A character's father raised their child believing that they were biologically theirs, but a sudden reveal causes them to learn that they're not. This often doubles as a reveal of a 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 it's 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're Not My Father/Mother, 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.
Due to this often being used as a plot twist, beware of unmarked spoilers.
- 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 researchers 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.
- In Umineko: When They Cry, it's hinted in the episode 8 manga that Battler's mother Asumu eventually figured out she wasn't actually the biological mother of her son, since her husband actually performed a baby switch with his mistress' after their own child was stillborn. She seemed to have decided to love him as her own regardless.
- Subverted in Avatar: The Last Airbender - The Search. Ursa purposefully wrote fake Affair Letters saying that her son Zuko wasn't her husband's son. Ozai knew they were lies, but he began abusing his son just to spite his wife.
- In the Nightwing one-shot Alfred's Return, Alfred is dragged into an incident involving an old flame of his, who reveals she has a son and that he's the father. When what starts as an attempt to pay off gambling debts reveals that it's actually a cover from a bunch of high-ranking officials attempting to take over Great Britain Nightwing and Alfred spring into action to stop the plot, the son dying in saving Alfred from certain death. As they grieve, the woman reveals that, no, the man wasn't his son at all, but her deceased husband's.
- 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 The New Retcons, it's eventually revealed that John isn't the biological father of Michael; that child was the result of a failed attempt at The Baby Trap. Once this truth has come out, John starts treating them far worse.
- 3 Generations: It turns out that the reason Craig left Ray and Maggie is because he'd realized he really wasn't Ray's father as a result of Maggie's cheating with his brother.
- 28 Hotel Rooms: The woman is letting her husband believe that he's her daughter's birth father, who was actually sired by the man from their affair.
- The Count of Monte Cristo (2002): Supremely jealous, Fernand Mondego does quite a bit of backstabbing to have his best friend Edmond Dantes unjustly imprisoned for treason against the Crown. By the time Dantes escapes, Fernand has married Dantes's fiancée, Mercedes, and the two have a sixteen-year-old son, Albert, together. Albert doesn't look much like his father, having jet black hair and striking blue eyes like his mother. But as Dantes's revenge plan falls into place and Fernand's life is falling apart, Mercedes reveals that Albert actually does take after his father... Edmond Dantes. Mercedes only married Fernand because the unwed, pregnant lover of a condemned traitor had very few other options in the early 1800s.
Fernand: Now go and find my son.Mercedes: He's not your son.Fernand: I beg your pardon?Mercedes: Albert Mondego is the son of Edmond Dantes. Why did you think I rushed off to marry you so quickly after Edmond was taken away?Fernand: (slowly, in realization) ...Premature... Well, aren't you a piece of work? So, he's the bastard son of a dead traitor. He always was disappointing.
- 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.
- The Power and the Glory: Tom's suicide comes after he learns that not only is Eve cheating on him, but their baby boy is actually his grandson, Eve's son by her lover Tom Jr.
- Red Riding Hood: Cesaire discovers that Lucie is not actually his child by blood. He kills her in a rage but expresses remorse about it later.
- A Royal Affair: Struensee is the father of Caroline's daughter. They contrive to have Christian share a bed with Caroline as soon as she realizes she's pregnant in order for Christian to believe the baby's his. He doesn't catch on until their affair is revealed later.
- 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.
- 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.
- A father of Massive Numbered Siblings notices that his youngest child looks considerably different from the others and queries his wife on it. She informs him that their youngest has a different father. Naturally somewhat put out by this, he irately demands to know who the father is. "You."
- One joke (also available in song format) has a young woman despair that every eligible young man in town is her half-brother thanks to her father being a very successful ladies' man in his youth. In revenge, she goes to tell her mother... who reveals that sure, he may be their father, but he's not her father.
- One Arsène Lupin short story has a gender-inverted version with a young man with two mothers (due to the complicated circumstances of the birth, neither woman knows whether her child was stillborn). The two women had to raise the child together, hating one another for every minute of it. It's resolved when Lupin reveals there was a third child involved (and that both children died), dropped off by an unrelated man who told the midwife to switch out his child with one of the dead ones) and therefore they have no relation to him. Lupin actually made up that part, deeming it better that they all get on with their lives then continue to torment each other.
