Jenny: I'm a mama...his name's Forrest.
Forrest: Like me.
Jenny: I named him after his daddy.
Forrest: He's got a daddy named Forrest too?
Jenny: You're his daddy, Forrest.
A man's girlfriend leaves him and marries someone else. She and her new husband have a son together. The man, jealous, develops feelings of resentment and even hatred towards the boy, whom he believes to be the offspring of the woman he loves by another man. When they meet, the relationship between them is tense at best, homicidal at worst.
Then one day someone reveals the truth. The man was the child's father all along! He's spent years resenting someone who was in fact his own son!
This is a common variation on the Your Son All Along trope. This trope covers any situation where a man believes that a child is his love-interest's by another man, then discovers that he is in fact the true father. Basically, it's the inverse of cuckolding.
For obvious reasons, the parent in these stories is almost always male. For less obvious reasons, the child is usually male, too. In many cases there is an adversarial relationship between parent and child prior to the revelation, but not always.
Sub-trope of Luke, You Are My Father and Mama's Baby, Papa's Maybe. The distinguishing feature is that the parent originally believes the child is someone else's, and only discovers the truth later - sometimes not till after the child is dead! This is significant because in most examples of Luke, You Are My Father, the surprise comes from the man learning that he is a parent, with the potential responsibilities that that entails. In this trope, the surprise comes from the man learning that he is the parent of a particular child. This forces him to rethink his relationship with that child and also, potentially, with the child's mother.
Expect lots of unmarked spoilers.
- Indiana Jones and the Kingdom of the Crystal Skull has Indy finding out that Mutt is his son.
- Used tragically in the French duology Jean de Florette and Manon des Sources. In the first film, the protagonist wants the titular Jean to sell him his land, and plays many tricks on him to pursuade him to do so. It doesn't help that Jean is the son of his childhood sweetheart Florette, who left him and married another man while he was away at war. In the end, he successfully drives Jean to his death and takes the land. In the second film Jean's daughter, Manon, takes revenge for her father's death. It is only at the end of this film that the main character discovers that, contrary to what he thought, Jean was not Florette's son by her husband, but his own son, and that he has unwittingly caused the death of his only child and been in conflict with his granddaughter!
- Silent film West of Zanzibar, and its talking remake Kongo, both involve a Villain Protagonist whose wife cheats on him and leaves him for her lover, only to suffer a Death by Despair soon after and leave the husband with the daughter she conceived with the lover. The husband then sets out on a twisted Best Served Cold revenge plot which involves letting the daughter grow up, turning her into an alcoholic prostitute, and then presenting her to the Glorified Sperm Donor dad. This backfires spectacularly when the lover gleefully reveals to the husband that the young woman is actually the husband's daughter.
- The Amazing Adventures of Kavalier and Clay: Tommy's parentage, while known to the reader, was unknown to Joe until Rosa and Sam revealed the truth.
- Inverted in The Mayor of Casterbridge. Michael Henchard, the title character, drunkenly sells his wife Susan and infant daughter Elizabeth-Jane to a sailor, Richard Newson. Two decades later, Newson is lost at sea and Susan returns to Henchard, bringing Elizabeth-Jane along with her. It is only upon Susan's death that Henchard learns that Elizabeth-Jane is not the same child he gave up, but a different child fathered by Newson, Henchard's daughter having died a few months after the sale.
- The Thorn Birds. Although in this case, Ralph adored Dane the moment he saw him (only after Dane's death does he admit that deep down, he probably knew all along), and instead harbors some resentment towards his mother Meggie, assuming that despite their passionate interlude (when Dane was conceived), she returned to her neglectful husband Luke.
- Inverted on Days of Our Lives. Craig is floored when wife Nancy confesses that she had a teenage daughter (Chloe) that she gave up for adoption. He asks her how she could hide the fact that "they" had a child together (they'd been childhood sweethearts but lost touch when he left for college, reuniting years later) and is devastated when she tells him the girl is her daughter, not his, meaning that she had cheated on him. After a rough start, the three are able to bond as a family. Until the girl becomes ill and needs a bone marrow transplant, at which point Nancy confesses that she's her Child by Rape. However, when the rapist's blood type doesn't match the girl's, Craig realizes that the young woman who he's grown to love as if she were his own daughter. . .is his daughter.
