Season two ended with a massive tease of a plotline that had apparently been building for two seasons. Alexander Trepkos (the man who advised Peter Kingsley, the season's Big Bad) puts in a call to a German arms dealer named Max and tells him that "Plan B" is a go. (In a deleted scene on the boxset, it's revealed that Nina is also with Max, and that the German contact she was in touch with in the first season was Max's associate.) "Plan B" kicks off with the (attempted) assassination of President Palmer...and nothing else. In the third season, Palmer says that the people who tried to kill him were brought to justice. The Max plotline is half-heartedly resolved in 24: The Game (which was released during the fifth season-airing of the show and focuses on events between seasons two and three, long after everyone stopped caring), and never mentions what happened to Trepkos.
Famously, Behrooz Araz disappears three-quarters of the way through the fourth season after his mother is executed by Marwan (and after having a majority of the season focused on his survival under ridiculous circumstances). His fate is resolved in a deleted scene — he's rescued by Curtis before Marwan's men execute him (and learns about his mother's fate) — but most viewers likely wondered where he went during the original television airings.
The first 8 episodes of season 1 focus on an assassin who gets plastic surgery to obtain a new identity and get close to Senator David Palmer at a morning rally. Jack never meets the guy until their one and only encounter, but successfully throws the assassin off Palmer by implicating himself as an enemy. The assassin runs away when Jack gets arrested...and never shows up again, despite having plenty of chances afterwards to kill his target, in any of the episodes or seasons following this. The producers supposedly forgot about the character.
There's a whole arc in the second season where Presidential aide Lynne Kresge tries to get incriminating evidence on Mike Novick to David Palmer. She falls down several flights of stairs while attempting to outwit one of Novick's guards, and has to be wheeled into an ambulance (while still having the knowledge). She never appears in the series again, and fans continually complained about it (pointing to Novick's duplicitous nature) in every season that followed. The producers even yo-yo'd on whether she would be brought back in a later season or if she died as a result of her injuries.
The original series itself is likely an example of this. 24: Live Another Day ends with Jack being hauled off by a bunch of goons to a Russian gulag to be tortured due to his actions over the course of the previous day. Likewise, a DVD bonus feature called "24: Solitary" has Tony Almeida (who is still in prison after being arrested at the end of season 7) being busted out by a female government agent, ostensibly so he can go rescue Jack. In 2015, this setup was scrapped when it was announced that the series would be rebooted as a new story called Twenty Four Legacy, which had no connection to the original series outside of CTU and no involvement from Kiefer Sutherland (who stated that he was done with the show). However, the confirmation that Tony would appear in Legacy showed that his arc would at least be continued.
In season 3 of 30 Rock, Liz Lemon decides to adopt a child after a pregnancy scare at the end of last season. This plan becomes nonexistent after being an integral part of a handful of season 3 episodes, though it was lampshaded in the season 5 episode "Operation Righteous Cowboy Lightning".
Kenneth: I couldn't put the memo in your mailbox because it's full of unread adoption materials. Liz:(uninterested) Yeah.
There was a lot of buildup about Diana's relationship with her father: in the first episode, she calls him when she thinks Earth is about to be destroyed, but he doesn't pick up. Later, she tells Tom that she wishes her father was as good to her as he was to Kyle, and at one point she comments "when you lose your trust in a person, especially a parent, you can never get it back." This arc is promptly dropped. In fact, in one episode where the characters all see people from their pasts, she sees an ex-fiance she's never mentioned before, while Tom sees his father, who had also never been talked about.
In "Suffer the Children", Shawn begins helping out a group of homeless people and quickly befriends one of them, a teenage girl named Liv. They eventually find out about his Healing Hands in spite of his best efforts to keep his ability a secret. In the next episode "As Fate Would Have It", Shawn heals Liv when she has a drug overdose and recruits her into the 4400 Center so that she can have a better life for herself. In "Hidden", Liv goes on a date with Danny and has a relapse but Shawn allows her to stay at the Center. After this, Liv is never seen nor mentioned again even though the implication was that she and Shawn would eventually start a relationship. Shawn instead began a relationship with Isabelle in Season Three.
In "Voices Carry", Kyle befriends his English literature professor Wendy Paulson. There is an immediate attraction between them and she soon becomes a close confidante. She is the only person other than his cousin Danny that he tells about his episodes of Missing Time. Wendy is never seen nor mentioned after her fourth and final appearance, which was also in "Hidden", in spite of the fact that it seemed as if she and Kyle were going to have a potentially inappropriate Teacher/Student Romance.
In "The Truth and Nothing But the Truth", Shawn decides to run for a seat on the Seattle City Council on the advice of Senator Roland Lenhoff in order to give the moderate members of the 4400 a voice. His campaign continues in the next two episodes "Try the Pie" and "Till We Have Built Jerusalem" but the only reference to it after that is the brief appearance of several posters and flyers towards the end of the Series Finale "The Great Leap Forward".
Lots and lots on Alias, like the idea that Sydney's whole life had been tinkered with as a top secret CIA project set up by her dad. Not only did the final cut of the episode where it was introduced cut out most of the more obvious references (which mistakenly ended up in the ABC.com recap of the episode for a few hours), but it was dropped completely in the next season opener. Well... sort of. They changed it into something else that contradicted what we DID learn from what we saw of the incriminating file. And they deleted a scene that explained much of what happened that season, only to address it vaguely in the next episode as if we should know what they meant (those who went to ABC.com during another brief window found out). Oh, and the whole plot of Season 3 was cut in half and resolved with Sydney being an idiot but still destroying centuries-old magical semen. AND the driving plot behind the series was suddenly dropped in the middle of the second season to make the show more accessible.
Kate Lockley's storyline was dropped from Angel after Elisabeth Rohm got a better job offer from the folks at Law & Order.
Another notable one is the cyborg plot that was in "Lineage". We never saw any more of it or found out where they really came from. The only clue was that the cyborgs bore the symbol of the Circle of the Black Thorn — but that doesn't tell us which member sent them or why.
Planned storylines for season 6 were either dropped or expanded in After the Fall. This resulted in some season 5 stuff also being compressed or eliminated altogether.
Babylon 5, the archetypal Arc show, had several of these over the years, usually as the result of actors leaving the show. To their credit, the important parts of those arcs were relocated and reassigned to other characters.
One aborted and unaborted arc: Lyta Alexander was in the pilot episode and had mental contact with Kosh; she was supposed to get closer with the Vorlons, rebelling against Psi-Corps, and the other things that happened to her character later. When Pat Tallman didn't return after the pilot, Talia was invented, and a new mechanism (a gift from Ironheart) to give her enhanced telepathic powers was created. When Andrea Thompson left the show and Tallman came back, the substance of that arc was handed back to Lyta. With the way Talia was removed from the show, however, her personal arc hit a brick wall and died, after being kept alive in the viewer's mind for so long.
The whole The Mole arc, which was originally intended to revolve around Laurel Takashima (who only appeared in the Pilot) and was shortly revisited on "Spider in the Web", was transferred to Talia, with Ivanova acting as a Red Herring. There were plans to bring back the "real" Talia via Kosh's memory storage device, but when Andrea Thompson left the show, this was dropped. Furthermore if you believe in the Word of God, Takashima was originally intended to shoot Garibaldi, which was then transferred to his right-hand officer.
The original plan for "Sleeping in Light" involved Commander Sinclair returning to Babylon 4 to travel back in time and become Valen as per War Without End. When Captain Sheridan inherited the commander's arc, Sinclair's premature aging in War Without End, and Sheridan's limited lifespan post-Z'ha'dum were the patches allowing Sheridan to take Sinclair's place in Sleeping in Light.
