"Parasites" are tennis ball-sized creatures of unknown origin and number. They invade and take over human minds to survive. Shinichi Izumi is a 16-year-old high school student living with his parents in a quiet neighborhood. One night, a Parasite invades Shinichi's body, attempting to take control. However, it fails to complete the takeover and ends up inhabiting only his right arm. Consequently, both Shinichi and the Parasite—who calls himself Migi—retain their individual minds. Now in a peculiar "human and alien" relationship, Migi proposes they cooperate to survive. Shinichi has no choice but to agree, and he must cling to his morality as he and Migi face off against other bloodthirsty Parasites.