High school student Kato Masaru (voiced by Daisuke Ono) and his younger brother live a life of dependence. One day, the subway station where Kato Masaru is located was attacked by indiscriminate murderers. In order to save people, Kato Masaru was stabbed several times and died. When he opened his eyes again, he found himself standing in a strange room intact, with a black strange ball in front of him. With Kato Masaru, there are also three people, Shimohira Reika (voiced by Saori Hayami), Nishi Joichiro (voiced by Tomohiro Kuo) and Suzuki Ryoichi (voiced by Shuichi Ikeda). Like Kato Masaru, they are all dead. Reika told the confused Kato Masaru that the black ball is called "GANTZ", and what they have to do is to wait for the order from the ball. This time, the four were teleported to Osaka, and what they have to face is a monster called "Slipper" and a grand night parade of one hundred demons. Only by killing all the monsters within the time specified by GANTZ can Kato Masaru get the opportunity to return to his brother again.