Harry would be heartbroken. He'd likely first try to jog their memories in simple ways, like showing them things that were important to their family. He might be in denial at first, thinking it's just a temporary thing.
In such a fanfic, perhaps a villain manipulates time and alters the memories of James and Lily. They might be living in a different reality where Harry doesn't exist in their minds. Harry could then embark on a journey to find the source of this alteration and restore his parents' memories. He could encounter various allies and enemies along the way, all while trying to come to terms with his parents' current state.
One possibility could be that Lily uses some kind of magic, perhaps a very powerful and rare spell. Maybe she does it out of a sense of protecting James from the pain associated with Harry. In the fanfic, it could lead to a lot of confusion for James. He might start a new life without the knowledge of his son, and then as the story unfolds, there could be a series of events that slowly make him start to remember bits and pieces. For example, he might encounter people or objects that were related to Harry, which would trigger his subconscious.
Well, it would change a lot. Harry would grow up without his father's influence, which could make him even more of an outcast in the Dursley household. Without James' presence in his life, Harry might not have the same sense of self - worth that he got from knowing his father was a brave Gryffindor. Also, the relationship between Harry and his mother Lily would be very different. Since she made James forget Harry, there must be some underlying reasons that could affect how Harry views his mother.
In some fanfics, James and Lily giving Harry away might be due to a prophecy that requires Harry to be raised away from them for a certain power to develop. So they make the heart - wrenching decision to give him to a family that has some special connection to an ancient magic that can safeguard him.
It could be a story that focuses on the relationship between James and Lily through their portraits. Maybe it shows how their love story is remembered and revered in the wizarding world even after their death.
One possible plot could be that the portraits of James and Lily are somehow damaged or fading, and Harry has to find a way to restore them. He goes on a quest to find ancient spells or magical artifacts that can help.
One great Lily and James fanfic is 'All the Young Dudes'. It delves deep into their relationship, from their school days at Hogwarts to the challenges they faced. It has well - developed characters and a captivating plot that stays true to the essence of the Potter universe.
The plot could involve James and Lily giving Harry away to a powerful wizard who has the ability to hide Harry's magical signature completely. This wizard is an old acquaintance of Dumbledore, and they believe that under his care, Harry will be safe from Voldemort's clutches. They sacrifice their parental rights to ensure Harry's safety.