A ChristmasEpisode of the show (and what would have been the final episode of the entire series).

After the Thanksgiving dinner, Marge set the family up for holiday photos much to their charign. She warns Bart and Lisa that they'll want to do this for their own kids someday though Bart and Lisa are doubtful they'll have any.

The photo is taken and we see the years pass via the Christmas photos as the Simpsons kids grow older. Bart tries to go to college but flunks out a year after attending and moves back in with his parents before eventually being kicked out. He married his girlfriend from "Future-Drama" (Jenda) and has two kids, but ends up divorced. Lisa, not surprisingly, is more successful in her college endeavors and her love life. She has a string of lesbian affairs in college before settling down with Milhouse and having a daughter. Maggie pursues a career in music and becomes a world famous rock star, despite that we the viewers still don't hear her voice. Homer and Marge remain married through the years.

In the current timeline, Bart is living in a broken-down apartment complex (that used to be Springfield Elementary) and barely keeping rent. His two sons are beamed over as Bart's ex-wife wants Bart to spent time with them, something both the kids and he aren't entirely happy about. Bart figures to just dump them with ''his'' parents. Lisa lives moderately with Milhouse and their now teenage daughter, Zia who spends all her time on the Neural-Net (The internet you plug into with an implant). Due to Milhouse's allergies, he suggests taking Zia to Lisa's parents house while he nurses his sinuses. Over in London, Maggie, while on tour, is revealed to be pregnant with her baby due soon. The doctor orders her to not sing or use her voice as he explains "recent studies have shown the umbilical cord is also a vocal cord".

Marge and Homer get "B-Mail" (Brain Emails) that the kids are all coming home to be with them during the holidays much to Marge's joy. Bart and Lisa arrives with their kids but Maggie is delayed as she is advised not to teleport while pregnant and has to take the commercial airlines which have descended into Mad Max-style brand of flight. Lisa tries to spend time with Zia but her daughter isn't willing, Marge suggests trying to be more "hands off" and give Zia some space. Bart likewise isn't having any luck with his kids, falling into depression when the boys reveal their mother remarried. Homer instead take the two boys out on a trip to Downtown Springfield.

That night Bart and Lisa meet up in the treehouse and have a few drinks while discussing their problems. Both agree to try their best with their kids. Maggie arrives in Springfield but no sooner then she hails a taxi, her water breaks and she has to be taken to the hospital. As the hospital is full, she has to be put in the manger wing. Lisa tries to take her mother's advice but can't go through with it and enter the Neural-Net to find her daughter. She manages to locate her thanks to Google and finds that her personal page shows that Zia aspires to be like Lisa. The two reconcile. Bart tries to locate his sons and finds that Homer took them to go see Grandpa who is located in a cryo-lab located in a graveyard (Homer revealing Abe was frozen when he came down with a disease. They found the cure for it but Homer finds the Cryo-lab cheaper then a retirement home.)

During the visit, Homer teaches the boys that, though Bart may not be a perfect father, he still loves them all the same and to give him another chance. Bart arrives shortly after and apologizes to his sons, promising he'll do better and they too reconcile. Homer likewise reconciles with his father. Marge is informed of Maggie giving birth and is there for the birth of her baby. The family all reunite for Christmas and gather together for a Christmas photo, this time taken by their pets who have evolve into intelligent beings.

---
!!''Holidays of Future Passed'' has the following tropes:

