Kei Kurono is a perverted and very to his own high school student. After a day of fairly normalcy he bumps into an old friend at the train station, one who he still finds slightly irritating. His old friend Masaru Kato was always the better looking and got the girls, but Kato always looked up to Kei no matter what wanting to become just like him. As they exchange a few words a homeless man stumbles onto a set of train tracks right in front of them. Kato leaps down first and tries to free the homeless man by himself but soon is unable to and the train is coming up quickly. Kei not wanting to made look the fool or coward an irritated that Kato is able to risk his own life for someone he never knew leaps down an attempts to help. They soon are able to boost the homeless drunk up into safety but soon realize they themselves are not able to get off the tracks. They attempt to make a run for it but soon are sucked into the train an decapitated, as their heads fly through the air Kei comments at how stupid Kato looks just before he closes his eyes. Seconds later they awake in a small room among other individuals who have also just "Died".
They soon find out that if they want to return to normalcy they must participate in a game called Gantz or (Gantsu), and in participating they must kill their target which is a Onion alien. But soon they find out the game is much more than simply finding their target, and that this "Game" is very much real.