Kei Kurono and his childhood friend Masaru Kato attempt to save a man who has fallen onto the train tracks but are run down by an oncoming train. However, rather than finding themselves dead, they are transported to a strange apartment in which they find a mysterious black orb known as `Gantz". Along with others there, they are provided weaponry and sent on missions to battle alien beings. Is this world, which tests your will to survive, a game or reality?