Shinichi Izumi, an ordinary high school student, sees his life completely changed when a strange alien creature attempts to take control of his body. Unable to reach his brain, the parasite instead settles in his right hand and develops a consciousness of its own. Forced to coexist, Shinichi and the creature he names Migi discover that other parasites have successfully taken complete control of their human hosts and now prey on other people. Caught between two species, Shinichi must learn to survive while trying to preserve his humanity. As the confrontations escalate, the boundary between human and monster becomes increasingly difficult to define.