A plot arc that was incomplete and forgotten, and then suddenly brought back out of the blue to continue the plot arc itself. This is distinct from the Continuity Cavalcade and Continuity Porn tropes in that it's not just a Shout-Out type situation - the plot arc is resumed, essentially where it left off. In short, what distinguishes Plot Archaeology from similar tropes is that it's not just a one-episode reappearance, either as a Shout-Out or a full-fledged Sequel Episode (which may be considered a Sub-Trope). Do not confuse with an archaeology plot.
- X-Men: Mystique reveals her true form to Nightcrawler, who is shocked that they look very similar. She mentions his mother's name. 10 YEARS (or more) later, it's revealed that she is his mother. It hadn't been even mentioned in the comics in-between those two points.
- The Third Summers Brother. In 1993, Mr Sinister casually refer to Cyclops having "brothers", then corrects himself. This arc got aborted when Fabian Nicieza left the X-titles before he could reveal the third brother was Adam-X, the X-Treme. Over the next ten years, the concept was never referred to. (Robert Weinberg thought it was Apocalypse, but also left the book before he could say so). In 2004 Chris Claremont suddenly brought it up again, and revealed it was Gambit, but that was in an Alternate Continuity. Then, in 2006, Scott and Alex finally meet Gabriel Summers, aka Vulcan. Unfortunately, Vulcan's backstory makes it impossible that Sinister could have known about him in order to make that casual offhand reference. Fans now wonder if there's a fourth Summers brother. Or perhaps even more than that; Sinister didn't mention a number, just that Cyclops has more than just the one brother he was aware of at the time.
- Spider-Man: The true identity of the Hobgoblin. The villain first appeared in 1983; his true identity, fashion mogul Roderick Kingsley, had first shown up three years prior. His creator Roger Stern planted a number of Red Herrings but maintains- and is corroborated by other writers- that he intended Kingsley to be the Hobgoblin all along and to reveal him after a couple of years, taking advantage of the fact that Roderick had a servile identical twin that Stern managed to sneak in without being noticed. The problem? Stern was fired not even halfway through the Myth Arc, and subsequent writers either disliked the Kingsley idea or didn't know about it. Doesn't help that at least two characters were framed as him- Flash Thompson, and a thug named Lefty Donovan, before Ned Leeds was Killed Off for Real and then, in an Ass Pull, "revealed" as the "true" Hobgoblin all along because the writers had gotten tired of the mystery and decided to introduce a second Hobgoblin, mercenary Jason Macendale (hitherto a minor villain known with the alias Jack O'Lantern), who quickly and unintentionally became a Big Bad Wannabe. It wasn't until 1997 that Stern was invited back and allowed to wrap up the story as he originally intended it- quite remarkably, the Kingsley idea was still internally consistent to the story and caused no real plot holes (if you pay attention, at least). For the whole story in painstaking detail, check out these essays.
- Star Trek II: The Wrath of Khan was all about digging up a one-episode villain from the first season of the original series and answering Kirk's question about what would happen with the "seed" that the Enterprise crew planted.
- In the Anita Blake series, the events of book 11 resulted in the vampire serial killing group Anita was after not actually getting caught. Fans bitched and complained when the next few books didn't mention it at all, and then in Book 17 LKH went back to it and we finally get to confront the Big Bad. A few other loose threads from that were also left hanging in that book and the one right after it which are still waiting to be picked back up though.
- BIONICLE Adventures #10: Time Trap, from the end of 2005 revisits several issues that were left hanging in '04, six books earlier, such as the recovery of the lost Mask of Time and the Shadowed One's reaction to the deaths of two of his servants at the hands of Makuta. The Mask of Time plot was briefly continued in an online serial three years later.
- In the first season of Heroes, Hiro falls in love with a waitress named Charlie. It turns out she has a brain tumor and is going to die. Hiro accepts that he can't save her, gets some Character Development, and the plot moves on. Several seasons later, Hiro regrets that he didn't save her, and the arc focuses on her again as Hiro goes back in time to try and save her, but she gets kidnapped and held hostage by the Big Bad.
- NewsRadio had a storyline where Lisa decided she wanted to have a baby with Dave. This went on for a few episodes, then was quietly dropped. In a later episode the same season, there is a brief conversation about how the moment had passed. The writers hated continuing story lines, which the network continually tried to force on the show.
