The plot centers on a seventeen-year-old teenager named Shinichi Izumi, who lives with his parents in a quiet neighborhood in Tokyo. One night, worm-like creatures called "parasites" appear on Earth, making humans their host by invading their brains by entering through ears, noses, etc. One of these parasites tries to enter Shinichi ear while he sleeps, but is unsuccessful as Shinichi is wearing headphones, so he decides on another path to enter his body and reach his brain: the right arm. of the. Because Shinichi was able to prevent this parasite from reaching his brain, it is limited and completes all its maturation in the invaded arm, causing both beings to retain their separate intellect and personality.
