A man lives his life.  He is a seemingly normal-looking individual.

This man also has an overactive imagination.  He is one who indulges himself in innocent children's entertainment, and is always pining for the innocence of these stories.

This man has an accident which causes a head injury, and is put into a coma.

When this man wakes up from his unconscious state, he realizes that things are not as he knows.

For, you see, this man is no longer the man that got into the accident, but is now a man who firmly believes he has a completely different past and personality than he used to have, and knows nothing about the current world he lives in or the person he once was.

What if you got into an accident and woke up believing you were a completely different person?  And you found out that the person you now are waking up as is nothing more than a character of fiction, and yet you remember these fictitious events as clearly as if they really happened?

To put it simply, what if your imagination took over your mind during a state of vulnerability and completely erased and rebuilt your memories into fake memories, creating a person who falsely believes they lived a completely different life?