Parasites are creatures that live off their host. They have absolutely superhuman strength. Sent by heaven, they must penetrate the human brain for complete power over the wearer. But whoever is lucky. It is the failure of one parasite that saves the life of the schoolboy Shinichi Izumi. But this same failure also makes him the bearer of this mysterious creature, dooming Shin'ichi to a constant struggle for survival with other, more powerful parasites.