- The First Evil from Buffy the Vampire Slayer makes a one-off appearance in season 3. It shows up four seasons later as the Big Bad. A script involving the First that they didn't use for Season 5 became the basis of one of the Xbox games.
- Stargate Atlantis: Stardrives were mentioned very briefly in an early episode and didn't become massively important until the finale three seasons later.
- Doctor Who:
- The Third Doctor casually mentions Metebelis III in a throwaway line at the beginning of "Carnival of Monsters". In the last story of Season 10, he takes a quick and mostly comical trip to Metebelis III while sulking about Jo's departure, which mostly serves the purpose of giving him Green Rocks to use as a deus ex machina. The final story of Season 11 is set partially on Metebelis III, and it turns out the planet is extremely important in his eventual downfall.
- Arc Words sometimes get planted before they're even used in a this-is-leading-up-to-something fashion. We get a briefly-seen newspaper mention of "Saxon" being ahead in the polls midway through new series Series 2. Nothing is said of it until "Mister Saxon" becomes a mystery figure of Series 3.
- River Song was introduced in "Silence of the Library" and quickly dropped, then brought back with the Eleventh Doctor.
- There's a gag mentioned in the Season 4 Specials that Queen Elizabeth I, due to getting involved with the Doctor in some fashion, is "no longer known as the......". This plot thread is used prominently in big chunks of "Day of the Doctor", which details this involvement.
- "Into the Dalek" deals with dropped plot points introduced in "Genesis of the Daleks", a Fourth Doctor story made way back in 1975.
- In Star Trek: The Next Generation, Wesley Crusher is built up as being a very special person, and it is suggested that he would do something remarkable, eventually. Then he was phased out of the show, and was gone for something like three seasons. Come the final season, the Creator's Pet returns, and the plot arc completes itself.
- At one point, Ugly Betty had a plot arc about Amanda trying to discover the identity of her biological father. After a few episode, the plot disappeared with no resolution. In the show's Grand Finale, Amanda finds her father, out of the blue and completely by accident.
- In Sliders, the Kromaggs were a season two Monster of the Week who got one mention in season three. Then comes a Channel Hop... and their return as the franchise Big Bad.
- On The Wire, McNulty's FBI buddy gives him the results of an unfinished investigation that showed Lt. Daniels was dirty when he was on the Narcotics task force. McNulty doesn't trust him for most of the first season, but eventually the men grow to respect each other. This is not mentioned again until the series finale, when now-Commissioner Daniels is forced to resign rather than be manipulated by the threat of revealing that very same investigation.
- On Babylon 5, much of Captain Sheridan's character arc in Season 2 centers on the death of his wife and how he needs to let it go and move on with his life. Fast forward to the end of Season 3, and Anna Sheridan shows up on B5, with a message for her husband from the Shadows.
- In Durarara!!, the slasher is mentioned early on a couple times but not addressed until much later.
- Quite a few minor plots in the earlier Mass Effect games come back in Mass Effect 3, sometimes in unexpected ways. An entire DLC of the third game, with profound revelations about the entire setting, is a Call Back to a throwaway bit of World Building fluff text in the first game. A villain from one of the first game's DL Cs can end up being recruited as an ally in the third game, and so on.
- Avatar: The Last Airbender: one early episode introduces Bato from Katara and Sokka's father Hakoda's fleet, and suggests the kids can meet up with the fleet. When this fails, no further attempts are made to meet up with Hakoda or the fleet until the end of the second season.
- At one point in Superman: The Animated Series, Brainiac blasts former partner Lex Luthor with a ray that (like many instances of Family-Friendly Firearms) doesn't seem to hurt him much for what appeared to be intended as a killshot. Much much much later in Justice League Unlimited, we find that that's because it actually wasn't. Brainy put his data inside Lex in case he was destroyed, and this backup of himself was now needed. Enter... Brainithor! The Powers That Be have said that they'd actually planned it the day they wrote the zapping scene, they just didn't have the right opportunity to use it for a long, long time. Lucky for them the DCAU lasted long enough to do it.
- In Transformers Animated, Porter C. Powell makes a single not-terribly-memorable appearance in the first season and it doesn't look like he's intended to return. In season two, he does... with a vengeance.
- Unicron in Transformers Prime first appears and is dealt with at the end of season 1. After being mostly forgotten for the following two seasons, he reappears to follow up on his plans in the series finale movie Predacons Rising.