High school students Kei Kurono and Masaru Kato find themselves in a life-altering situation when they die after being struck by a subway train while trying to save a drunken homeless man who had slipped onto the tracks. Only moments later, they awaken in a desolate Tokyo apartment alongside a group of bewildered participants who share the same fate. In this strange environment, they are introduced to a mysterious black sphere known as Gantz, which serves as the catalyst for their new reality.
Once activated, Gantz assigns them perilous missions where they must hunt and eliminate alien entities hidden throughout Earth. Armed with state-of-the-art weaponry—including enhanced suits and energy weapons—the participants are thrust into deadly confrontations. Each mission transports them to the designated locations, where they must finish their tasks before time runs out, or risk being trapped forever. The chilling aspect of their reality is that ordinary humans are completely oblivious to their presence, leaving the participants to battle against formidable alien foes in secrecy.
Survival earns them points based on their kills. Accumulating 100 points grants participants a crucial choice: return to their previous lives with memories wiped clean, acquire a powerful weapon, or bring back a fallen teammate. After enduring their first hazardous missions, Kurono and Kato are pushed to their limits in fierce battles that inflict heavy losses on their ranks. In a moment of self-sacrifice during their third mission, Kato bravely gives his life to eliminate the last enemy, allowing Kurono to escape with life-threatening injuries. This event becomes a turning point for Kurono, transforming his character and igniting a new sense of heroism within him. Determined to revive his fallen comrades, he takes on the mantle of leadership, guiding a new squad of Gantz players and showcasing extraordinary combat skills.
As fate would have it, Kurono eventually revives Kato, but his victory is short-lived as he confronts another lethal enemy: a group of vampires. As the narrative unfolds, the missions evolve, bringing with them twisted changes that allow ordinary humans to witness the battles while simultaneously increasing the strength of the aliens. The participants soon engage in a collaborative mission against a rival Gantz team originating from Osaka. In these critical moments, Kato emerges as the primary force to resurrect Kurono, culminating in an epic showdown with the formidable alien Nurarihyon. Though Kato triumphs in the end, he is gravely injured.
An experienced member, Joichiro Nishi, uncovers a secret "catastrophe countdown" tied to the Gantz sphere, revealing an impending apocalypse. Just a week later, Earth is plunged into chaos as an overwhelming alien force arrives with the intent to eliminate humanity. Faced with this existential threat, Kurono and his allies rely on Gantz technology to orchestrate a defense. It's soon disclosed that a superior alien species gifted humanity with Gantz as a means to ensure survival, although their true motivations remain shrouded in mystery.
As the climactic battle approaches, the alien leader Eeva challenges humanity, issuing an ultimatum: the destruction of Earth unless Kurono steps forth to confront him. With the world watching anxiously, Kurono, with Kato at his side, rises to the challenge and ultimately overcomes Eeva, safeguarding the world from obliteration. The saga culminates with Kurono and Kato returning as celebrated heroes, forever altered by their harrowing journey and the friendships formed along the way.