The latter half of Season 3 was going to have a story arc about the Sagitarrons. The story goes that during the New Caprica arc, the rest of the colonials had run low on food, but the Sagitarrons, being close-to-nature, had grown enough. The Colonial government made the decision to seize their food in order to feed everyone, a kind of reverse ant-grasshopper parable. The only remnants of this arc are: the episode "The Woman King," and Baltar whispering to Gaeta during Baltar's imprisonment, which was supposed to tie into this arc. The latter was repurposed for the minisodes "Face of the Enemy." It might also explain what Tyrol was protesting immediately after the Time Skip — and given a hint to the decision made by Apollo in the finale.
Bulldog's mysteriously one-off appearance — originally, the character was intended to recur, but scheduling issues prevented that from playing out.
From The Bill: Very near to the end of Paul Marquess' time as executive producer of the series (2005), there is an episode where PC Kapoor and PC Valentine work together to uncover the corruption of a uniformed Sergeant over at Barton Street nick. The episode even ends with a Sequel Hook, with PC Valentine informing her that he'll support her if she chooses to follow through with her allegation, although it will likely drag both of them through the mud (coppers who grass on other coppers, even ones who are genuinely in the wrong, tend to be looked on very unfavorably by their colleagues). Given the heavily serialized nature of the program at that stage, you might have expected this storyline to be followed up in further episodes, exploring the fallout of this affair. But it wasn't. It was dropped completely. The (off-screen) outcome did get a belated mention in PC Kapoor's final episode three years later, though.
The Gormagon arc was originally going to be longer on Bones, but the writer's strike shortened the season forced it to be compressed.
Brothers and Sisters strongly implied in its third season finale that Rebecca was bipolar (even tying in the events of the episode to her otherwise forgotten backstory). They promptly dropped any references to the storyline in the fourth season.
In Season 2, it was hinted that Mr. Snyder was conspiring with Mayor Wilkins to eliminate Buffy as a threat by bullying her, and later by expelling her from school on trumped-up murder charges. Season 3 revealed that while he was doing some work with the mayor, Snyder was still as much in the dark about what was going on as the rest of the adults of Sunnydale.
Also in Season 2, Willow suddenly becomes a lot stronger in magic and seems possessed while performing the curse to restore Angel's soul. The other characters are notably frightened. Despite this being a good starting explanation for Willow's developing magical powers, the possession is never mentioned again.
Once more, with feeling: in Season 2, the Anointed One — a prepubescent child who'd been made a vampire in the first season — was meant to be the main villain of the season. The problem was, while vampires don't age, the actor playing the role had had something of a growth spurt and clearly wouldn't be able to hold up as an immortal, ageless vampire. As a result, his storyline was scrapped and he was killed off rather anticlimactically — if satisfyingly — by Spike three episodes into the season.
Season 2 of Burn Notice had, in the first half, a mystery as to what the mysterious organization was having Michael do (personal shopper for a sniper), then what that guy was doing (killing someone). Then at the midseason finale, everyone dies and Michael gets half blown up. And we never hear about this plot again. Kind of justified in that, by the end of the season, the people involved are dead and the organization has abandoned Michael. And Michael's handler might have been acting on her own, without the blessing of Management, for that mission.
Originally it was planned that Zoe created an avatar of Ben as well as herself. (It was in fact the double-whammy of this reveal and the fact that Zoe was still inside the Cylon that won a lot of people over when only the script for the pilot was available.) The scene was filmed, and is included on the DVD, but essentially nothing in it is canon: Lacy didn't tell Clarice about Zoe-A right away and there was no avatar of Ben. Furthermore the scene ends when Lacy hears her cellphone ringing in the real world and takes her holoband off; it's dramatically revealed later on that holoband users don't receive sensory input like that from the real world.
Deleted scenes from the pilot also had Tomas Vergis appearing earlier than he did in the series, and played by Roger R. Cross (better known as Joshua in First Wave and Travis in Continuum) instead of by John Pyper-Ferguson, who played him in the series. In this storyline, Amanda was was having an extramarital affair with him; the showrunners cut it to make her more sympathetic.
The original idea behind Amanda's "hallucinations" of her brother in Season 1.0 was that Tomas Vergis had hired an actor to drive her off the deep end, but the subplot was dropped and a scene explaining it was deleted. Now it just seems that she had temporary relapse.
The fourth season of Charmed where Phoebe becomes impregnated with the Son of the Source of All Evil had such potential for exploring the morality of killing a child (albeit a psychopathic killer baby, which is such an awesome plot point in itself: imagine them attending school) to prevent future evil, or even an Aesop about redemption (or lack thereof). Instead the child is disowned, retconed into a surrogate child, casually dispatched and never mentioned again — all within the last few minutes of the penultimate episode of the season. It could have been such a cool Big Bad.
During one of the later seasons of Cheers Kirstie Alley got pregnant, so a story arc was created in which she and Sam decided to have a child together (while still just being friends). Kirstie Alley had a miscarriage and the arc was abruptly dropped.
Cold Case has several storylines in Season 6 unresolved due to cancellation. These include new love interests for Lilly, Kat, Vera, and Stillman, Lilly receiving a job offer from the FBI, and Scotty's quest for justice for his robbed and raped mother ultimately leading to his becoming accessory to the murder of the perpetrator.
Peyton's return in the end of the sixth season of CSI: NY was hyped as the beginning of a love triangle. The season would have ended with Mac trying to choose between the feelings he still had for Peyton and the early-stage relationship he was beginning with Aubrey. Who he picked would have been revealed in the beginning of season seven. However, Claire Forliani got a part on Camelot and couldn't return for more episodes, plus Melina Kanakaredes decided to leave the series, pushing the writers to put aside that plot to focus on the newly-arrived Jo.
Around episode 300, the original Dark Shadows had a storyline where Victoria and Burke were going to move into a house, Seaview, after they get married. The house was strangely unoccupied and Elizabeth agrees to sell it even though the deed says it shouldn't be sold. The popularity of Barnabas Collins probably led to this arc being canceled; it turned out she wasn't allowed to sell the house after all and what was wrong with it was never followed up on.
In fact, Victoria's entire presence on the show is one big Aborted Arc. In the very beginning, the show's main storyline was the mystery around the real family of Victoria, who was an orphan who received financial support from an unknown person. Besides many hints that Victoria was somehow linked to the Collins family, the mystery was never resolved. The original series bible held that Victoria was Paul Stoddard's daughter, conceived in an affair he had while married to Elizabeth Collins, and Elizabeth out of a sense of obligation was the one supporting Victoria. Later it was decided that Victoria was actually Elizabeth's daughter, but this was never revealed...except in a tape that Joan Bennett, the actress playing Elizabeth Collins and while speaking for her character, made for fans.
In Dawson's Creek, infamously annoying Eve Whitman was introduced in season 3 as a new Love Interest for Dawson who later finds out she's Jennifer's half-sister. Dawson tells Jenn's mother about his discovery and it's never heard of again. Eve disappears by the end of the season. Lampshaded in one of the series' final episodes, when a character who wasn't around for season three asks about her:
Audrey: Who the hell is Eve? Jack: Long story. Ambiguous ending.
The Dead Zone television series started an arc concerning the villain from the book of the same name, Greg Stillson — a racist, sociopathic, corrupt President who ends up starting a nuclear war that causes The End of the World as We Know It. Later, the television writers tried to downplay the arc, as they thought viewers would prefer a Monster of the Week format where they wouldn't have to watch episodes in a certain order or keep track of story arcs at all. The Stillson Arc was increasingly downplayed until he pulls a HeelFace Turn (which later turns out to be a trick masking his true evil agenda). This was a result of Executive Meddling — they were finally allowed to get back to the arc right at the end of Season 6, and the series was promptly cancelled.
