Kei Korono is an ordinary high school student who keeps to himself. He isn't well-liked, but he doesn't care, as he values no one but himself. One day, when he sees an alcoholic fall onto the tracks at a subway station, he stands by as usual, doing nothing. At that moment, his childhood friend Kato Masaku, who doesn't recognize him, can't bear it and jumps onto the tracks to help. Reluctantly responding to Kato's call for help, Kei pulls the alcoholic off the tracks. They both find themselves trapped under an oncoming subway train. Kei, whose body should have been completely shattered, suddenly finds himself in an empty room. The room is not entirely empty; Kato has also arrived there, along with others. And there is a black sphere. They cannot escape the room, and at that moment, the black sphere - Gantz - speaks. What is the secret of the room? What is the black sphere hiding? In this life after death, Kei and the others are faced with a challenging battle.
