You’re Kei Kurono, a 15 y/o teenager. Your best friend is Masaru Kato. You both are hit by a subway train in an attempt to save the life of a drunk homeless man who had fallen onto the tracks. Following your deaths, you and Kato find yourselves transported to the interior of an unfurnished Tokyo apartment, where you meet Joichiro Nishi, a Gantz veteran, and other clueless participants. You soon realizes that they are not allowed to leave the apartment. At one end of the room there is a large black sphere known as "Gantz".

After some time in the room, the Gantz sphere opens up, revealing a bald naked man with a breathing mask and wires attached to his head, and various weapons for them to use. These include the custom fitting black suits which give them super-human strength, speed, stamina and damage resistance, a controller which acts as a radar and stealth unit, X-gun, X-Shotgun, Y-Gun.

When the Gantz sphere opens, green text appears on its surface, informing those present that their "lives have ended and now belong to him". A picture and brief information is shown of some of the Gantz' targets, Gantz orders you and the rest to go and kill them. Except for a single mission, all the targets are aliens living on Earth, which take on a wide variety of forms. During the mission, normal people cannot see the players or the aliens. Gantz transports you and the rest to the area of the mission, and they cannot leave or return until all the enemies have been killed, or the time limit has run out. If they survive a successful mission, each individual is awarded points for the aliens they have killed. Once a participant has scored 100 points, a "100 point menu" will appear. The menu offers three options:

1: The participant can return to their normal life, never having to be summoned by Gantz again. As a price, their memories of Gantz and the missions will be erased. 2: The participant obtains a unique and extremely powerful weapon. 3: The participant can revive someone who has died