So the story is.. two guys died in a train accident and when they regain consciousness, they found themselves in a room with this big black sphere (Gantz) together with some other people who are also supposed to be dead. These people are part of a semi-posthumous "game" in which they are forced to kill aliens.
Mind-blowing effects and there's a fair bit of disgusting gore in the first half an hour of the movie. Some nudity too, which in fact was toned down a lot in the movie version.
This movie is more like an introduction to the whole concept of Gantz and also the 'rules' of the game. There are some stuff which were left unanswered, hopefully it'll get answered in its second part, Gantz - Perfect Answer - which should be showing by end of this year.