After Kei and his childhood friend, Kato, are torn into bloody pieces by a speeding train, they awake in an unfamiliar room, surrounded by strangers. In the middle of the room is a bizarre black sphere: the Gantz.

The Gantz opens, revealing unusual weaponry and odd suits. The strangers are sent into the city, ordered to kill an alien that roams the streets - and then the carnage begins.

With the small alien dead, the large alien exacts his revenge, slaughtering many with the group. When the creature pursues Kei, the young man quickly discovers that his physical abilities are enhanced by the suit he wears.

The scores are tallied and Nishi explains some of what he knows about the rules of the strange world of the Gantz. The unusual situation grows even odder the next day when Kei wakes - and finds that this nightmare is no dream.

Seemingly back among the living, Kato must protect his brother from their hateful aunt. Meanwhile, Kei’s special suit gives him an advantage against the bullies at school. But will it help him when Kishimoto appears at his door?

Despite his best efforts, Kei remains unsatisfied during Kishimoto’s overnight stay at his apartment. At the place Kato calls home, more ugly evidence of abuse piles up.