearms, Buford, still angry at Doc for saving Marty, plans to shoot the blacksmith using a small derringer hidden in his jacket. He presses the gun against Doc and says that not only will no-one hear the shot, but he will linger for a few days before dying in great pain. Clara tries to distract Buford by offering to dance with him and then kicking him in the shin, but this only provokes the furious Buford into aiming the gun at Doc's head. Thinking fast, Marty uses a Frisbie pie plate as an improvisation frisbee, hitting Buford and causing his only bullet to hit Doc's hat. Marty tells Buford to leave Doc alone; the enraged Buford challenges Marty to a duel. Marty refuses, but once Buford calls him "yellow", Marty accepts and arranges the duel for Monday morning, the day he and Doc are planning to leave. Marty is congratulated by the rest of the townsfolk for standing up to Buford and presented with a Colt Peacemaker and bullets by a Colt representative. Only Seamus and Maggie are unimpressed, telling Marty the cautionary tale of Seamus' brother Martin, who wouldn't let anyone call him a coward and paid for this failing with his life during a saloon brawl in Virginia City. Meanwhile, Doc and Clara watch the stars at her house and discover a shared fondness for the works of Jules Verne, deepening their affection for each other.
The next day, Marty realizes that the name on Doc's gravestone in the photo has vanished and assumes this means he is now safe, but Doc reminds him that since the gravestone is still there, as is the original death date, one of them may end up dead. When the town undertaker measures Marty for his coffin, noting that betting odds are against him in his duel with Buford, he and Doc realize that it might now be Marty who is buried in the grave. Doc warns Marty not to get angry when someone calls him names and even accidentally foreshadows the car accident in the second movie that caused his life to enter a terminal downward spiral. Marty is shocked and asks for details, but Doc refuses to tell him, but advises him "Marty, we all do things that affect the course of our lives. You gotta do what you gotta do, and I gotta do...what I gotta do." Doc reveals that he has fallen in love with Clara and has decided to stay with her in 1885. Marty doesn't like this idea, but suggests that instead they bring Clara back to 1985 with them; Doc is equally opposed to the potential ramifications to the timeline this would cause, but when Marty notes that Doc staying in 1885 could also adversely affect the timeline, he agrees to return to 1985, but would at least like to say goodbye to Clara.
Unfortunately, Clara senses that Doc is hiding something from her when he tries to say his farewells, and asks him to tell her the truth, but the "truth" sounds so far-fetched to her that she accuses him of spinning a ridiculous lie to break up with her. Heartbroken, he heads into the saloon, where Marty finds him the next morning, telling the other customers tales of a future of horseless carriages and running for pleasure, which they write off as drunken ravings. Doc and Marty prepare to leave to avoid having to fight the duel with Buford, but Doc downs his whiskey and immediately passes out. The bartender revives him with wakeup juice, although it still takes a few minutes for Doc to fully revive. Meanwhile, Seamus arrives at the saloon, saying "something told [him]" he should be there, and Buford and his gang arrive soon after, demanding that Marty come out and take part in the duel. Buford tries baiting Marty into coming out by calling him names, but when Marty sees Seamus looking at him and remembers the story of his brother Martin, he decides that his life depends on no longer caring what Buford, or anyone, thinks of him, and that he will not go through with the duel.
Doc finally revives, and he and Marty try to escape out the side door of the saloon, but Buford's gang catch Doc. Buford threatens Marty, telling him that if he doesn't show up Doc dies. With no choice, Marty decides to duel Buford. However, during the showdown, Marty intentionally drops his weapon, suggesting they fight each other like men. Buford simply shoots him in the chest, shocking the audience and Doc, and laughs at his naïveté and stupidity. But as he decides to finish him off, everyone is shocked when Marty kicks the gun away from his hand, standing up as if nothing has happened to him. Buford punches him in the chest but does far more damage to his hand, as Marty has used a stove cover as a bulletproof vest (taking a cue from the man who inspired his alias as seen in A Fistful of Dollars), which he then removes to hit Buford, sending him hurtling into the tombstone and shattering it. Marty then gives Buford a final punch, knocking him face first into a cartload of manure. Everyone cheers for him, even Seamus, until Marshal Strickland's deputy and his men arrive. Buford's men release Doc, seeing Buford is under arrest for a stagecoach robbery. Buford comments "I hate manure!" before being taken away. Marty and Doc are ecstatic to see that the tombstone from the photo has vanished, and after Marty throws his Colt Peacemaker to Seamus (who says he might trade it for a hat to replace the one he gave to Marty), they head to the train tracks to put their plan to return to 1985 into action.
Meanwhile, Clara is about to leave town on the train to San Francisco, but when a passenger behind her talks about the heartbroken man he met in the saloon the previous night, lamenting how much he hurt a woman called Clara, she stops the train and rushes back to Hill Valley. Unfortunately, she arrives a couple of minutes after Doc and Marty leave, and she decides to catch them with a horse. After 'borrowing' the train, Doc and Marty plan to push the DeLorean to 88mph with the train by using special logs to boost its speed, at the cost of the train's engine being destroyed once it reaches its maximum speed. Marty sits in the DeLorean monitoring its speed, while Doc is in the engine putting the logs in the firebox. However, by the time Doc is about to move forward to the DeLorean, Clara catches up with them and gets Doc's attention by pulling the train's whistle. Doc is stunned but delighted, but as the train has now passed the point where it can be stopped before reaching the bridge, he and Marty now have to bring Clara back to 1985. Doc goes back to get her, but the explosion of the engine from the power of the logs causes Clara to fall, dangling from the train. With no way to stop the train, Marty grabs the hoverboard from 2015 and sends it back to Doc and Clara, who are able to ride it away to safety at the cost of being unable to get to the DeLorean before it reaches 88 mph, leaving Marty no choice but to leave without Doc. The train flies off the bridge, destroying itself after it drops down the ravine.
