Kei Kurono, a high school student, feels detached and disillusioned with humanity's cruelty, which leads him to keep his distance from others. One day, he witnesses his classmate, Masaru Katou, risking his life to save a drunken man who had jumped onto the subway tracks. Driven by an inexplicable impulse to help others, Kei joins in, but tragically succumbs to death alongside Katou.
Upon regaining consciousness, Kei wakes up in an unfamiliar apartment with strangers who shared his fate. Gantz, a dark orb, appears and commands them to eliminate a bizarre monster, equipping them with advanced gear and weapons before sending them on their mission.
Despite facing unforeseen obstacles, Kei manages to survive and earn points, which he can use to return to his regular life. However, he is vulnerable to sudden summons back to the apartment, where Gantz assigns them a new target to assassinate.
While Katou dreads returning to Gantz, Kei finds purpose in completing these missions. He develops feelings for his comrades and becomes embroiled in a world of escalating monstrous threats, gradually infiltrating his daily life outside of Gantz.