Over the years, Degrassi has had a string of abandoned plot holes that have never been resolved or acknowledged at whatsoever, but the most infamous arc up to date is Clare's internship job at the Toronto Interpreter in Season 12. Several episodes in the first half followed her trying to please her boss, Asher Shostak, by editing a good story for the Toronto newspaper and one episode has her being sexually assaulted by Asher in his car. That became one of the most jaw-dropping moments of Degrassi history. In the first half finale, Clare is confronted by Asher's former intern, who admits that she was also sexually assaulted by him and convinces her that they should both go to the police together about Asher. The whole storyline was completely dropped from the twelfth season and we never really find out if she went to the police about it or not.
Another one often referenced is from the Season 7 episode "Talking in Your Sleep". Paige ends up sleeping with Griffin. Later she finds out he has HIV. She gets tested and is told by doctors that she won't know the results for 6 months. Griffin unceremoniously disappears from the show shortly after this revelation when Paige reveals she is moving out, and there is never any follow-up on Paige's results. Actress Lauren Collins who played Paige has referenced this plothole jokingly a few times on her Twitter.
Maria LaGuerta has a crush on Dexter in the first episode of the first season, something that is never mentioned again. This is most likely based on the first book where this plot point is fully explored and ended rather badly for her.
Also, after Debra kills one of the Fuente brothers, the other Fuente brother is mentioned to still be on the loose, but he is never brought up again.
At the Season 4 ending, there is an implication that Debra is about to find out Brian Moser had a brother while at the same time suspecting that Dexter is hiding something. Come Season 5, that plot thread is weakly relegated to Quinn.
The end of Season 6 dropped some heavy hints that Louis knew about Dexter being a serial killer and that he was the next big threat on the horizon. In Season 7, Louis is shown to not actually know about Dexter being a serial killer, and he is at best a nuisance to Dexter for a couple of episodes, before he is quite unceremoniously killed by The Mafiya, the actual main villains of the Season. The fact that Louis' actor, Josh Cooke, had landed a gig in a major Broadway production between seasons is the most likely reason for this.
Deb realizing she has romantic feelings for Dexter in season six was mostly dropped in the following season after intensely negative fan reaction. The arc that replaced it — Deb finding out Dex's secret — turns their entire relationship upside down. The person Deb loved essentially never existed, so by the time the season six arc is finally brought up again, it's a moot point.
The First Doctor drops a heavy hint in one story that he doesn't reallylook like a human being. This was ignored forever afternote Except possibly in the Doctor Who New Adventures novel Sky Pirates!, which suggests that not only does he have "another body" that can only be seen by beings who are really in tune with the Time Vortex, but that's not his true form either, which is even worse. Unless the Doctor meant his Time Lord organs and organ systems, which are vastly different than those of a human's, and are unable to be seen beyond his exterior.
The concept behind the relationship between the Fourth Doctor and Leela was that he was going to educate her from being a Nubile Savage into a Proper Lady and his perfect companion, drawing inspiration from Eliza Doolittle in Pygmalion (in fact, an early concept for her character was that she would be a Victorian street girl). Unfortunately, a forced change in showrunner, a leap in the Lighter and Softer direction, re-editing the sequence of her next season and the addition of a new companion, K-9, who shared her Super Senses and Minored In Ass Kicking gimmicks meant that this element to their relationship was dropped, and she ended up being more of a generic, quirky-naive friend-type companion. The Expanded Universe does a lot to fill in the gaps, particularly the Big Finish Doctor WhoFourth Doctor Adventures line which deals very heavily with Leela's education and some of the darkersubtext of the Doctor doing this to someone.
The malevolent unknown force in the middle of the TARDIS, mentioned by the newly regenerated Fifth Doctor in the story "Castrovalva". This was intended as a reference to another story that would follow it in that season (where the Doctor and crew would indeed discover that there's something evil hidden at the very centre of the Doctor's ship), but the script for this other story was eventually dropped. The reference to it in "Castrovalva" wasn't, and it remains unanswered to this day (even in the Expanded Universe).
Then there was the Cartmel Masterplan that was supposed to introduce more mysteries about the Doctor's origin and nature. The Old Series was cancelled before anything could come from it, but the Expanded Universe saved some plotlines. It is, however, debatable just how much of a Masterplan there was; some fans say that the Doctor Who New Adventures based on it were exactly what it was going to be like, especially Lungbarrow, but the TV version of "Lungbarrow" had already been abandoned and turned into "Ghost Light". Cartmel himself never said he had a Masterplan and Marc Platt, who was heavily involved in Cartmel's vision of the series (and wrote Lungbarrow and Cat's Cradle: Time's Crucible, the two novels most often cited as evidence of the Masterplan) called it "more of a mood and direction".
The season-long "Trial of a Time Lord" introduced Mel, a future companion of the Doctor who showed up to rescue him at the end despite them not having actually met from his point of view. The producers had planned to show this first meeting, but were forced to just plow ahead with Mel as the current companion when Colin Baker was fired. Once again, the Expanded Universe takes over and fills in the empty spaces.
In "The Empty Child"/"The Doctor Dances", Captain Jack ruminates on years of his life that were presumably deleted from his memory by the Time Agents. These were initially intended to be explored in a hypothetical Season 2, which would have kept Jack on as a companion, but when Christopher Eccleston decided to drop the role it was decided that David Tennant should establish his version of the character without having to compete with another strong male lead more established than him. As a result, Jack's missing years, involving the Time Agents, are explored via Expanded Universe material.
"Victory of the Daleks" introduced, with great fanfare, a new kind of Daleks, the Paradigms, with the intent that they would take over as the standard Daleks, and have plenty of stories about them. Due to their intense unpopularity among the fanbase, however, this was quickly dropped, and the Paradigms were reduced to cameo appearances for the next couple years before vanishing altogether, allowing the older bronze Time War-era Daleks to resume being the main models.
Series 7 kickstarted a storyline where the TARDIS refused to cooperate with Clara Oswald. When quizzed in 2014, Steven Moffat teased, "It's almost like it's all building to something... Oh! What's this I'm writing today?" The last we heard of it would be a throwaway line in Series 8's "Kill the Moon". Moffat finally explained this away in a Doctor Who Magazine Q & A column in 2017 when a fan brought it up: The TARDIS was sour with her because, being able to see all of time at once, she knew Clara and the Doctor were the Hybrid and that he would suffer greatly for their relationship. This references the Series 9 Story Arc. As Moffat and company did not initially plan for Series 9 to involve Clara at all, it's a Retcon, not something he had in mind from the the start.
The first season finale of Dominion confirms that Uriel is playing Michael and Gabriel against each other, and plotting with Arika to destroy them both, so that they can then take control of Alex for their own unrevealed plans... all of which gets thrown out the window when Uriel gets a bridge dropped on her in the Season 2 premiere, being killed offscreen by the human bombing run on Gabriel's lair. After that, she's dismissively mentioned exactly once by Arika and a handful of times by the other angels, but is then never brought up again.
A plot thread running through the first two episodes of Enemy at the Door involves Peter's attempt to sneak away from German-occupied Guernsey by boat, which is interrupted without him being identified, and the Germans' subsequent efforts to figure out who he was so he can be made an example of. The second episode ends with Kluge reporting to Major Richter that he is now certain it was Peter — and then the whole thing is dropped, perhaps because the showrunners decided they wanted to keep Peter on as a regular. The Germans never mention it again, even when Peter gets into other kinds of trouble, and Peter himself only brings it up again much later in the episode that writes him out, when he resolves to make another attempt to escape the island.
ER had a notorious one involving the return of Anna Del Amico's supposedly reformed junkie ex-boyfriend, and a romantic rivalry developing between him and Carter. Problem is, this was all set up at the end of the season. When Maria Bello didn't return for the following season, the writers had no choice but to drop the whole thing.
