Shinichi has survived, not just high school (so far), but being invaded by an intelligent and deadly parasite. Instead of taking over his entire body, the creature stopped at Shinichi's right arm. Thus, he's nicknamed it Migi (“Righty”), and now they are dependent on each other. Shinichi has family, and he has Satomi, a best friend who looks at him with longing… But in a world where humans can't possibly understand, and successful parasites fully control their hosts and hunger for human flesh, what place is there for a being that is half and both at the same time?