* AbusiveParents: Thankfully averted with Bart and Lisa who, despite having problems with their kids, don't do what Homer and Marge did in terms of parenting -- mostly because they're better than that, and mostly because Homer's strangling of Bart became such a problem that a law was made to forbid child abuse (particularly strangling of any child) once and for all (called "Homer's Law").
* AbsurdlyYouthfulMother: Lisa doesn't look much older than her teenage daughter.
* AwayInAManger: Slightly parodied with Kearney as Joseph and helping Maggie (both the Mary expy as well as the "star") to the hospital and her having to be taken to the Manger...Wing.
* BabiesEverAfter: All the Simpsons kids have their own kids and Maggie having first daughter. [[SubvertedTrope The "ever after" would have been true had the show ended at that point, but it's not]].
* BaitAndSwitch: Aside from the FoodPills parody, there's also a cab driver (Kearney) who drove Maggie to the hospital. When his cab's computer took his order to drive to the hospital ("Computer, hospital") as an order to go to the [[ExactWords computer hospital]], he said he'd have to do it the old-fashioned way. He took a pair of gloves and ordered them to drive to the hospital.
* BookEnds: The first ''Simpsons'' episode was a Christmas episode ("Simpsons Roasting Over an Open Fire"). If this actually aired as the series finale, it would have ended with a Christmas episode.
* CallBack: Bart is shown to have married (and divorced) Jenda from "Future-Drama".
** Homer accidentally getting Edna killed echoes what happened to Maude, though, unlike Maude, we don't see how Homer had a hand in killing Edna.
** HarsherInHindsight: A year after this episode aired, Edna's voice actor, Marcia Wallace, passed away, and her character, Edna Krabappel, will never be seen again.
* ChalkOutline: Several of them at Moe's. One of them was implied to be of Sideshow Bob (with a rake). Another is the Capitol City Goofball from "Homer at the Bat".
* CloningBlues: Ralph is revealed to have made clones of himself...who get killed with alarming frequency.
* DrowningMySorrows: Bart and Lisa do this in the treehouse after Zia won't spend time with Lisa and after Bart's sons prefer spending time with Homer then with him.
* ExperimentedInCollege: Lisa has several girlfriends in college (and at one point, two girlfriends at once), and then gets engaged to Milhouse soon after graduating.
* FamousFamousFictional: A variation in this exchange:
-->'''Lisa:''' My daughter thinks I'm some sort of dictator, like Hitler or Prince Harry.\\
'''Homer:''' Ah, Bloody Harry. He brought back beheading in a big way.
* FoodPills[=\=]InstantMassJustAddWater: Parodied. When Marge wants to make a cake, she takes out a tablet and adds water. The tablet turns into... a recipe card, which she uses to make a cake with real ingredients.
* FreakyFridayFlip: Carl and Lenny are revealed to have swapped brains in the future. One of them has an ex-wife who swapped brains with a monkey.
* GenerationXerox: Bart's kids have a strained relationship with their dad, but get along fine with their grandfather. Homer points out that this is par for the course with every son.
** Lisa tries to understand a daughter that seems very different from her and can be condescending at the drop of a hat.
--->'''Lisa:''' You have no idea what that's like.\\
'''Marge:''' [[DeadpanSnarker Uh-huh.]]
** Maggie's daughter sucking on a pacifier.
** Apu and Manjula's Octuplets work at the Kwik-E-Mart with their dad. They also each have their own set of octuplets.
* HotMom: Lisa. By the end of the episode, Maggie's one as well.
* HumanPopsicle: Along with Grandpa, other Simpson regulars are cryogenically frozen.
* NeuralImplanting
* NoodleIncident: Lenny and Carl switched brains so Lenny could get back together with his wife, who was sleeping with Carl.
-->'''Carl:''' Turns out she had switched brains with a monkey on a Japanese game show, and it just got weirder from there.\\
'''Lenny:''' Ah, I found it quite normal.
** Homer somehow got Edna killed.
* NotSoDifferent: Bart and Lisa bond in the treehouse over some beer and their parenting difficulties.
* TheNothingAfterDeath: Maude's ghost claims as such to Ned, though he doesn't seem to notice.
* OlderThanTheyLook: Lisa is almost 40 in the episode. She looks 10-15 years younger. Boy, did she age well.
* ProductPlacement: Google
* {{Reincarnation}}: Apu thinks his brother was reincarnated as a cow, but it insists it's just an animal that died 600 years ago.
* SeriesFauxnale: This episode was conceived as a DistantFinale in case the show really was going to end.
* ShoutOut: In the montage, Bart and Lisa dress up as [[ComicBook/VForVendetta Guy Fawkes]] and Franchise/SailorMoon (or a ''Sailor Moon''-inspired magic girl superhero) respectively during their teen years.
** Selma's [[{{Sexbot}} lovebot]] looks like Gigolo Joe (Jude Law's character) from ''[[Film/AIArtificialIntelligence AI: Artificial Intelligence]]''.
** [[Series/DoctorWho Daleks]] are the police in London.
** Maggie is staying at the [[Series/TheBennyHillShow Benny Hilton]].
** The planes are from ''TheRoadWarrior''.
** ''Film/{{District 9}}'' is a dystopia, but District 10 is upscale.
** Dr. Hibbert is wearing [[Series/StarTrekTheNextGeneration Geordi's VISOR]].
* SubwaysSuck: In the future, the interior of planes look and sound like subways and teleportation is the transportation of choice for the poor, the elderly, and pregnant women.
* TalkingToHimself: Nancy Cartwright voices Bart's sons.
** Likewise Yeardley Smith (who doesn't voice anyone else except for Lisa [and, on rare occasions, girls who look or act like Lisa, like Lisabella from "The Last Tap Dance in Springfield" or the Pacific Islander girl Homer names "Lisa, Jr." on "Missionary Impossible"]) voices Zia.
* TimeSkip: Unlike previous episodes that take place in the future, there's no framing device here. The only present day scene is the one in the beginning with the family photo.
* TooDumbToLive: Ralph Wiggum's clones.
* TookALevelInKindness: Homer by the time he's a grandparent, he's a really good grandfather and has learnt from his mistakes.
* UncannyFamilyResemblance: Per usual, the next generation of Simpson kids greatly resemble their parents.
* VictoriousChildhoodFriend: Milhouse manages to marry Lisa
* TheVoiceless: Maggie, in continuing with the RunningGag that was set up on "Lisa's Wedding."
* WhatCouldHaveBeen: As mentioned in SeriesFauxnale, this episode was slated to be the final episode, as FOX was having budget issues with the show (the show was becoming too expensive to advertise and ''Bob's Burgers'' was becoming more popular, blowing both this show and most of SethMacFarlane's shows out of the water). Once everyone agreed to take a pay cut in order to keep the show afloat for at least two more seasons (the 24th season and the current 25th season. On October 4, 2013, it was announced that ''The Simpsons'' would have a ''26th'' season, and Al Jean has said he wanted the show to end after 30 seasons), this was set as a Christmas episode.
* YourCheatingHeart: Implied.
-->'''Bart:''' Nelson still calls you?\\
'''Lisa:''' Let's just say someone calls someone.