Despite The Artifact being one of the central plot points of the first two seasons, everything concerning it and The Consortium was inexplicably dropped, without any sort of closure or explanation. Fridge Logic makes this even worse, due to one of the characters explicitly saying that "Power of that magnitude doesn't just disappear." Ed Quinn (Nathan Stark's actor) actually left the show due to this, as Nathan Stark's obsession with The Artifact was his defining character trait.
The fourth season brought back The Consortium but with new plot line about having to send someone from the future (aka Jack) or Dr. Grant who was from time displaced from the past in order to save the history of the consortium. It was also teased twice that Stark had returned, but he never did.
Beverley returned for the end of an episode and then disappeared again, despite being a Big Bad for the series. Allison's son was also all but deleted from the series, also possibly in part because of the dropping of The Artifact arc.
The Nebari are built up to be huge threats — one of their "standard host vessels" (since the Nebari have highly Orwellian overtones, that might just be an unusual euphemism for a warship) took out the Peacekeepers' strongest Command Carrier; their "Establishment" deals with contentious citizens by infecting them with a sexually transmitted virus that will throw worlds into chaos and them sending them into the galaxy at large; they're apparently capable of blowing up planets; and they wear lots of eyeliner. And we never hear of them again after "A Clockwork Nebari". Though it's later implied that the Nebari aren't really interested in anyone who doesn't get in the way of their forced utopia and the ones we see are just out tiding up loose ends.
During several episodes in the first season, it was clear that Pa'u Zotoh Zhaan had some romantic feelings (or at sexual attraction) for John Crichton, and Crichton was actually beginning to respond. And then the entire budding romance went away with no further mention of it at all in favor of the John Crichton/Aeryn Sun relationship.
They had plans from the beginning to get Rygel back home and get his revenge on his traitorous cousin, reclaiming leadership of the Hynerian Empire. Unfortunately, an episode involving just one other Hynerian presented all kinds of logistical issues, making it clear that setting an episode on a whole planet of them would be impossible. Fortunately, they were able to wrap up the story properly in the comics, which have no such restrictions.
Starting from Greyza's introduction at the end of season three, leading a Luxan delegation, the show implied all the galaxy's prominent species would be drawn into the looming Peacekeeper/Scarran war, and that the factional politics among the various planets (including Hyneria, and possibly Nebari) would be crucially important to the story. However, since the promised fifth season was gutted from twenty-two episodes to a four-episode miniseries, it amounted to pretty much nothing in the end, aside from Jothee's Luxan commando squad showing up at the eleventh hour.
In The Stinger of "Power Outage", Thawne takes a blood sample from the dead body of Blackout, musing that his ability to drain The Flash's powers will come in handy. However, this is never brought up again for the rest of the season.
Captain Cold slowly forming the Rogues has met an abrupt end since he's been transplanted to Legends of Tomorrow. And now, with his death, it seems this arc will never be completed.
After the end of the Gorilla Grodd storyline near the end of season 1 General Eiling tells Barry that he is aware of his Secret Identity and that this isn't the last time he sees him. Despite that Eiling has yet to appear in the Arrowverse since said episode.
In Season 3, Mirror Master and the Top were meant to be replacements for Cold's gang but given the lukewarm reception of their debut they haven't been seen since.
In Season 3, Savitar says to Jesse Quick that he has plans for her when they meet. This is never addressed or brought up ever again. And with Savitar's death/erasure from existence at the end of the season, it's likely that it will never be addressed again.
During Frasier's 10th season, an arc was slowly built up where it was suggested that Roz had feelings for Frasier and was jealous over his relationship with Julia Wilcox. In the first episode of Season 11, the old writers from earlier seasons rejoined the show and quickly ended the arc by saying Roz's father had remarried and thus she was scared of losing Frasier as a friend. The characters made up, and nothing more was ever said.
In the fourth season of The Fresh Prince of Bel-Air, they introduced a new character named Jackie Ames, played by model Tyra Banks. Jackie was one of Will's childhood friends and ex-girlfriends from his hometown West Philly, who received a basketball scholarship to attend the University of Los Angeles, which is coincidentally Will's choice of college too. Their relationship is at first tense, since Jackie was deeply upset about Will not keeping in contact with her after he left for Bel-Air, but soon enough, they exchange in a little "will they, won't they" type of relationship, where there are obvious signs of attraction and chemistry between both characters, but nothing really coming out of it. They almost come close to going out in the ninth episode, but Will ruins it by choosing the Playboy Mansion over Jackie. The arc goes on to make you wonder what route their relationship will be going, until in 'You've Got to Be a Football Hero', wherein Will and Jackie's boyfriend Hank Farley exchange in a drinking contest over their manliness, and over Jackie. Jackie, fed up with their childish behavior, chooses to leave in the middle of the event and asks Carlton to drive her home. This is the last time we see of her, and is never brought up in conversation again.
When season 2 of Friday Night Lights was cut short by the writers' strike, the showrunners decided to cut their losses and abandon several storylines, including Smash Williams' decision to play football for a small, academically-minded historically black college after losing his scholarship, Tim Riggins and Lyla Garrity's will they or won't they subplot (season 3 began with them together), Lyla Garrity's Christian radio career, and the entire character of juvenile delinquent-turned-star defensive player Santiago Herrera.
This is lampshaded in Friends, where Joey gets a job working at the coffee house, and then simply stops working there. A few episodes later, he and Gunther realize this, and Joey reveals that he quit but forgot to tell him.
The first season set up a subplot about Peter having a shady past with the mafia, and lots of ominous foreshadowing about mafiosos spying on him. It amounted to absolutely nothing and was quickly forgotten.
FBI Agent Amy Jessup is brought in during the Season 2 opening and looked to be bringing in some religious interpretations to the Fringe cases. She appeared in two episodes then was dropped without a word of explanation. This is probably because she was invented by the writers as a temporary fill-in for Agent Dunham for the whole of season 2 but test audiences wanted to see more of Dunham.
When Sam Evans' original planned coming-out arc was dropped in favor of the developing relationship between Kurt and Blaine and the subjective "chemistry" Dianna Agron and Chord Overstreet shared.
Allegedly, Chord Overstreet was supposed to be Kurt's love interest, but Darren Criss seemed a better fit. With the Sam character's entrance, Kurt hounds him about his bleached hair (Sam denying that it's bleached). Sam later admits to Quinn that he thought a surfer dude would be cooler and allow him to make a better transition as a transfer student. And then... we never hear of it again, even though Sam's hair stays blond.
In the penultimate episode of season 2, Sue's Evil League is completely abandoned, Terri leaves the show and actually manages to help New Directions. To top it off, in the same episode, Quinn gives a vague threat of doing something to sabotage the club in New York only to just get a haircut the next episode.
Quinn and Puck's entire relationship (including him confessing his love for her) was completely dropped once they gave their daughter, Beth, up for adoption. The relationship (or, really, Beth in general) would not be brought up again until Season 3, where the show treated their romance as a one-off fling.
Quinn and Joe were starting to admit to some attraction and develop a relationship, but after the prom episode all sexual tension and ideas of romance between the two were completely dropped in favor of the Quinn/Puck relationship.
While Quinn's car accident was (too-neatly) wrapped up, her play date with Artie wasn't. It was heavily implied that Artie was right that Quinn was only pretending her prognosis included walking again, only she did walk again.
The entire arc about the twelve villains that were supposed to be the worst villains ever, but all the characters stopped caring after Arthur came back to life.
Knox said that all he thought about during his time in level five was revenge on Noah, the man that put him there — also dropped after his first appearance.
Then the Eclipse mini-arc, which was dropped almost as quickly as it was picked up.
