A character's father raised their child believing that they were biologically theirs, however a sudden reveal causes them to learn that they're not. This often doubles as a reveal of Cuckold, when the significant other wasn't aware of any straying. Another explanation is that the child was Switched at Birth or a changeling.
Compare to Chocolate Baby when it's obvious by the child's looks, Mama's Baby, Papa's Maybe for when its ambiguous, and I Am Not Your Father for when the dad reveals this to the child. Reverse to Oblivious Adoption, when it's the child that is out of the loop while the parent knows. See also Your Son All Along, You Are Not My Father, and Not Blood Siblings. Not to be mistaken for Not His Blood, which involves a character literally being splattered with blood that isn't their own. Compare and contrast Luke, I Am Your Father and Luke, You Are My Father, where a person previously thought unrelated is revealed to be another character's parent or child.
- Inverted in the backstory of Magical Girl Lyrical Nanoha Strikers. Quint Nakajima rescued two young combat cyborgs during a raid on an illegal research facility. Given the fact that they bore a resemblance to her (and they needed someone to take care of them) she decided to adopt them alongside her husband. It was only years later that she discovered that there was a very good reason they looked so much like her: the researches had gotten a hold of her DNA and used it as the genetic base for their cyborg project, meaning that Subaru and Ginga were biologically her daughters. Unlike most examples that would have this be the source of drama, this did not affect her relationship with them in any way.
- Lasting Fame: Pizzazz had told Eric for years that they had a son together. When he finally meets Pizzazz's kid face-to-face as adults, it's clear to Eric that the boy isn't his. He's obviously Rio's son.
- In Titanic (1953), Julia reveals to her husband Richard that her 10-year old son Norman was the result of a one-night stand. Her husband immediately becomes distant towards Norman and brushes him (and Julia) off the entire movie. In the end Richard stops his aloofness and confesses his love for his son, just as they wait to die together on the sinking Titanic.
- Late in Get Him to the Greek, Jackie Q confesses to Aldous that their son was actually conceived during a fling she had in Italy, and she covered it up for at least a decade. Touchingly, Aldous still wants to be a part of the boy's life, and the kid still thinks of Aldous as his dad.
- In Yellowbeard, Betty ends up telling her son that his father is not the Lord Lambourn, but is instead the son of infamous pirate Yellowbeard. When Lambourn finds out, he takes it very well on account of him being completely drunk at the time.
- Defied in A Game of Thrones regarding Robert and Cersei's children, all of whom were born of Twincest between Cersei and her brother Jaime. Jon Arryn was poisoned shortly after discovering the affair (by going through historical descriptions of past Baratheon-Lannister children, discovering that they normally had Baratheon looks, especially black hair instead of blond). Ned Stark discovers it later after replacing Arryn as Hand of the King, but Robert is mortally wounded by a boar and Ned can't bring himself to reveal the affair to his old friend on his deathbed.
- Warrior Cats: Happens to Brambleclaw when Hollyleaf reveals, to every cat attending the Gathering, that he's not her father and Squirrelflight isn't her mother, but that she and her siblings are instead the children of Leafpool, Squirrelflight's sister and the Clan's medicine cat, and Crowfeather, a warrior from a different Clan who had a legitimate son of his own. Naturally, he took the news pretty hard, but eventually got over it when he decided that, as their foster father, he still loved and was proud of Hollyleaf, Lionblaze and Jayfeather, and that their blood relation didn't matter.
- At the beginning of The Mayor of Casterbridge, the eponymous character gets drunk and sells his wife Susan, and their infant daughter Elizabeth-Jane to a passing sailor. She returns with Elizabeth-Jane many years later, him having sobered himself up out of remorse and turned his life around (although remaining a jerkass). After his wife passes away, he learns that the Elizabeth-Jane who returned with his wife is the daughter of the Sailor; named in honour of the daughter he sold along with his wife, who died soon after.
- Rick Grimes of television's The Walking Dead learns that his wife, Lori, has had an affair with his best friend and co-worker, Shane; Lori later gives birth to a daughter, Judith. Rick mentions to another character that Shane is Judith's father. (Paternity testing is likely a low priority in the zombie apocalypse, however.)
- Maury has carved its niche in television history by conducting genome testing on troubled couples. Roughly half the time, the alleged father hears Maury declare, "You are not the father," which leads to the "slut run" backstage to the "crash couch." Although nowadays, these "parents" are hired actors, early seasons were conducted with raw recruits willing to air their dirty laundry on stage in exchange for two nights' stay with meals at a nice hotel.
- Nip/Tuck: Sean McNamara discovers that his teenage son Matt was actually fathered by his longtime best friend and co-partner of their plastic surgery firm, Christian Troy, from an affair he had with Sean's wife Julia many years ago. Sean is extremely livid at both Christian and Julia while Matt starts lashing out more and more. Eventually, Sean forgives both of them (Christian sooner than Julia), and Matt comes to see both men as his fathers.
- The season two finale of It's Always Sunny in Philadelphia has Frank Reynolds finding out, while out to dinner with his ex-wife Barbara, that Dennis and Dee aren't actually his children and Barbara had an affair with a man named Bruce Mathis during their marriage, then tricked Frank into raising the twins anyway. This inevitably leads to a Frank meltdown in the middle of the restaurant, where he's told to not make a scene but ends up asking random strangers if they had "banged his whore wife" and then has a stroke.
- Crusader Kings II has an event chain where a character suspects that their wife was impregnated by another man and can hire spies to try and find the truth. If in fact this turns out to be the case, the child will be born a bastard and disinherited. It's also possible to use the Intrigue focus to start rumors that a character is secretly a bastard, which rightly or wrongly may then be used as cause to dethrone them.
- Fire Emblem Fates takes this trope and squeezes it nearly dry almost exactly before the end of the fourth chapter; your character, the Avatar, is revealed to not be the child of King Garon of Nohr, but rather that of King Sumeragi of Hoshido. Actually, Sumeragi isn't your father either, but that's besides the point. During the rest of the game, you struggle with the fact that Garon's kingdom and family are at war with that of your home nation and REAL family.
- In the animated Cyanide & Happiness sketch "Step on a Crack" a married couple is watching their daughter play while singing the childhood rhyme "Step in a crack/And you break your momma's back." At that moment the daughter steps on a crack in the pavement, and sure enough the mother's back breaks. The daughter then goes onto to the line "Step one a line and you break your father's spine"... and the next door neighbor keels over while shouting about his spine.
- The Simpsons: A non-canon example occurs in one Treehouse of Horror segment which has Maggie's father revealed to actually be Kang rather than Homer.
- Played with in King of the Hill, where it's painfully obvious that Dale's son Joseph is actually the lovechild of his wife Nancy and her Native American healer John Redcorn, yet Dale never even suspects. The only other person not to realize this is Peggy, and when she does and tries to tell Dale, Hank advises against it, as Dale is not the most emotionally stable person. Dale eventually does figure out that Joseph is not his son, but thinks that it's because Nancy was inseminated by aliens, and never even suspects Redcorn (who at one point muses that he's taking the fun out of cheating with Nancy).
- Young Justice: Between seasons one and two, it is revealed that Kaldur is the son of Black Manta, having spent most of his life believing his mother's husband - Calvin Durham - was his father. Kaldur's mother, stepfather, and Aquaman concealed the truth from him and everyone else in an attempt to protect him. The reveal of this and Tula's death led to his defection ...except that was that was just used as justification for him to be a Fake Defector.