Arriving back at October 27, 1985 in Eastwood Ravine (now named for Marty's alias since he is believed to have perished in the ravine), Marty has to quickly get out of the DeLorean as it is crushed into scrap metal by an oncoming train. Marty looks at the wreckage, knowing that Doc's wishes have been fulfilled but upset that he will never see him again. He checks in on his family, who are back to how they were at the end of the first film (while Biff is back to being an auto detailer instead of a corrupt casino owner), then drives in the Toyota pickup to where he left Jennifer. He wakes her with a kiss and takes her for a drive. As they reach Lone Pine Mall, Needles challenges Marty to a drag race. Jennifer realizes this is the incident she heard about that destroyed Marty's music career and with it, his life, and she pleads with him to ignore Needles. Needles seems to succeed in baiting Marty into racing by calling him "chicken", but once the traffic light turns green, Marty throws the truck into reverse, telling Jennifer he isn't "stupid enough" to race them. As he sees Needles nearly crash into a Rolls-Royce pulling out of a side road, he realizes that if he had taken part in the race, he would have crashed into the car. Jennifer checks the "YOU'RE FIRED" paper she got from 2015 and sees the words vanish.
Marty returns to the DeLorean's wreckage site with Jennifer, depressed that he will never see Doc again. While Jennifer tries to comfort Marty, suddenly the railroad crossing lights activate with no train in sight. The two teens are pushed back by an explosion as a train appears out of nowhere. They are greeted by the driver, Doc, who tells them that this time machine train runs on steam with the Flux Capacitor located on the front. He is now married to Clara and they have two sons, Jules and Verne. He came back to let Marty know that he was all right, to pick up Einstein, and to present Marty with the framed photo of the two of them in front of the courthouse clock with the inscription "To Marty, Partner in Time. September 5th, 1885". Jennifer asks about the paper that she received from the future that just faded. Doc assures them that it means their future can be whatever they want it to be. "So make it a good one, both of you!" As the Browns prepare to leave, Marty asks if Doc is going back to the future; Doc tells him that he has already been there. The train takes off, just like the DeLorean at the end of the first movie, and vanishes into thin air.
- Back To The Future The Animated Series S 1 E 1 Brothers
- Back To The Future The Animated Series S 1 E 2 A Family Vacation
- Back To The Future The Animated Series S 1 E 3 Forward To The Past
- Back To The Future The Animated Series S 1 E 4 Witchcraft
- Back To The Future The Animated Series S 1 E 5 Roman Holiday
- Back To The Future The Animated Series S 1 E 6 Go Fly A Kite
- Time Waits for No Frog / Einstein's Adventure
- Back To The Future The Animated Series S 1 E 8 Batter Up
- Solar Sailors
- Back To The Future The Animated Series S 1 E 11 Gone Fishin
- Back To The Future The Animated Series S 1 E 13 Claras Folks
- Back To The Future The Animated Series S 2 E 1 Mac The Black
- Back To The Future The Animated Series S 2 E 2 Put On Your Thinking Caps Kids Its Time For Mr Wisdom
- Back To The Future The Animated Series S 2 E 3 A Friend In Deed
- Back To The Future The Animated Series S 2 E 4 Marty Mc Fly PFC
- Back To The Future The Animated Series S 2 E 5 Vernes New Friend
- Back To The Future The Animated Series S 2 E 6 Bravelord And The Demon Monstrux
- Back To The Future The Animated Series S 2 E 7 The Money Tree
- Back To The Future The Animated Series S 2 E 8 A Verne By Any Other Name
- Back To The Future The Animated Series S 2 E 9 Hill Valley Brown Out
- Back To The Future The Animated Series S 2 E 10 My Pops An Alien
- Back To The Future The Animated Series S 2 E 11 Super Doc
- Back To The Future The Animated Series S 2 E 12 St Louis Blues
- Back To The Future The Animated Series S 2 E 13 Verne Hatches An Egg
Six months have passed since Doctor Emmett "Doc" Brown disappeared with his family and Time Travel Train in Back to the Future III. After a very vivid dream about Doc being erased from existence, Marty wakes up in his room. His mother realizes he is awake and reminds him that the estate sale of Doc's belongings has already started and that he should join his father, George, to pick up some of his stuff before everything is sold. Realizing he's late, he quickly goes to Doc's garage to stop it, but is too late to prevent some of Doc's stuff from being sold. Since Doc has been missing for so long, the city wants to sell his house and turn it into a parking lot; George volunteers to supervise the sale to ensure that Doc's stuff will be treated with respect. Luckily, Marty manages to take back Doc's notebook before Biff, unaware of its significance but sensing its importance to Marty, can take it back home. Suddenly, he hears a familiar sound from outside and is surprised to find the time-traveling DeLorean showing up with Einstein as its only passenger. Doc's faithful dog is accompanied by an unknown shoe and a tape recorder. Marty plays the tape and hears Doc explain that he had built a safety device which automatically returned the car to the present, presuming he might get into trouble in the past or future and trusting Marty to save him. Unfortunately, Marty cannot obey Doc's instructions to go to the 'Last Time Departed' as indicated on the console, as it suddenly malfunctions when Marty checks it. With the unknown shoe as the only clue, he asks Einstein to find its owner.