Adam was dug up, because Angela said he was the key to everything; turns out that was a lie as well, since nobody even bothered looking for him after Arthur killed him.
There's also the issue of Peter's season 2 girlfriend Caitlin, who got lost in an alternate future that no longer exists. They kind of completely forgot about her after that, and Peter doesn't seem too concerned with getting her back. (In an interview, one of the writers jokingly said that no, Peter didn't really care, then backpedaled and said that she was originally meant to be rescued in the second half of season 2. "But sadly that will never happen...")
Also, when it was decided that the show would continue following the central characters of season 1 (and not a new group each year, as Tim Kring had planned) numerous possible future arcs were hinted, but ultimately never came to be. Many of them can be seen in Isaac's paintings, such as one of Hiro facing down a T. rex (obviously, the show never had the budget to do that one). That one actually was wrapped up; right after stealing the sword, Hiro runs into a T. rex display in a museum.
Highlander's first season made frequent mention of The Gathering as in the films. It was the very reason Christopher Lambert's Connor came to see Duncan in the first episode with that very title. The murder of Darius and the Watcher/Hunter storyline took over from this, and the emergence of new Immortals in later seasons seemed to belie a final battle being close at hand.
On Home and Away, the toxic waste buried in the construction site probably being the cause and justification of all the cancer causes of the series, plot line ended with Belle being hospitalized and then shifted over to her drug-abuse arc. The toxic waste has not been touched upon again, not even when Belle herself was dying of cancer!!
An interesting variation in House of Anubis with the tear of gold. In Season 2, the arc was followed through to the end and played a pretty good role in the over all story- until the cliffhanger at the end of the season, where Victor discovered he had a tear of gold with him. Fans were excited and wondering how it would come into play next season... but it never did. It was completely forgotten in Season 3. Needless to say, the fans were not too happy about that.
JAG: Harms Russian half-brother Sergey is not heard from anymore post season seven.
Kamen Rider Den-O had to drop Hana's planned arc when the actress quit suddenly. We still got the reveal of who Hana is and why she is significant to the plot; but her replacement, Kohana (a time-switched version of Hana at ten years old) took a back seat and Airi largely took over as the show's female lead.
The Movie Wars crossover between Kamen Rider Double and Kamen Rider OOO introduced the concept of homunculi in the OOO universe and hinted that they could reappear in the show, but this didn't happen. Word of God confirmed that Date was to have died and been resurrected as a homunculus, but this was changed due to the character's popularity.
Towards the end of an arc, the Corrupt Corporate Executive was defeated and killed, and his (apparently) dumb-blonde trophy wife Mindy was last seen saying that she would be in charge from now on, with an implication that perhaps she had been the prime mover all along. She made one subsequent appearance (again successfully framing someone else for her crimes), and was never mentioned again.
There's a story arc involving two characters named Nikki and Paulo, who actually had quite a bit of backstory and plot setup to them. They were introduced as background characters who had suddenly acquired more dimension. The problem? The creators didn't use the extras that had long been on the show, any of the recurring minor survivors (e.g., Steve), or any mentioned but unseen characters (Tracy, Neil). The intricate plot set up for the two was condensed into a single episode, and the two were summarily killed off.
Another aborted arc resolves around Libby, who was revealed to have been in the same mental hospital as Hurley. The next episode, she was killed. The Powers That Be originally said her story will be told, later said her story is over and no longer relevant, then reversed again when they brought her back in season 6 where her time in the mental facility starts to make more sense for the story.
The producers have revealed that they had an intricate four-season arc planned for Eko, until the actor decided to leave. Parts were given to other characters, but the main thrust of it—the conflict between Eko and Locke for the position of "spiritual leader of the Island"—was lost. That said, given the conflict between Eko's Catholicism and Locke's shamanism, it's not hard to see how Locke and Ben's arc in season five was rejiggered to take the place of the arc, with Eko as the seemingly-messianic Jesus figure and Locke as the spurned believer who murders his "god" in a fit of jealous rage, meaning it wasn't totally abandoned.
The War between Widmore and Ben hinted at during seasons 4 and 5 was replaced with the conflict between Jacob's followers and MIB's followers. Widmore returned to the Island without mentioning Ben once, focusing on MIB, until the penultimate episode.
The producers also said that Walt would only be seen again in a DVD-exclusive. Indeed, many hinted-at arcs from earlier seasons were forgotten in favor of Jacob and MIB.
Many people found Ilana's promotion to series regular baffling, given that she had almost no role in the plot outside of "Dr Linus", appeared only once in the flash-sideways and got blown up randomly. According to Zuleikha Robinson (though her account may be wrong), she originally had a longer arc that would have featured her as Jacob's daughter. Sadly, a lack of time made them decide to focus on the original characters more, explaining her quick departure.
The people on the outrigger who shoot at the main characters in season five was never explained, despite repeated assurances from Lindelof and Cuse that it would be. However, given hints from the production crew (namely that most of the principals were killed off), it's exceedingly likely it was supposed to be Ilana and her followers. However, due to the circumstances mentioned above, when they axed Ilana, they unfortunately also axed this unresolved plot point. Jorge Garcia has said the script reveals who it was, and Lindelof and Cuse floated the possibility of a charity auction, but as of 2016 nothing has come of it. A (non-canon) excerpt from the Black Rock's log included with some of the box sets also explained it away as the Black Rock's away crew.
The first half of the fourth series of Merlin deals with Morgana hearing a prophesy that states a sorcerer called Emrys will be "her destiny and her doom." Naturally she freaks out at this, and puts a lot of time and effort into figuring out just who this mystery man is and how she can destroy him before he destroys her. After episode seven, she abruptly turns her attention to other schemes.
At the end of the two-part series premiere of Mortal Kombat: Conquest, Vorpax tells Shang Tsung that Shao Kahn has imprisoned hundreds of powerful warriors in his mines over the years. Shang Tsung vows to free the captive fighters and raise up an army, but this plot point is never mentioned again in any of the subsequent episodes.
The first episode of the second season of The Musketeers makes a huge thing about how the dead Cardinal Richelieu has set all kinds of plans in motion for posthumous revenge on the Musketeers. Absolutely nothing of the kind ever develops.
Before that, the first season finale dropped hints that the Cardinal had some clue about Aramis' affair with the Queen. However this obviously didn't lead to anything, since the Cardinal was killed off after Peter Capaldi left the show for Doctor Who.
During the Sci-Fi Channel era of Mystery Science Theater 3000, Season 9 started a plot where Pearl Forrester sought to become a mad scientist in the same vein as her son Clayton. This is abruptly dropped when it became clear that Season 10 would be the last season.
The season 2 finale of The Nanny is a cliffhanger where the Sheffield Mansion is robbed, but this is never resolved or mentioned again when season 3 begins.
NCIS spent a few seasons setting up a plot in which Director Jenny Shepherd learns that her apparently dead father was, in fact, still alive (despite the fact that he'd shot himself in the head and she was the one who found his body). Just as she's beginning to accept it may be true, she's killed in a gun-battle related to one of her first cases as an agent, and so the whole story is now apparently done, with no resolution either way.
This was actually explained in a blink and you'll miss it scene. Midway through the season Abby speculates that this was a plot by someone (most likely from the CIA) to make Director Shephard look mentally unstable. Leon Vance's remark that Abby is smarter than she looks is about the only answer we'll ever get. It is believed that the reason this plotline was swept under the rug was due to Donald Bellasario stepping down as showrunner and his successor deciding to drop the arc quickly.
NewsRadio's writers made a point of intentionally abandoning every arc the network forced on them, since they preferred stand-alone episodes to arcs. For example, the storyline about Lisa wanting a baby plodded along for several episodes, never went anywhere, and was quietly dropped.
