The origin and population of the Parasites, which are the size of tennis balls, remain unknown. By controlling their hosts through infiltrating their minds, they force them to reproduce. One night, Shinichi Izumi, a regular 16-year-old residing in a peaceful district with his family, is affected by one such Parasite. However, instead of overtaking his entire body, the invader is confined to his right arm.
Rather surprisingly, both Shinichi and his invader, named Migi, retain their own consciousness in this bizarre "human and alien" alliance. Migi proposes a collaborative arrangement for survival, which Shinichi reluctantly agrees to, leading him to confront ethical dilemmas as they come across other dangerous Parasites.
