Mari's quiet everyday life falls apart when she meets a mysterious boy. Her uncle returns after a long absence and tells her that earth is under alien control and that Mari is half human and half alien, while the strange boy confesses that he is trying to escape because he has stolen the key to liberate human beings from Mari's alien uncle. Mari, who only half believes their stories, meets her sister though she is not supposed to have one. Where is Mari heading for in her upside-down life?