Possibly the most jarring example of this trope was in the "Andrea" arc. Station owner Jimmy James hires an "efficiency expert" named Andrea, who proceeds to fire Matthew, demote Dave from News Director to reporter and fill the vacant Director's office by promoting Lisa. It was also not-so-subtly implied that she was a lesbian with a fairly violent criminal history and a crush on Lisa. After four episodes, Andrea disappeared without so much as an off-camera farewell, Matthew was un-fired by Mr. James and given back his old job, all other changes made by Andrea were undone (except for Dave and Lisa's job-swap, which took a few more episodes to resolve) and not a single explanation was ever given.
During Season 6, there was an arc featuring Dwight trying to hatch a scheme to legitimately get Jim fired, which included forming an alliance with Ryan and planting a bug in Jim's office. It would be one thing if this was Ryan on his own accord (who crossed into Jerkass territory two seasons prior), but the writers apparently forgot that there was a begrudging respect between Dwight and Jim despite their rivalry, from Dwight preventing Roy from attacking Jim to the two actually having a successful traveling sales run, and co-running the Party Planning Committee together for a while.
Jim destroyed Dwight's respect when he went over Michael's head to Jo and talked himself into the Regional Manager job that Dwight thinks belongs to him. In Season 8, Andy gets the same job after Dwight screwed up his chance and Dwight immediately turns on him too.
Dwight makes very clear that he didn't view the event as special and didn't even understand why Jim was grateful. Roy could have gone after anyone and Dwight would have done the same thing. The real dropped arc is the scheme itself. Despite all the build up mentioned above across several episodes, eventually Jim steps down from his position willingly for unrelated reasons and Dwight and Ryan dissolve the alliance thinking they succeeded in bringing him down.
Parenthood: In season 4, Adam and Kristina lie to their daughter Haddie about a major medical issue. They claim Kristina is 100% cancer-free when she isn't. They do this, presumably, to convince her it's 100% fine for her to fly back across the country to the college she's attending instead of take a semester off to help out at home. This is clearly something that should bite them in the butt, especially if something goes drastically wrong. But when something does, and Kristina is in the hospital around Christmas and might die from a severely compromised immune system, Haddie shows up and simply hugs her mom. The lie and potential for Haddie to be either very angry and betrayed, or forgiving and understanding is 100% ignored/forgotten.
Power Rangers had quite a few of these after it started regularly using Story Arcs.
Scorpina was a major villain in Season 1, who (thanks to Japanese stock footage, mostly) seemed to be a close friend or romantic interest of Goldar's.note In the Japanese show they're married. She appeared in a single Season 2 episode, at the end of which she swore she would return... and then she was never seen or mentioned again, even in "Countdown to Destruction." Years later, the Soul of the Dragon graphic novel explained that she'd been banished to Lokar's dimension (dubbed the Talos Dimension) as punishment for betraying Rita Repulsa and seeking more power for herself; she eventually returns as Lokar's servant and tries to sacrifice Tommy's son J.J. to him in exchange for greater power, only for J.J. to escape; Lokar kills Scorpina for her failure.
A few episodes in Season 1 dealt with Rita's discovery and use of "Super-Putty," a new material she used to create near-invincible Putty Patrollers. Tommy and Jason had to go on a quest to find weapons that could destroy them, which they did. However, after that episode, neither the Super-Putty nor the new weapons were ever seen or mentioned again, and the putties reverted to their normal weak selves with no explanation.
The multi-part Season 3 premiere served as a Backdoor Pilot for another Saban action show, Masked Rider, and ended with that show's villains and hero heading to Earth. This episode was referenced exactly once later in the season and then forgotten about, with no further crossovers or indications of what happened — and even on Masked Rider, this episode was non-canon.
Power Rangers Zeo used Billy as a Red Herring for the Gold Ranger's true identity; he would often be mysteriously absent working on some sort of "project" whenever the Gold Ranger showed up. Once the Gold Ranger's real identity was revealed, his project was never brought up again.note According to Word of God, had David Yost not left the show, the project would have been revealed to be the Turbo powers.
Zeo had a huge number of aborted arcs. Here are some other examples:
In "The Lore of Auric", Tanya finally discovers and rescues her long-lost parents, archaeologists who had been lost on a tropical island. After that one episode, they're never seen or mentioned again.
The same episode introduced Auric the Conqueror, a powerful ally of the Rangers. He appeared in a few more episodes but was conspicuously absent for the finale, and was never seen or mentioned after Zeo ended.
Partway through the season, Bulk, Skull and Lt. Stone were fired from the Angel Grove Junior Police and opened up a private detective business. In the season finale, Bulk and Skull were offered a case in Paris and quit Lt. Stone's agency to pursue it. As soon as Turbo starts, all three are back on the Junior Police force, with just an off-hand comment about how they're lucky to have their jobs back. The trip to Paris is never mentioned.
In the last third of the season, Prince Gasket and Archerina, two new villains, take over the Machine Empire. They're driven away just before the finale by the returned King Mondo. After this, they're never seen or mentioned again and their fate remains unknown. Behind the scenes, real life wrote the plot when the costumes for he and Archerina fell into disrepair and all but disintegrated, leaving the complete suits unusable (though Gasket's boots were salvageable and were incorporated into the costume for one of the Mut-Orgs in Power Rangers Wild Force).
The season ends with not the Rangers, but Mighty Morphin villains Lord Zedd and Rita Repulsa destroying the leaders of the Machine Empire. Zedd remarks that they're finally back. However, they never became the main villains again and, excepting a joke cameo in the movie, weren't shown or mentioned at all in Turbo. They did eventually return in In Space, but no mention was made of where they had been or why they hadn't attacked Earth since Zeo.
Power Rangers Turbo had Dimitria's missing twin sister. She was implicitly Divatox, judging by the fact that they were both played by Carol Hoyt... then again, she was a replacement while the actress who played Divatox in the Pilot Movie, Hilary Shepard Turner, was on maternity leave. When Divatox is "purified" in "Countdown to Destruction", she's wearing an outfit identical to Dimitria's, which is as good an answer as we're ever going to get about the twin sister thing.
Turbo also left the identity of the Phantom Ranger and his apparent budding relationship with Cassie up in the air.
The first series of Primeval ends with Nick Cutter going through an anomaly into the Permian era, and coming back out to find that his actions have somehow altered the timeline so that his love interest Claudia Brown has become a different person named Jenny Lewis, as well as a few other changes. The second series makes many references to this mystery (as well as wondering why relatively little has changed) but never explains it. In Series 3, the still unexplained arc is apparently abandoned as Cutter is killed off and Jenny leaves the show: apart from a few brief references, it has not been touched upon since.
Red Dwarf ended its second season with Lister (a male character) falling pregnant to his female Alternate Universe counterpart. The writers had planned to spend an episode on it, but found their proposed script was misogynistic and — more damningly — not very funny. Season 3 wrapped up the storyline with a Star Wars-style text opening that scrolled so quickly it was unreadable unless viewed in slow motion on tape/DVD. This text also halfheartedly explained the sudden recasting of the characters Holly and Kryten. (Neither case was inconspicuous: the former gave himself a sex change; the latter was an obscure one-off character "rebuilt" into to a permanent cast member, acquiring a new look and personality in the process.) This gave the distinct impression of missing several episodes of major character developments; perhaps even a season. (This was given slightly more detail in the Red Dwarf Smegazine comic strip, in which an accident sent Lister from before the accident into traumatic moments from his future. One of them had him about to give birth, while Rimmer explained that it was going to be the Skutters doing the caesarian, "I know you wanted Kryten to do this, but he's still walking funny and talking with a Canadian accent.")