Marty follows Einstein to the apartment of the elderly Edna Strickland, a former reporter and Vice Principal Strickland's older sister. Returning her shoe, Marty finds a newspaper revealing that Doc, posing as "Carl Sagan", was stranded on June 14, 1931, charged as an arsonist for burning down a speakeasy run by mob boss Irving "Kid" Tannen, and killed by Kid's gang the next day as soon he was released. Upon returning to the DeLorean, Marty is confronted by George - but his father doesn't try to stop him from leaving, even though Marty tells a strange cover story about the 1931 clothes he is wearing. George only tells him to be careful, and that he trusts Marty to deal with his problems.
And so Marty and Einstein travel to 1931 in the DeLorean, arriving in the middle of a car chase/gun fight between mobsters and the police that results in the mobsters escaping and the police car running off the road, and Marty meets Doc in jail. Doc assumes he's safe since he's going to be released the next day. But once he sees the newspaper, which Marty swiped from the elderly Edna, Doc is shocked, and after some rejected suggestions by Marty, Doc suggests that he seeks help from his 17-year-old self, who works at the courthouse, to complete a Rocket Powered Drill that will get him out of jail. Doc tells Marty not to tell young Emmett what the drill is for or anything about time-traveling. During his 'quest', Marty meets Arthur McFly (his young grandfather), Irving "Kid" Tannen, and a younger Edna Strickland. With a bit of help from Doc, Marty manages to convince young Emmett that he's come from the patent office to check the Rocket Powered Drill and wants to present the drill to their office before they give the honor to 'Dr. McCoy'. However, Marty realizes that in order for Emmett to show the drill, he needs to subpoena his grandfather (an accountant for Kid Tannen's illegal speakeasy business) and get some alcohol for the fuel.
After getting those done Emmett is free for the rest of the day and he's able to build the Rocket Powered Drill to get Doc out of jail (despite an argument between Emmett and his father, Judge Brown, over the ruse they played to get the alcohol of claiming that the local temperance society, led by Edna Strickland, could meet at the Brown mansion, as well as Emmett's insistence on pursuing a career in science instead of law). However, out of guilt, Marty decides to tell the truth about breaking his friend out of jail, but young Emmett doesn't object. When Marty reaches the station, Edna tells him that Doc has been moved to another facility. Marty uses the Rocket parts of the Drill on Edna's bicycle to catch up with the police vehicle transporting Doc - which he discovers is being driven by Kid Tannen. After a struggle, Marty manages to rescue Doc while Kid becomes one of the many Tannens Marty has sent crashing into a manure truck.
As they are about to return to the future, both are relieved their misadventure is over - at least until Marty starts disappearing...
Marty goes back at 4pm, avoiding his other self, and manages to save Arthur from Tannen's goons when they are about to rat him out below the soup kitchen. After reuniting with and rescuing Doc from the policeman holding him at gunpoint, they hide Arthur and tell him to lay low. Arthur then asks both if they are "G-Men." Deciding to just play along, Marty and Doc declare that they ARE G-Men and tell him look out for a suspect named "Sylvia" (which is the name of Marty's grandmother, Arthur's future wife; Marty and Doc worry that if Arthur leaves Hill Valley, he and Sylvia will never meet) since she's the girl for him before leaving him.
Upon returning to 1986, Marty is horrified to discover that his father is wheelchair-bound, Biff has two other brothers, Cliff and Riff, and the elderly Kid Tannen is still alive. Worse, he finds out that Hill Valley is run by the Tannen family, the fifth most dangerous crime family in California. To prove it, Biff shows Marty a gun lighter presented by Don Valenti, leader of the Valenti family, the third most dangerous crime family in the state. After outwitting the Tannen brothers, Marty is shocked to see an elderly Kid in a limousine, pointing a gun at him. Luckily, he is rescued by Doc and driven away in the DeLorean. Doc informs Marty that in that new timeline, somehow Kid Tannen avoided arrest and the Tannen crime family now runs Hill Valley and endlessly harasses everyone there including the McFlys. The two time travellers decide to head back to August 25, 1931, the night when Kid Tannen was supposed to be arrested.
Doc explains that Kid Tannen was originally arrested by a rookie cop named Danny Parker (Jennifer (Marty's girlfriend)'s grandfather) when Trixie Trotter, his beautiful girlfriend and the singer at his speakeasy, turned on him by giving some incriminating evidence. With the help of a mobster disguise and the lighter he picked up from the Tannen brothers, Marty manages to enter the speakeasy by posing as a member of the Valenti gang.
Before entering the speakeasy, he meets up with Edna, who has written a song called "You Should Care" to perform at the Stay Sober Society meetings; she "treats" Marty to a performance. Marty also encounters young Emmett, who is testing a prototype rocket-powered flying car with Einstein. Ever since Judge Brown discovered that the "soup" being served at the Stay Sober Society meeting at his house was actually bootleg alcohol (a scheme orchestrated by Marty on his last visit to 1931), Edna has been harassing young Emmett and even writes bad articles about him in the newspaper. But it seems that after young Emmett's argument with his father over his choice of career, the timeline has accelerated, as Emmett quit his job at the courthouse the following day and began devoting himself to science; he thanks Marty for the inspiration. Doc is surprised to see himself inventing the car so early, but he tells Marty that the prototype will be a failure due to its reliance on rocket power, and that he will later go to see Frankenstein and be inspired to use electricity instead, just as Dr. Frankenstein used lightning to animate his monster. He has kept the ticket stub from the screening ever since as a memento of this landmark night in his development as a scientist.
