James and Lily had a daughter who unfortunately passed away at a young age, but they kept her memories to keep a piece of them. At a similar time an unrelated boy who looked remarkably like James was having a discussion around what to do with him after he was the only survivor of a very violent incident he caused. In the end the boy has his memories wiped and is physically reverted back to a child and gets Harriet's memories on the condition that Lily and James get to raise him as their son. Obviously that doesn't go well, and Harry never gets to learn about himself, that is, until he was supposed to go to Hogwarts.