- In Maurice Level's short story "The Father," a young man receives a letter written to him by his dying mother, in which she reveals that he is not the son of the man who raised him, but of a rich man with whom she had an affair, and that his biological father is interested in establishing a relationship (which would also make him rich). After some intense Bastard Angst, he burns the letter and gives the father who raised him a big hug.
- 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. This ends badly for Ned.
- 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; she was named in honour of the daughter he sold along with his wife, who died soon after.
- 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.
- Star Trek: In the novel "The Captain's Daughter" an old enemy of Hikaru Sulu tries this on him after kidnapping Demora and claimed Demora was his daughter. That falls apart when Sulu reveals that Starfleet had ran a genetic analysis on Demora years ago and confirmed she was Sulu's daughter. Later, Demora asks Sulu if the genetic analysis had shown him not to be her father if he still would have agreed to raise her as his daughter. Sulu tells her that yes he would have honored her mother's last request even if he was not biologically her father. This marks a turning point in their relationship, which had been tense for a number of years.
- The Aliens: It turns out that Lewis's father is not biologically related to him, as his mother had an affair. Later he meets his biological father, who had been in prison for years.
- Better Things: In Season 4, Sam learns her maternal grandfather was not the man she had thought he was biologically. He'd actually been a milkman who'd lived near her mom's childhood home in England. Phil, her mom, is flippantly indifferent, saying they're all dead anyway, so who cares?
- Part of Brasss backstory in CSI. He found out before the series that his daughter Ellie was conceived during an affair his wife had while he was Married to the Job. He doesnt tell Ellie but she finds out anyway.
- In Cursed, it's revealed that Jonah abandoned Nimue after he learned that she wasn't biologically his daughter (his wife had a relationship with another man shortly before their marriage), and also because he was afraid of her powers. Nimue later figures out her real father is Merlin.
- The Family: This is invoked by Willa to explain why Ben's DNA doesn't match John, claiming he's the result of an affair which Claire had. It isn't true though.
- In The Haunting of Bly Manor, Dominic eventually realizes that his daughter Flora isn't his. He'd been led to believe that Flora was born prematurely and was relieved that she was healthy, but later came to realize she was conceived months earlier than he'd thought, while he was away on business and his wife was staying with his brother Henry. Dominic confronts Flora's biological father and declares that in spite of this, he still regards Flora as his daughter and wants Henry to keep his distance.
- In Hold Tight, the police discover via a DNA test that Anna's missing son Adam isn't biologically her husband's child and they ask her if she knew this. Anna herself is shocked. When Michal confronts her, she says she genuinely believed that he was their son's father, though he's initially so angry he half-sarcastically asks if their daughter also isn't his (in front of her). Anna later explains what actually happened; she didn't cheat on Michal, but was drugged and raped at her bachelorette party. Because of the timing, either her rapist or her new husband could've been the father, and she desperately wanted to believe it was the latter. After calming down, Michal is only upset that his wife never told him she was raped and it's made clear he still views Adam as his son.
- 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.
- 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.
- The Mick: In Season 2, his mom reveals that Chip's dad isn't his birth father. Both of them react poorly to the revelation. This is subverted when it turns out the other guy wasn't actually Chip's father. It turns out he's a different man whom his mom had sex with back then.
- 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.
- Roswell, New Mexico: It turns out that Rosa isn't Arturo's biological child; his wife had an affair with Jim Valenti, and he is Rosa's biological father.
- Rick Grimes of television's The Walking Dead (2010) 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.)
- War of the Worlds: Sarah tells Bill that she cheated on her husband in the past, and due to this Emily was really fathered by another man. Though he still loved Emily since, Jonathan never entirely forgave Sarah.
- 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. During the rest of the game, the Avatar struggles with the fact that Garon's kingdom and family are at war with Sumeragi's. Sumeragi turns out to have not been the Avatar's father either, but that's beside the point.
- In the animated Cyanide and 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 on a line and you break your father's spine"... and the next-door neighbor keels over while shouting about his spine.
- 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).
- 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.
- Young Justice (2010): 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 just used as justification for him to be a Fake Defector.