In many ways, the series aborted many promising story arcs introduced in the second season finale, such as Charlotte's pregnancy, Nolan being arrested for terrorism, Conrad being a member of the Initiative and moved on with only brief explanations of what happened.
In the first and second series of Robin Hood a huge amount of time and effort is put into two very distinct plotlines: the assassination attempt by Guy of Gisborne to kill King Richard and Robin's attempts to expose him; and the sheriff's conspiracy to help Prince John usurp the throne by mustering the Black Knights and having them sign the Pact of Nottingham to ensure their loyalty (a MacGuffin that one regular character actually dies for). Then in the season finale of S2 Marian is murdered by Guy of Gisborne. By the third season the Pact, the Black Knights, and the assassination attempt have been dropped entirely to deal with the repercussions of Marian's death. But then, even this is aborted in favor of melodramatic family dramas and convoluted love triangles thanks to the introduction of Isabella and Archer and Kate. The political ramifications of the time period and the basic "rob from the rich to give to the poor" mantra are simply afterthoughts.
Season 2 has something halfway between an Aborted Arc and Red Herring Twist. Vorenus and Pullo's friend and gang colleague, Mascius, builds up visible resentment over the course of the mid-season, annoyed that he remains junior to them despite putting in more work and sacrifice. This appears to happen to explain why, when Vorenus and Pullo realise that there is a mole in their gang betraying them to a rival gang, they suspect Mascius. But the mole is not Mascius, but Vorenus' daughter Vorena (which the viewer knew all along), and Vorenus only briefly suspects Mascius before realising this. The next time we see Mascius, all his resentment appears to have mysteriously gone.
The first half of Season 2 also has an arc centred around Timon, Atia's Jewish bodyguard and previously a peripheral character, rediscovering his religious devotion. Word of God says this was supposed to begin an arc showing his interactions with Jesus and the rise of Christianity, but the show never got that far, so Timon just abruptly leaves Rome for Judea mid-season and is never seen again.
The 4th season of Sabrina the Teenage Witch introduced the character of Dreama, a naive young witch Sabrina was supposed to be tutoring for her Witches License. The story line never really got off the ground and Dreama mostly spent her fairly limited screen time as Greek chorus for Sabrina and a Suspiciously Similar Substitute for Sabrina's former best friend Valerie. In her final two appearances her character arc was not mentioned at all and she vanished without trace in the penultimate episode of the season, without the show ever bothering to mention if she got her License and what happened to her then.
Scandal: The whole thing with Cyrus being a secret evil mastermind seems to have been dropped in exchange for Hollis Doyle. Cyrus is still morally ambiguous, but no more so than Olivia at this point.
Seinfeld's transgression had Jerry and Elaine get back together at the end of Season Two, then started Season Three with them apart (with no explanation). Creator/executive producer Larry David had always hated the idea of the two of them being together and preferred them to have a purely physical relationship, only attaching them due to editorial mandate, and had only written that episode that way because he thought it was going to be the series finale.
Silicon Valley: The first season makes a lot of references to Peter Gregory and Gavin Belson being former friends, with the implication that Pied Piper's fate will become a highly personal proxy war between two rival billionaires who will ultimately be forced to reexamine their relationship. Due to the actor's death, Peter Gregory is written out and replaced by a Suspiciously Similar Substitute who has no prior relationship with Gavin, so Peter and Gavin's falling out goes unexplored and Gavin becomes a more straightforward villain motivated by greed.
The disjointed, episodic nature of the series made it easy for them to follow up good ideas or drop bad ones with each new reality. They could even tease an interesting idea by giving a glimpse of it in an upcoming world or one our heroes just escaped, but never have to flesh out the details. Many abortive arcs came from the network shooting down creator Tracy Torme's attempts to inject continuity into the show — and being shown out of order, they lost what little continuity they had.
One episode suggested that Professor Arturo had been replaced by his Evil Twin, but this was never followed up on in-show, aside from being teased at in a couple of episodes, and the plot effectively ended when he was unceremoniously killed off. Tormé did eventually confirm, in a 2009 interview, that the wrong Arturo had indeed slid. Arturo's terminal illness from another episode was mentioned once again and then forgotten.
Quinn's evil female double in another episode was meant to be a recurring character, but was never seen again.
A new member of the group was added in the first-season finale and then written out in a single line at the start of season 2, due to the network's meddling.
The Kromaggs were dropped after their initial appearance and not revisited until the show moved to the Sci Fi Channel two seasons later (and they were heavily retooled there from their original form, less monsters and more Nazis).
The third Season Finale ended with Quinn and Maggie getting separated from the others, and sliding into a city of big glowy crystal-like buildings and flying cars. "I think we just slid into the future!" exclaims Quinn. The entire "future" concept is never even referenced again, with the next season picking up several slides later. (However, this wasn't the first time the series had used a similar idea - the second season had featured an episode set on a world where time had moved slower, so the Sliders had arrived a decade or so in their own pasts.)
So Weird, the Disney Channel's version of The X-Files, took this a step further — it abandoned the entire Myth Arc which had been mapped out for three seasons when the lead actress left the show after season 2. After this, she was replaced by an unrelated character and Executive Meddling ensured everything that had built up was quietly dropped with little explanation in the span of a single episode. Floating around on the Internet is a Word of God summary of how season 3 was supposed to go, and it was the culmination of the Myth Arc of the first two seasons.
The first two seasons of Sons of Anarchy build up David Hale as the man who will take over the Charming police department, purge it of corruption, and lead the cops in a final showdown against SAMCRO. Then Taylor Sheridan decided to leave, so Hale was killed in a drive-by shooting.
The fourth season of Spin City has the Mayor run for senate. The arc is quietly dropped later in the season, even though it had been used to justify bringing Caitlin aboard.
Stargate SG-1 is infamous for introducing characters, races, and enemies that are never seen again. Examples: Nem (an advanced alien who befriended Daniel in the first season), Nyan (a man who supposedly became Daniel's assistant), the Re'tu faction (who supposedly wanted to wipe out all humans), and the general idea of the great alliance introduced in the "Fifth Race" (Asgard and Ancients get their share of plot, but Nox are never to be seen after helping free Skaara; the Furlings are more a running gag than anything). All planets whose Stargate has been lost/destroyed have not been mentioned again even after the SGC started building ships (such as Heliopolis, on which the "meaning of life" machine is housed).
Forgetting Nem becomes infuriating in the later seasons. Nem's entire motivation was to find out the fate of his wife, Omaroca, who is revealed by Daniel to have been killed and torn apart by Belus. If this sounds familiar, it's because Belus and Omaroca are also known by the slightly less obscure names of Marduk and Tiamat. In show, Marduk is a Goa'uld who has spent the past five thousand years locked in a ziggurat, and the Eye of Tiamat is part of the same set of MacGuffins as the Eye of Ra. Since Marduk is in possession of the Eye when he's released, one could assume that Omaroca tried to use the Eye as a weapon against the Goa'uld occupying Earth, but failed and was killed by Marduk, information that Nem would certainly want to hear. This connection is never made.
Strangely, though the Re'tu are never seen again, they are mentioned practically every instance when someone is attacked by something invisible or when there is a threat of unknown origin.
His planet gets a guest appearance on SGU, he doesn't. Blink and you'll miss it — his planet is mentioned as one of those which fell to the Ori in season 9 or 10. Season 2 of Stargate Universe even has his planet as the focus of an episode (it's one of the only planets that can safely dial Destiny), but Jonas is again absent.
Stargate Atlantis was far from better. What effect did Sheppard's "Blending" with an ascended being have? None. What about the last Asgards? Or the travelers after their brief help in fighting the Asgards. Or what about Lt. Ford, who Sheppard was convinced had survived? Or that Ancient-worshipping cult that hoarded a ZPM? The list goes on...
