Alien parasites have invaded Earth and taken over the minds of ordinary people. Shinichi has also been infected with a parasite, but he can still think and act for himself, and that makes him different. He's already been approached by two mysterious victims of the invasion: Ryoko Tamiya, a schoolteacher, and her former lover, A. While Ryoko may yet be an ally or foe, Shinichi barely escaped A's attack with his life intact. Now, there's a new parasite in town, and he's a student in Shinichi's school! Just what do these parasites want with humanity...and with Shinichi?