A typical teenager named Shinichi Izumi lives with his parents in a quiet neighborhood until alien parasites invade. They are worm-like creatures that enter human hosts through the ears or nose and take control of their brains. One parasite attempts to infiltrate Shinichi's ear and control his brain while he sleeps, but it is stopped by his earphones and accidentally enters one of his hands. With separate intelligences, he and his parasite, Migi, fight against the invasion of the city.