On the subject of the Asgard Outcasts, The heroes have an Omniscient Database containing all of the achievements, both scientific and cultural, of the mainstream Asgard race, something that could be used to negotiate an alliance with those jerkass Asgards who have been reduced to using vastly inferior technology to their extinct mainstream counterparts. There could have been a whole plot on the rebirth of the Asgards. Presumably the series just ran out of time to tell it.
Two of these plot threads were given closure in the Fandemonium Stargate Atlantis novels Unascended and The Third Path which both take place after Season 5. Ford was found by the team living among the Travelers and has a family and returned with them to Earth after being honorally discharged from the military. The Vanir got the help of ascended Asgard named Ran who was born prior to their species cloning and genetic degradation began and descended to help the Vanir fix their genetic degradation issues and use her eggs to hopefully restore the Asgard species.
The episode "Conspiracy" introduces a race of mind-controlling slugs that threaten to infiltrate The Federation. The episode ended with the revelation that the aliens had sent out a homing signal into deep space, presumably as a prelude to a full-scale invasion. They were never seen nor heard from again in any TV series (though they showed up in the non-canon novels as being tied to the Trill). They were intended to be a way of introducing the Borg, who were later introduced by other means. The expanded universe goes back to this one sometimes; in the comics, Geordi stumbles on their second invasion attempt, they're trying to start slower by going after a less-advanced race. In the Shatnerverse, Captain Raddison explains to Kirk that her super-secret division exists to protect The Federation from things that would keep even him up at night. Among her list of incidences, "Parasites of unimaginable power that have three times tried to take over the Federation from within. Ask Picard to tell you about the time he knows about."
The Season 1 finale involved outposts along the Romulan Neutral Zone being mysteriously destroyed, with each side at first thinking the other was responsible. The Borg were meant to be this new threat, but that doesn't track with their debut appearance the following season. In "Q Who?" it's explicitly suggested, if not outright stated, that the Borg destroyed the Neutral Zone outposts. On the other hand, later Borg retconsalso don't track with that debut appearance, and make the Neutral Zone thing more logical.
There were also the extradimensional abductors in the episode "Schisms", who released a probe into "our" universe which the Enterprise lost track of in an obvious attempt to establish them as a continuing menace. They were never seen nor mentioned again (although these guys, like the above-mentioned parasites, were followed-up upon in the comics).
Also, there was the whole "warp drive damages reality" problem they introduced in the sixth season of Star Trek: The Next Generation, and then only mentioned once the following season. This had the potential to be either really cool or really stupid, but we will never know which. Word of God says that the "variable geometry pylons" on U.S.S. Voyager were The Federation's stopgap solution to the problem, thus explaining why the Enterprise-E and other ships produced thereafter has fixed pylons. Was later retconned to having to do with a Star Trek TOS episode involving the Federation trying to make an omega molecule and it destroying subspace in the entire sector. And by the time of The Next Generation it's a closely guarded secret in the hands of only a few that the Federation destroyed this entire sector of space's subspace field.
In the season 2 episode "Loud As A Whisper", there is a scene in which Geordi La Forge goes to see Dr. Pulaski in order to have his visor adjusted, and Pulaski brings up the possibility of restoring his sight. This arose out of actor LeVar Burton's desire to ditch the visor, which he felt restricted the expressive range of his face by obscuring his eyes. Nothing ever came of it until the series finale years later, where his eyes were restored as a result of anti-time distortion. (It probably didn't help that this scene came in the middle of an episode about a deaf character seeking a way to cope with being stuck without his interpreters.)
A subplot in the Season 6 two-part episode "Birthright" was Data discovering that his programming is now enabling him to dream and trying to understand the dreams he is having (which involve him meeting a younger version of his creator/father Dr. Soong, among other things). This subplot was contained entirely within the first part of the episode and completely absent from the second, all without really going further into the implications of Data's newfound ability to dream. Due to fan demand, the story of Data's dreaming was followed up on in Season 7's episode "Phantasms".
The series made a big deal out of the integration of the Maquis and Starfleet crew early on, but the network insisted on the show adopting a Monster of the Week format a la TNG rather than DS9's complex plot arcs. This lead to only a single episode being devoted to the subject and Chakotay acting like a Starfleet officer as soon as the third episode with no transition (though he was previously a Starfleet Commander).
One part of the plot survived in the form of Seska, a Maquis-cum-saboteur who chose to work with the Kazon rather than integrate into the crew.
Averted with the episode "Year of Hell". They wanted it to be an entire season, but the network refused. So they made it a stand-alone episode, which ended with a literal Reset Button.
The Janeway holonovel plot that was featured in a few early episodes was dropped because of a lack of fan interest. A conclusion was written but never filmed.
In Strange Luck, Chance's brother mentioned that he'd made a friend in the FBI who could help them. His name was Mulder. The series was canceled before this crossover could happen (although a character suspiciously similar to Chance Harper later appeared in the X-Files episode "The Goldberg Variation").
In Strike Back: Shadow Warfare, Colonel Locke's personal quest to find the man who killed his son is never resolved by the end of the series, and Locke is resigned to never getting closure.
The entire high school element of Terminator: The Sarah Connor Chronicles Season 1, with incomplete arcs involving a mysterious suicide, implications of teacher-student sexual exploitation, and a male student lusting after Cameron, was just dumped with no explanation at all once Season 2 started. Word of God says that the creators decided that it was unnecessary and that the show worked better if the central characters weren't even trying to pretend to have a normal life. Also there was a writers strike.
Fez and Laurie's marriage on That '70s Show. At some point, Red and Kitty wanted the marriage to end, but to no avail, and decided to postpone the divorce when they started receiving wedding gifts. Next season, Fez is acting like a single again, Laurie is said to be in Canada, and the marriage is never mentioned again. The fact that Laurie was recast and given much less screen time didn't help. There's also Grandma Bea who was invited to stay at the Formans', but vanished a couple episodes later.
On The Unit, Tiffy, Kim (reluctantly), and whoever Grey or Hector was seeing at the time, got to digging around in Molly's past and found out that she is not who she says she is. Molly found out and confronted the nosy bunch, even telling them that she was going to tell Jonas but before that could happen, the storyline was dropped the very next week, never to be mentioned again.
In a second season episode, the White House is politically out-maneuvered by the Republican Majority Leader's new Chief of Staff, played by Felicity Huffman. The episode ends with White House staffers realizing that the majority leader is running for President, with Huffman's shrewd, capable character set up as a major antagonist. However, Huffman never appeared on the show again, and the majority leader's presidential bid was later dispensed with in a cursory fashion.
The West Wing was somewhat notorious for this — the arcs of a number of major characters abruptly ended without resolution and with the characters disappearing without explanation. Fans called the phenomenon "being sent to Mandyville". It happened to characters as major as Mandy, Sam and Amy Gardner.
Wizards of Waverly Place strongly hinted that the Russos will be responsible for a world where magic is commonplace. But when the moment came, Disney Channel chickened out.
During Season 6-7, the Syndicate was destroyed at the hands of the Alien Rebels. The writers at the time spoke of their plans for a new Syndicate, headed by Alex Krycek and Marita Covarrubias, which was essentially set up in Season 7 finale "Requiem" (Krycek and Covarrubias deliberately disobey CSM's orders and eventually try to kill him) but never resurfaced. Covarrubias disappeared, and Krycek was killed in Season 8.
At the end of the first movie, Scully talks about the vaccine which saved her, saying that it could cure people from the alien virus. It would seriously mess up the upcoming alien invasion and might help to save mankind. But rather than trying to retrieve the conspiracy's research on this or trying to develop the vaccine at the FBI labs, the agents decided to pursue other things. Somewhat justified because they were Reassigned to Antarctica.