Exponential drift

Imagine a story in which the hero begins to consider himself an alien, which has possessed the body of a person. He can't explain why or how, but the memories that it claims as its own, for him - memories of another world. He observes the life of humans not with your eyes, and the body in which he lives, seems to him not his body. What can he do? Where his home world? Is there a possibility to interact with the world, to ask for help? For people around - he is mad...