After dying in a brutal subway accident, Kei Kurono and Masaru Kato awaken in a locked apartment alongside other people who have recently died. At the center of the room stands a mysterious black sphere known as Gantz, which forces them to take part in deadly missions against alien creatures hidden among humans. Equipped with suits and weapons whose capabilities they barely understand, the participants must survive, earn points, and try to uncover the rules of a game whose true origin remains unknown. As the missions grow increasingly violent, Kei discovers a world where survival, fear, and individual choices expose both the weaknesses and the strengths of those trapped within it.