Marty finds Danny Parker getting drunk at Kid's new speakeasy and learns that he was driving the police car in the chase/gun fight that Marty interrupted with the DeLorean when he first arrived in 1931, and was also the policeman who was trying to arrest "Carl Sagan the arsonist" at the beginning of the episode. For losing track of the gangsters and a key witness (Arthur McFly), as well as failing to arrest "Carl Sagan" and telling a far-fetched story about a "space car" that vanished into thin air, he was demoted by his superiors, sent to a psychiatrist by his family, and dumped by his girlfriend, Betty (Jennifer's grandmother). Convinced that he can win Betty back by earning some money, he has started taking kickbacks from Kid Tannen in exchange for turning a blind eye to key pieces of evidence being destroyed and bootleg gin being shipped to the speakeasy. However, with the aid of certain songs in Trixie Trotter's set for the night, Marty makes Danny realize he has made a huge mistake, and the unfortunate cop sobs with grief. By slipping "You Should Care" into the sheet music Trixie is performing, Marty restores Danny's confidence and persuades him to get the evidence he needs to arrest Kid from Trixie.
However, Trixie is hesitant to betray Kid Tannen since the one person she trusts isn't here... Arthur McFly. Luckily, Marty sees him returning to Hill Valley by streetcar to watch a movie. Marty and Doc tell Arthur to meet Trixie Trotter to turn on Kid; Arthur sees where they are going with this, but still refuses to reveal the evidence he has against Kid. Unfortunately, one of Kid's goons spots "Carl Sagan" and tells Kid. To make matters worse, when Trixie manages to reunite with Arthur, Kid catches them and prepares to kill Arthur until Trixie saves him by promising she won't sell him out, planning to destroy any incriminating evidence that she was going to turn over to Danny after the speakeasy closes. But Marty manages to convince Trixie to give Officer Parker the evidence - accounting books that prove that Kid is guilty of tax evasion - by tricking her into believing Kid broke his promise not to kill Arthur, as evidenced by a caricature of Arthur (drawn at Marty's request by Kid's associate, Zane, from a photo of George, who looks just like his father) on the 'Wall of Fame' (a gallery of all of Kid's murder victims).
After solving both problems, Officer Parker tells everyone to get out of the speakeasy, but they soon realize Kid is on the loose thanks to a tip-off from his goons that someone is plotting to blow up the speakeasy with dynamite. After Kid is found, he is seen taking Edna as hostage. Having also realized that Marty isn't really a member of the Valenti family, Kid opens fire on Marty and Doc from an upstairs window with a Tommy gun. Marty is able to distract Kid for long enough to allow Edna to escape, and thanks to Emmett's flying car (controlled with a bit of help from Doc), Kid is brought in and arrested (after getting dumped in the back of a manure truck yet again), causing Edna to develop an interest in Emmett, while Trixie is relieved to find Arthur alive.
As Marty flies the DeLorean to 88mph to return to 1986, Doc realizes that something is terribly wrong when his Frankenstein movie ticket begins to fade from existence; back on the Courthouse Square, we see Edna diverting Emmett's attention from buying a ticket for the film, and they walk off together instead. Doc tells Marty to stop, but it's too late as they have already hit 88mph and end up in another alternate 1986. A panicked Marty crashes into a billboard and is shocked to see Doc is gone. Just then, he hears Doc's voice from the poster - or, rather, the voice of First Citizen Brown, telling him to relax: they have everything under control...
Once in Hill Valley, he meets alternate versions of his mother (back to being an alcoholic), his father (still a voyeur, but now monitoring a bank of surveillance cameras), and Biff (who has been rendered polite and harmless, incapable of swearing or violence, thanks to "Citizen Plus" treatment). It is also revealed that Emmett and Edna got married, resulting in a drastic change in the former, and as a result, everything that happened in the movie trilogy never occurred. Marty even realizes that his and Jennifer's personalities and roles have been reversed, and she has dumped Marty for being a square and started dating someone else. But worse, once Marty checks on his father's work, it turns out that nobody likes the way Citizen Brown runs the place. Marty decides he needs to meet Citizen Brown, and seeks advice from Biff, who says that if he breaks enough rules and racks up enough demerits to get Citizen Brown's attention, he will be taken to him.
Marty goes on a mission to break three rules. First he decides to find some booze. George, suspecting that Lorraine's drinking habit hasn't changed, asks Marty to check on her, but Marty's flimsy cover story causes his parents to argue. With the help of a statue, he blocks the camera while Lorraine takes a sip of booze from her flask. He is able to take the flask from her and promise to keep her drinking a secret as she breaks down crying, but he then purposely gets himself caught for alcohol possession by Jennifer Parker's dad (who's now the Chief of Police of Hill Valley), who confiscates the flask and gives Marty a demerit. Second, he manages to find Einstein and remove his muffler, resulting in Marty getting a second demerit after Einstein scares Edna, who is out for a walk. Oddly, she remembers Einstein and asks Officer Parker to throw him in the trash chute. And finally, he manages to convince Jennifer that he's the only one for her by beating her new boyfriend, Leech, in a rock showdown. He kisses her in Leech's workplace, and is caught by Edna. Not only does he get a third demerit, but Officer Parker orders Marty to stay away from his daughter. With three demerits, Marty is apprehended and sent to Citizen Brown.
