Stealth Sequel

When a Sequel or Prequel to a work isn't actually stated or advertised as a sequel/prequel. At first, it looks like a Spiritual Successor, Continuity Reboot, Alternate Universe, or some other closely related universe tie-in but it turns out to be a direct sequel. This might just be hinted at, or expressed outright near the end. See also Broad Strokes.

Anime & Manga

 * Suzumiya Haruhi. The internet exploded after new episodes suddenly starting airing without any previous announcement. Well they did make an announcement, they said it was a rerun of the first season (which is technically true, as the new episodes were shown chronologically with the old ones).
 * Rebuild of Evangelion. Maybe.
 * Rozen Maiden Tale started out as a What If manga that shows what would happen if Jun decided not to wind. Then the story transitioned into where the original continuity (which was Cut Short in the original manga) left off...
 * Blood-C at first appears to be unrelated to Blood: The Last Vampire but is later revealed to be a sequel.

Film

 * Alice in Wonderland, the movie, isn't too stealthy about it since Alice is older, but she starts off her adventure in nearly the same way as the original book. Then, while Alice is struggling with changing size, the Dodo remarks, "You'd think she'd remember all of this from the first time."
 * The 2009 Star Trek film functions as both this and a reboot, with.
 * The main stealth part comes from the fact that the movie is not a full reboot, but rather an Alternate Timeline,, that supersedes every previous movie and series but one.
 * It also offers a conclusion to the Federation's conflict with the Romulan Empire, which had been gradually developing since Star Trek: The Original Series:.
 * Catwoman is a stealth sequel to the Batman films, notably, Batman Returns. Her origin is identical to the one played by Michelle Pfeiffer, the Catwoman persona is said to be a Legacy Character, and in the pictures of former "Catwomen", we see Michelle Pfeiffer's original character.
 * This is debatable though, considering Michelle Pfeiffer's character does not have actual superpowers, as opposed to Halle Berry's character.
 * The Ridley Scott film Prometheus is a stealth prequel to Alien.
 * Similarly, the first Alien vs. Predator movie could be seen as a prequel to the Alien series proper. It starts out looking like a mindless slash-em-up crossover, but it goes on to explain the origin of the Xenomorph species as well as some details about The Company's genesis.
 * Basement Jack is a loose sequel to Evilution, the two film featuring the same old apartment building and creepy manager and his equally creepy display room of murderous artifacts, which he adds more weapons to at the end of both films (a vial of alien virus in Evilution, and the eponymous character's sword in Basement Jack).
 * At the end of Final Destination 5
 * Indiana Jones and the Temple of Doom takes place before Raiders of the Lost Ark, and it's only indicated by the date at the beginning of the film.
 * The Good the Bad And The Ugly takes place before the first two movies in the Dollars Trilogy.
 * Nanny McPhee and the Big Bang. The only connection to the first movie is revealed
 * The 21 Jump Street film is actually a sequel to the original TV series.

Interactive Fiction

 * Planetfall has a few references to Infocom's previous sci-fi work Starcross that imply that it is set within the same universe several centuries later.
 * Narcolepsy by Adam Cadre isn't advertised as a sequel to Adam Cadre's previous work I-0, but it isn't long before you run across a place mentioned in the prior game. Also, I-0's protagonist Tracy Valencia makes a somewhat in-joky cameo.
 * Dinner with Andre by Liza Daly has a twist ending where the PC turns out to be the same character from Liza's previous IF work Bloodline several years later and sees someone she knows from that time.
 * Masquerade by Kathleen Fischer has a number of endings, one of them causing the game to become a prequel to her other work, The Cove.

Literature

 * If you did not pay attention, you may not have noticed that the elder and protagonist in Gathering Blue are the protagonist and baby from The Giver.

Live Action TV

 * The last episode of Newhart revealed that the entire series was the dream of the protagonist from The Bob Newhart Show.
 * This is one interpretation of the remade Battlestar Galactica ("All this has happened before, and all this will happen again")

Video Games

 * Shadow Hearts wasn't advertised as a sequel to Koudelka, and indeed some people still claim that they don't have any real story links, despite.
 * Shadow of the Colossus was a Prequel to Ico, but you'd never realize until the very end. And even then you might not realize unless you were a big fan of Ico.
 * Vagrant Story was a sequel to Final Fantasy Tactics, although this wasn't apparent until the whole Ivalice Alliance was fleshed out with more games.
 * Captain Commando is a futuristic sequel to Final Fight. The game is set in Metro City, Ginzu the Ninja is Guy's future successor in the ways of the Bushin school, and a bust of Mike Haggar can be obtained as a power-up.
 * Dragon Quest III looks like a Continuity Reboot, but late in the game you find yourself on a very familiar world map, and the ending names you as the famed ancestor of the first two games' heroes.
 * Dragon Quest VI has thematic links to the other "Zenithian Trilogy" games (IV and V), but there are hints that it's actually a prequel.
 * Xenosaga may or may not be a prequel to Xenogears. Likewise, Xenoblade being a sequel to both. While Blade doesn't have much in common with the other two apart from some themes and design elements, and takes place in a different universe

Web Comics

 * For quite a long time, Black Ops Civil Service webcomic Skin Horse by Jeff Wells and Shaenon Garrity looked to be the spiritual successor to Garrity's earlier Mad Science comic Narbonic, but readers suspected it was actually a sequel. It was three and a half years before the connection was officially made.
 * The actual degree of continued story is pretty marginal, but Word of God confirms the connection was planned from the beginning and not just a Sure Why Not.