An ordinary teenage boy 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 their ears or noses, and take over their brains. A parasite attempts to crawl into Shinichi's ear and take over his brain while he sleeps, but is stopped by his headphones and accidentally burrows into one of his hands instead. With separate intellects, he and his parasite, Migi, fight the invasion of the town.