Citizen Brown greets Marty coldly and acts as his psychiatrist even when Marty calls him "Doc". Marty tries to tell him about time travel, the DeLorean, and that he has met his younger self under a different name, but Brown dismisses this as the ravings of a lunatic. Not giving up, he reminds Brown of the scientific work he did as a teenager before he fell in love with Edna, but this makes Brown accuse Marty of trying to confuse him. Brown retaliates by showing Marty his "achievement" photo where he captured Kid Tannen single-handedly and met the woman of his life, Edna. But Marty manages to point out himself and the original version of Doc in the photo, and a stunned Brown realizes Marty is telling the truth. Marty explains the difference between his original timeline and the current timeline now, but Brown is enraged when Marty says that the original timeline is vastly superior to the police state Hill Valley has become. Marty says he can prove that no-one in Hill Valley likes how the town is run; unbeknownst to Marty, Brown believes him, as he lets him leave once the conversation is over. Meanwhile, Edna has been eavesdropping on their conversation.
Marty goes back home and finds an unconscious George on the floor near his monitors. He checks the surveillance video and sees Biff knock George out with a baseball bat, and his Citizen Plus Treatment watch flashing and showing X:11 while he's bashing the surveillance equipment. Worse, George's surveillance tapes were stolen, which is the proof that Marty needs to prove his point to Brown. He confronts Biff, but Biff doesn't remember the event ever occurring, and he can't break rules as even swearing causes him to feel physically sick thanks to the Citizen Plus treatment. Marty plays a bit with his watch and notices that the X:11 setting turns Biff into a mindless zombie. Marty asks Biff to retrieve the tapes and Biff jumps into the trash chute.
As Marty follows him into the trash chute, he is surprised to end up in Kid Tannen's old speakeasy and discovers that Edna's the real mastermind behind all of it. She modifies Biff's watch to X:11 and orders him to catch Marty. Marty manages to snap Biff out of it by showing him many of his favorite things that are hidden in this room (beer, porn, cars...) The sheer stress makes Biff collapse and he passes out. Unfortunately, it is too late, as Edna has already erased the tapes during Marty's struggle with Biff. Just then, Citizen Brown arrives after hearing the report from Officer Parker about Biff's jump into the trash chute. He even can't believe that Edna is using him and even overheard her confession. He is more furious that Edna uses Citizen Plus as a way to turn the people of Hill Valley into mindless puppets with no free will. Edna tries to charm her way out saying this was what he wanted, but Brown has had enough and tells her to wait at home.
Marty can only watch as Brown wails in anguish over how his dream for Hill Valley has gone so badly wrong. Brown finally says that what's done is done, as there's no way to change it, but Marty gives him Doc's old notebook, which includes the drawing of the flux capacitor. Brown realizes that he incorporated into the the logo of Hill Valley and gets a "Eureka" moment with a flash in his eyes, obviously overjoyed to have a way to correct his errors. Brown tells him that he would be happy to fix the DeLorean as he exits the speakeasy, and Marty frees a caged Einstein. But then Biff wakes up, angry at Marty for making him jump into the trash chute, and decide to take out his anger on him before going after Edna. With the help of Einstein and the equipment from the speakeasy, they manage to knock out Biff and leave the room.
Marty goes back to Brown's office, only to be greeted by Edna. Fed up of Marty's meddling, she reveals she is preparing a Citizen Plus Treatment for BOTH Brown and him to make them obedient again, and tells the guards to take Marty away...
Marty and Brown escape by hiding inside the dustbin and realize that Brown has hidden the damaged DeLorean at his secret lab near Clayton Ravine (back to its original name since Marty and Doc never went back to 1885 to save Clara Clayton after Brown abandoned his plans for a career in science) before he was captured by Edna. They soon witness Marty's parents getting captured by the guard when George tries to rescue Lorraine. Brown reassures Marty that none of this will have ever happened once they restore the timeline, and he decides to get the DeLorean prepared. He tells Marty he will be back in a few minutes, since Marty's help fixing the DeLorean could have catastrophic effects.
As soon Marty exits the bin, Edna appears and explains to him that her husband's not coming while mocking his inability to do anything without her. Just then Brown appears in the DeLorean, explaining that the repairs took him six months and required that he spend the remainder of his family's fortune and make a shady deal with some Libyan terrorists to get the rest of the money and materials, but that he's back in time, although he made a slight miscalculation with the timing. He coldly rejects Edna's attempts at affection, seeing them as a ploy to stop him from going with Marty. Brown insists that Marty call him "Doc" again, and the duo escape back to 1931 in the DeLorean while Edna swears revenge.
Unfortunately, they arrive on the day of the opening of the Hill Valley Science Expo at the high school instead of several months earlier, so that young Emmett is already madly in love with Edna and cannot be persuaded to change his feelings for her and, worse, wants to propose marraige to her that night. Even worse than that, Doc discovers that the DeLorean's time circuits are malfunctioning, as it has jumped to a different timeline despite being given the correct time co-ordinates. The two time travellers are stuck in 1931, as Doc doesn't want to risk doing a time jump again, concerned they might end up in a prehistoric age. He suggests they try talking to the more level-headed half of the couple: Edna.
Marty meets up with Edna, who is arguing with Arthur for forgetting to include Emmett's name in the expo. He greets Marty and tells him that Emmett (with Edna's help) has built a Mental Alignment Meter which tests everyone's personality. Marty decides to ask her what she likes about Emmett to find a way to break them up. She tells Marty that Emmett is a genius and, as demonstrated by the machine, he is a model citizen, and he doesn't chase other girls. She also notes that Emmett is wearing her grandfather's suit, and says she hopes he doesn't ruin it. Marty decides to head to Emmett's house, but Edna is already going there, and Doc has to stall her so that Marty can get there ahead of her.
