The story begins when a pair of high school students (Kei Kurono and Masaru Kato) are seemingly run over by a subway train, after saving the life of a homeless drunk who had fallen onto the tracks. Following their untimely deaths, Kei and Masaru find themselves transported — alive and well — to the interior of an unfurnished Tokyo condominium, where they're unable to leave, from which the Tokyo Tower may be seen, with a number of people who have also just died. At one end of the room is a featureless black sphere we later learn is called "Gantz". After a short amount of time, a postwar Japanese exercise radio begins to play from the sphere. 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 have ended. What you do with your new lives is entirely up to me. That's the theory, anyway." (The ADV translations have it as "Your lives are over, you bastards. What you do with your new lives is entirely up to me. So there you have it.") The sphere then opens up to reveal suits that, when worn, amplify the user's attributes, such as strength, speed and agility. Along with the suits are various high-tech weapons with different uses for different situations. Next, 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.
Much is still left unknown about the room. It appears to be isolated from the rest of the world through a dimensional barrier. Its door will only open under the condition that the mission is completed. According to the manga, there are reports of similar rooms in Germany, United States and Israel. The appearance of another Japanese team set in Osaka indicate that there is more than one of these in Japan.
In addition to the weapons, suits, and scanners, Gantz holds what appears to be a muscle-bound bald man on life support in the center of the ball. The true identity and purpose of this man is never revealed; but there has been no small amount of implication that he is in some way responsible for the running of the game, selecting players, scoring the individual players' performance, and monitoring the team both during missions and during the 'off hours' in between.