- On General Hospital, Alan and Monica spent several years believing that their son AJ was the result of Monica's affair, before finding out that he was Alan's son after all. Unfortunately, crucial bonding time had been lost between the two, resulting in years of AJ being The Unfavorite (to the son conceived during Alan's affair) and him being an alcoholic failure. Decades later, AJ (correctly) suspected that he was the father of Carly's unborn child, but she lied and told him that his brother Jason was, fearing that either AJ or her boyfriend Tony would take the child away from her. Years later, Jason's girlfriend Robin finally told AJ the truth.
- My Name Is Earl: Joy believes that Li'l Chubby is Dodge's father, but she's afraid of him, because she witnessed him mistreating another woman who claimed to be pregnant with his child. Earl wants her to come forward to Chubby, because he can see that Dodge (and Earl Jr.) are suffering due to their living in Perpetual Poverty. Joy tells him to drop it, but he persists, and gets a Daddy DNA Test, against all odds (with a little help from Darnell). The test shows that Dodge is not Li'l Chubby's...but Earl's. It turns out, Earl and Joy conceived him at a Halloween party, and did not recognize each other afterwards. For the six years that they were married, Earl had been led to believe that Dodge's biological father was a stranger from a concert. In another Shocking Swerve, the test also shows that Earl Jr. is not Darnell's, as previously thought. It's never stated who his father actually is, due to the series being canceled, though Word of God says it was supposed to be a celebrity.
- Scream Queens (2015): A DNA test proves that Weston Gardner is the father of Boone and Hester.
- In Scrubs when Jordan becomes pregnant the first time she claims that the father was someone she had a one-night stand with, but is unclear which one. It is actually Dr. Cox's baby, but she doesn't tell him in order to hold their relationship together. Eventually it leaks to Dr. Cox that Jack is his son, at which point Dr. Cox is angry at Jordan for keeping it a secret from him.
- In Dallas, both Sue Ellen and JR believed that her son was the result of an affair with Cliff Barnes, JR's arch-rival. When it's discovered that Cliff might have a hereditary condition that could be passed to offspring, it is found out that John Ross III was, in fact, JR's son.
- Shakespeare & Hathaway - Private Investigators: A Gender Flipped example in "Ill Met by Moonlight", Lady Bede had an Arranged Marriage that became a Sexless Marriage to escape her tyrannical parents. She was impregnated with her daughter by the gardener, who remained unsuspecting and worked with her family for years.
- In Love Never Dies, the Phantom realizes at the end of Act One that Gustave is his son, not Raoul's.
- At the end of Metal Gear Solid 4: Guns of the Patriots, in a Throw the Dog a Bone moment it's reveal that Rose's son is Raiden's (her marriage to Campbell was a sham to prevent the Patriots coming after the kid to get leverage over Raiden). He then hammers it in by doing the same moves Raiden does with a toy sword.
- While both V and Vergil from Devil May Cry 5 are aware that Nero has the blood of Sparda within him, they naturally assume that it's Dante who fathered him, when in reality, it is Vergil himself who is Nero's father. And man-oh-man, is it ever a shock for Vergil when he finds this out.
- A rare female version can be found in Umineko: When They Cry when it's confirmed in Ep 8 that Battler's biological mother isn't Asumu but Kyrie. A baby switch was made by Rudolf when Asumu's child was a stillbirth, as he thought it would be easier since he and Asumu were married while Kyrie was a lover.
- Throughout the first half of The Venture Bros. Dermott Fictel is assumed to be the illegitimate son of Brock Samson, but in Season 4 it was revealed he's actually the third Venture brother and his mother isn't even his actual mother, but his grandmother.