Determined to break up the two lovebirds, Marty tells Emmett that Edna has sent for him, but Emmett is still resolutely planning to propose to her that night. While Emmett is distracted with his Mental Alignment Meter, still wearing the helmet that takes the readings for the personality test, Marty creates fake results and exchanges them with Emmett's real results. Emmett unknowingly puts the fake results in the box he is planning to take to the expo. Next, Marty tries to ruin Edna's grandfather's suit by "accidentally" spilling oil on it, but Emmett has already created a cleanser that cleans stains out of the suit instantly. When Emmett explains that the cleanser only works for twelve hours, after which the cleanser would cause the bacteria in the cleanser to eat the suit like a pack of moths, Marty steals his cleanser and puts it inside the DeLorean to make it defective. Finally, he seeks Trixie's help by purposely showing an incriminating photo of her to Edna (since she despises Trixie for her background as a speakeasy singer) and gets Trixie fired by Arthur after he discovers that she is a Canadian, and the Expo only allows American participants. With some items requested by Trixie, Marty is all prepared for Emmett to show up.
However, before Marty can put his plan into action, Doc stops him out of concern that Edna will end up old and lonely, as she did in the original timeline, since his affection for her as his wife is still part of his personality even after his conversations with young Edna. Doc proposes a third option: Marty should find a way to ensure that Emmett keeps his passion for science without having to break Edna's heart. Marty refuses, saying that they have already done enough damage to the timestream, and a furious Doc drives off in the DeLorean. And so once Emmett arrives, Marty spills oil on his suit, and Emmett uses the tampered cleanser on it, causing it to disintegrate in front of a horrified Edna. Trixie arrives and pretends that she has been having a relationship with Emmett behind Edna's back that she is now ending. Finally, Edna sees the fake personality test results that show Emmett to be a degenerate criminal like Kid Tannen, causing Edna to break up with him and storm off while Trixie looks smug. Unfortunately, the plan works too well, as Emmett's self-confidence is destroyed by the encounter.
At night, Marty finds Emmett on the courthouse roof and is worried Emmett is planning to jump to his death after the day's events. But Emmett reassures him that this is where he comes when he needs his time alone. After much reflection, Emmett realizes that his ongoing misfortunes are all Marty's fault, as every time he shows up, something bad happens - the argument with his father, his "fake hero" status, and the demise of his relationship with Edna. Marty is able to use reverse psychology to get Emmett angry enough to reignite his love for science. A bolt of lightning strikes the courthouse roof near them, and Emmett regains his passion and an idea for another source of "fuel energy" for his flying car. Unfortunately, the same lightning dislodges a statue next to the clock that leaves young Emmett about to fall to his death. Marty manages to save Emmett's life and reveals his true name: Marty. Emmett decides to ask Marty to help him out back in his lab before the expo opens the next day, and Marty is relieved to see Emmett back to his old self.
Meanwhile, Emmett's older self, Doc, picks up a crying Edna in the DeLorean and asks her to tell him everything she knows about Marty, having decided to do what he can to thwart his former friend's plans...
At the expo, Marty can't find Emmett, but he finds Edna and Officer Parker barricading Emmett's booth. Edna tells Parker to arrest him, identifying him as the anarchist "Yakov Smirnoff" that Brown has told her so much about. Officer Parker tells Marty privately that he believes that Edna is crazy, but he has to obey her because after her articles soared in popularity after Kid's arrest, he worries that she will write a series of articles alleging corruption in the police force if they resist her demands, plus the mayor and the Chief of Police love her articles and don't want to get into trouble. Parker suggests that if Marty wants the barricade to be removed, Marty needs to find some evidence that will tarnish her reputation.
As Marty looks for Emmett, the Science Expo's bathysphere owner Jacques Douteux mentions that he saw him going to the House of Glass with a bald old man - Citizen Brown. Marty gets into the House and witnesses Emmett being knocked out with chloroform by Brown, who notices Marty following them as he drags his unconscious younger self out of the house. Marty loses track of the older and younger Emmett and asks Douteux if he has seen them again, but Douteux answers in a very strange way, and Marty realizes that Brown has disguised himself using Douteux's diving suit. He orders Brown to free Emmett, but Brown plays dumb, and Marty nearly gets hauled away by Arthur (the organizer of the expo). Marty outwits him by asking Arthur to let him see the inside of the bathysphere. Suspecting that young Emmett is in the bathysphere, he crimps its oxygen line which causes Citizen Brown to choke, realizing that if young Emmett dies of suffocation, he too will vanish from the timeline. He eventually gives in and raises the bathysphere, revealing young Emmett inside and proving to Arthur that Marty is telling the truth about the fake Douteux. Brown escapes from the expo in the confusion while Arthur gives chase. Emmett regains conscious and realizes that his booth's been barricaded, whereupon he furiously confronts Edna.
With the help of a recording device shaped like a flower pot from Officer Parker's booth and the "phone of the future" to impersonate "Carl Sagan" in the House of Future, Marty gets proof that Edna is the real speakeasy arsonist, and she framed Doc for the crime after Einstein tried to stop her from starting the fire. She burned down the speakeasy because the law was doing nothing about it, and she hates the smell of booze and the fact that it dirties people's minds, so out of moral outrage, she "did what any rightful citizen would do". Marty plays the recording of Edna's confession to Officer Parker and Emmett. Edna denies it, but Parker, who has had enough of her behavior, wants to arrest Edna and intends to bring her down the station to interrogate her about her whereabouts during the night of the speakeasy fire. Edna pleads with Emmett for help, but he refuses after what she has done. Out of options, Edna creates a distraction and runs away from Parker. Emmett is relieved that with Edna and Parker gone, they can finally remove the barricade and show off his prototype.
