Manga: Somewhere in Tokyo, there is a room. In that room is a black sphere. Periodically, people who should otherwise have died are transferred to the room. There, the sphere gives them special suits and weapons, and sends them out on a mission to kill aliens here on Earth. While these missions take place, the rest of the world is largely oblivious to them. These missions are lethal - few participants survive them. The sphere calls the shots, and it's not the slightest bit nice. Its name... Gantz.

Anime: Kei Kurono is a typical high school student in a selfish world, not caring about anyone other than himself. However when, by chance, he was forced to help out his childhood friend, Katou Masaru, rescue a drunkard who fell into subway tracks, both he and Katou were killed... Or not. After being run over by the train, they were suddenly teleported to an enclosed apartment with others who recently died. And now their lives are controlled by a mysterious black ball inside the apartment called GANTZ, and they are forced to participate in a "game" of unprecedented danger and horror.

(source: ANN)