Summary [AniDB]: * Based on [Iwaaki Hitoshi]`s science-fiction horror manga by the same name.
17-year-old [Izumi Shin`ichi] lives with his [mother] and [father] in a quiet neighborhood in Tokyo. One night, worm-like parasitic aliens invade Earth, taking over the brains of human hosts by entering through their ears or noses. One such parasite attempts to crawl into Shinichi`s ear while he sleeps, but fails since he is wearing headphones; it enters his body by burrowing into his arm instead, taking over his right hand and eventually receiving the name [Migi] from him.
Because Migi matured in Shin`ichi`s hand, it wasn`t able to travel further up into his brain, and as such both beings retain their separate intellect and personality. As they encounter other hosts to these parasites, they capitalize on their strange situation and gradually form a strong bond, working together to survive. This gives them an edge in battling the other aliens, who have taken over their human hosts` brains and attack the pair upon realizing that Shin`ichi`s human brain is still intact. Shin`ichi feels compelled to fight the other parasites, who devour humans as food, and he enlists Migi`s help to that end.