Tragically, Kei Kurono and Masaru Kato – high school students -- are killed by a subway train while trying to save the life of a homeless drunk who had fallen onto the tracks. Immediately following their untimely deaths, Kei and Masaru find themselves transported—alive and well -- to the interior of an unfurnished Tokyo condominium. A number of other people are there, too, and all of them claim to have just died. They do not know where they are, but they assume they do based on their view from the window, which includes the Tokyo Tower may be seen—but none of them are able to leave the apartment.
At one end of the room is a featureless black sphere known as "Gantz". By way of green text that appears on the surface of the sphere, Gantz informs those present that their lives have ended. The following words appear on the black sphere's surface:
Your lives are over, you bastards. What you do with your new lives is entirely up to me. So there you have it.
Without explaining anything to them, the Gantz sphere opens up to reveal a bald man on life support in the center, and three racks that offer various items for them to use. Information on the Gantz Targets appear on the surface of the sphere, shortly before the Gantz Team are transported to the location of the mission, where they are expected to kill aliens. The series regularly introduces new characters. Most of them are killed off almost as quickly as they appear, though at times a stable cast of veterans forms.
The hunters cannot return from their mission until all enemies have been killed, or the time limit has run out. If the mission is successful, each individual is awarded points for the aliens they have killed. They are then allowed to leave, and live their lives as they see fit, Gantz summoning them back again when its time for the next mission. The only way to stop having to participating in the missions, is to earn a hundred points, and choose the option to be freed.