In Gantz, Kei Kurono, a high school student disillusioned with the harsh realities of life, finds himself drawn into a peculiar situation one fateful day. While waiting for the subway, Kei witnesses his classmate, Masaru Katou, leap onto the tracks to save a drunken individual who has stumbled there. Acting on a sudden urge to assist, Kei joins Katou in the act of rescue. Despite their efforts, both are hit by the approaching train and die.
Upon waking up, Kei discovers himself in a peculiar apartment along with Katou and other strangers, all gathered around a large, dark sphere with a glossy exterior. It becomes clear that everyone present has recently passed away. Instructions appear on the sphere, instructing them to eliminate an unusual creature. Equipped with advanced armor and weaponry provided by the sphere, Kei and the others embark on a perilous mission.
Despite the high risks involved, Kei manages to survive. The survivors are then transported back to the apartment, where the sphere, now dubbed "Gantz" by one of the participants, assigns them scores based on their performance. Kei is given the opportunity to return to his regular life, but he remains subject to being called back at any time to take part in further missions assigned by Gantz.
While Katou fears the constant threat of being summoned again, Kei embraces the missions, finding a sense of direction through the battles. As he grows more adept and begins to value his companions, Kei confronts increasingly formidable foes that start to infiltrate his daily life outside of Gantz.