Teenager Shinichi Izumi is a high-school student who lives in a quiet Tokyo neighborhood with his parents. His life changes when wormlike creatures, called Parasites, appear on Earth, bent on taking over the brains of human hosts by entering their ears or noses. One such Parasite tries to crawl into Shinichi's ear while he's sleeping one night, but his headphones block entry, so the creature -- who gets the name Migi -- burrows into Shinichi's arm instead, taking over control of his right hand. Shinichi and Migi eventually form a bond as they work together to survive their unique circumstances.