Their victory is short-lived as an outraged Judge Brown arrives, ordering Emmett to stop this at once. Caught in the crossfire between father and son since neither will listen to the other, Marty becomes the peacemaker and "mouthpiece" and sorts things out. During the conversation, Judge Brown and Emmett realize that they are turning out like their own fathers; Judge Brown's father never approved of his decision to immigrate to America since he was almost penniless and couldn't speak English, while Emmett realizes he's displaying the same hot-headedness and stubbornness he resents in his father. Judge Brown calms down as he tells his son that even though he knows it will turn into a disaster, he is still there to support him. Emmett climbs into his flying car, and after an initial stumble, it begins to work, to Judge Brown's delight. Marty is relieved... until he looks out through the window to see Edna confronting Citizen Brown before stealing the DeLorean, with Officer Parker giving chase.
Marty rushes out to check on him, as Brown sighs that he thought he could change Edna but made things worse instead. Marty tries to help, but soon the timeline catches up as Brown begins to fade and tells Marty not to help. Suddenly, Marty sees Edna driving straight toward him in the DeLorean, with Officer Parker in hot pursuit. At the last second, Brown pushes Marty out of the way, saving his life at the cost of his own. The dying Citizen Brown starts to fade from history and asks Marty to show him his 1986 newspaper. It is revealed that Doc has been presented with the key to the city of Hill Valley now that Marty has corrected his timeline. Citizen Brown smiles and disappears, while Marty kneels down, closes his eyes, and has a moment of silence for his late friend.
Meanwhile, there is a bright flash and the sound of an explosion from the school windows, and Emmett emerges through the doors in a cloud of smoke, surprised that Marty missed the show. He explains that the experiment failed and he's banned from the expo for the next fifty years (it is implied that the car exploded after crashing into the House of Glass). Far from being discouraged, Emmett begins brainstorming modifications to his car's design and begins to show much more passion. Just as Emmett begins to ask Marty to follow him, he notices Marty seems sad. Marty tells him it is fine, but Emmett realizes that if there's something bothering Marty, it's always something to do with him. Emmett sees the 1986 newspaper but Marty refuses to show it to him. Having an idea, Marty gives Emmett a folded piece of newspaper from 1986 making him promise to never look at this until he is given the key of the city in the future. Emmett is confused at first, but ultimately agrees. Before his final farewell, he asks Marty whether he'll be seeing him again. Marty replies, "I guarantee it." Emmett waves goodbye and enters the school to meet his father.
Suddenly, the DeLorean arrives out of nowhere and Doc Brown (now back to his normal self) is in the car to meet up with Marty, and the two embrace. Doc came to check up on Marty after receiving the key after a long ceremony and couldn't believe that the boy who helped him in 1931 and the boy he helped in 1955 were the same person, cleverly praising him for finding a way to send messages without disrupting the time stream. Doc was curious why he was here in the first place, but when Marty discovers that Doc doesn't even remember being in jail, he decides to drop the topic. As soon he's going to ask about any other time anomalies Marty might have accidentally triggered, William McFly (Arthur's father) arrives in his Hill Valley Mercantile delivery truck to find his son - who has entered into a Citizenship Marriage with Trixie Trotter.
As William goes into the school, Marty panics as he fears the change of timeline. Doc reassures him there's nothing wrong, but as he's about to explain, Officer Parker stops in front of the DeLorean, wondering that if this is a joke to send him to the nuthouse since he just saw Edna and the DeLorean vanish into thin air. A shocked Doc asks Marty if he remembers the time she jumped to, but Marty says that Edna's DeLorean is malfunctioning and could jump into any timeline. Doc panics and hopes that Edna has ended up in the future rather than the past, where she could cause irreparable damage. Just as Parker asks for an explanation, he vanishes, and soon the whole town vanishes around them, leaving them standing by a dirt road in the middle of the desert. Marty asks if they have just left Hill Valley; Doc replies, "Hill Valley has left US."
They are surprised when William McFly drives up in his delivery truck (now bearing "Haysville Mercantile" livery), and they ask him what happened to the town, but William only recalls that his father told him that while there was once a town where they are now, it was gone by the time they arrived in California. He adds that nobody lives here now except "Scary Mary". Doc and Marty deduce that "Scary Mary" might know something about this and decide to find her after William gives them directions. Upon arriving they find out that "Scary Mary" is actually an elderly Edna Strickland who has a rusted out DeLorean outside her porch and goes by the name 'Mary Pickford'. By gathering clues to re-kindle the elderly Edna's memories, Doc and Marty learn that Edna ended up in 1876 and was happy to live in that era since there's no booze and debauchery, and she even met her grandfather, the local United States Marshall. The happy times ended with the arrival of newcomer Beauregard Tannen, who bought a plot of land on which he built a saloon. Furious that no-one seemed interested in doing anything to stop it, Edna burned down the saloon on July 17, 1876 - and, by accident, the rest of the town of Hill Valley as well. Thankfully, she still has the newspaper regarding the fire and hands it to Doc, but she then identifies Marty and Doc as "Yakov Smirnoff" and "Carl Sagan", declaring that they set her up so she could destroy Hill Valley, in revenge for which she plans to shoot both of them with her rifle. Luckily, William, who is making his monthly newspaper delivery to Edna, overhears everything and holds Edna off while Marty and Doc escape.
