A man discovers that a child he thought was his girlfriend's by another man is actually his own.
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 cuckholding. 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. 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.