The two time travellers head back to 1876 to stop Edna from burning down the saloon. Marty sneaks past the back of the saloon while Doc confronts Edna before she can light the fire. However, the commotion wakes Beauregard Tannen, who plans to shoot both of them for trespassing. Doc tells them that they're at a stalemate since if Beauregard shoots Edna, she'll drop the torch she is holding and set fire to the saloon, while if Edna sets fire to the saloon, Beauregard will shoot her before she can escape; irrespective of which one acts first, they both lose. Thanks to Marty's wits, Beauregard and Edna are disarmed at the same time, and Marty distracts Beauregard for long enough for Doc to knock him out. However, Edna escapes in her defective DeLorean, and Marty and Doc chase after her. Doc reassures Marty that the road in 1876 isn't smooth enough to reach 88mph so they have a chance to sync each other's DeLorean and send her back to 1931 by placing some radars onto the DeLorean's flux emitters, put there by Citizen Brown. With the aid of the hoverboard Marty picked up in 2015, they are able to attach the radars and send both DeLoreans back to 1931.
Edna's DeLorean crashes into Hill Valley's Police Station upon arriving in 1931. She is rudely woken up by Officer Parker and arrested for arson, resisting an arrest, and being an all-round pain in the ass. Edna insists she's a time traveller who has just returned from the past, but Parker doesn't believe her and drags her into the police station while Kid Tannen, seeing the commotion outside his jail window, thinks she's drunk and offers to let her bunk in his cell. Marty and Doc watch the scene from across Courthouse Square, and Marty asks what are they going to do with Edna's DeLorean. Doc mentions that now that they have synced the two DeLoreans, Edna's DeLorean will decay at an accelerated rate as the timeline catches up. As Doc checkes his watch, saying it should catch up "now", Edna's DeLorean disappears, to the amazement of a watching Kid, who tries to tell Officer Parker.
Marty remembers that he still has to stop Arthur from marrying Trixie instead of his grandmother, Sylvia, but as he sees them exit the courthouse, Trixie wearing a wedding ring, he worries that it is too late. His fear escalates to panic as Arthur and Trixie explain that they got married in Reno to make Trixie a U.S. citizen after she was fired for being a Canadian (and for posing for the racy picture they showed Edna to get her to break up with Emmett). Marty tells Arthur he's supposed to marry Sylvia Miskin, but Arthur says he did - Trixie Trotter is Sylvia Miskin! She adopted the stage name "Trixie Trotter" for her singing career. Marty is delighted, and tells them to have fun and not to worry about William until he shows up behind them. Marty assumes that he's angry about this, but William changes his mind after taking a good look at Sylvia and approves their marriage. Sylvia thanks him by calling him "Dad" and gives him a kiss. Before bidding Marty farewell, William tells Marty not to poke his nose into the McFly family business, Arthur shakes hands with Marty saying it's been a pleasure working with him, and Sylvia kisses him on the cheek, telling him she'll see him in the funny papers. As Arthur and Sylvia leave, Doc says he knew about Trixie's real identity and jokes that he has already taken some pictures of her; an unamused Marty tells Doc that's his grandmother he's talking about.
Returning to May 15, 1986, Marty is shocked to find Doc's items are still being sold as part of the estate sale, and he asks Doc to stop the bank from selling his house. Doc is confused; he tells Marty that it's not an estate sale, it's a garage sale. He, Clara, and their two kids are using their home part time in 1986 and need to clear out some space. George comes to greet both of them and asks Marty if he found what he's been looking for. Marty nods as George spots something interesting in Doc's garage sale. It turns out that after mending his relationship with his father, Doc established a foundation in his memory to encourage young people to enter the field of science, to Marty's amusement. Doc remembers something and gives Marty a book titled "The McFlys of Hill Valley" as an early high school graduation present. He has been absent doing research into Marty's family tree, from Seamus McFly onwards. Most of his research has been done "traditionally" but the only problem in the family tree book was Marty's grandmother Sylvia. Doc never knew she sang in a speakeasy under a stage name, which was the whole reason he went to 1931 in the first place. It was a way of saying thank you to Marty for being the person who saved him from making the worst mistake of his life.
They receive a huge shock when the elderly Edna arrives, now much more kindly disposed - not just toward people, but toward dogs, as she now takes Einstein for his daily walks. As if that isn't enough, it turns out that she has married Kid Tannen, who pulls up in a car to collect her and Biff, whom Edna addresses as her stepson. As the Tannens leave, George comments that they make a lovely family - who would believe they met in jail? It is hinted that after Kid discovered the 'disappearing' DeLorean, he and Edna were bunked into one cell and mended their relationship with each other and fell in love. After they were released, they got married, and have become much kinder and happier than their original counterparts. Doc tells Marty "Don't say it!" and suggests that they walk back into the lab, hoping that no other surprises are waiting for them.
Suddenly a DeLorean appears with a future Marty inside, telling Doc that he needs to go back to the future with him to save his and Jennifer's great-great-grandchildren... followed by a second DeLorean with a second future Marty telling him that the first future Marty's timeline was erased five jumps ago and that it's his timeline that needs to be fixed... followed by a third DeLorean and a third future Marty, who tells the other Martys that they've messed with his timestream for the last time. Confused, Marty asks Doc which one is the real Marty; Doc points out it's the one he's talking to now. While the future Martys start squabbling, the original Marty and Doc get into the DeLorean as Doc tells the three other Martys "The future can wait, we have a present to catch up with." Doc and Marty start flying the DeLorean as Marty asks, "Where to Doc?" Doc answers, "Mr McFly, thrill me!" They both depart to an unknown time and place.