From the girl who had surely commit suicide, came a death game. "On the other side of this door is the colosseum where you will kill each other" When the girl woke up, before her eyes was a space she had never seen before. Given a handgun and a knife, they were told to kill each other. A mercilessly planned game of death. Of the 30 people gathered, only 1 was allowed to remain--. When high schooler Hagiwara Yuuto picked up an app called 'Collesseum', he came to know the start of the death game 'Collesseum' as well as that central class figure who had surely commit suicide, Tsukishima Io, was participating. Knowing he can support her from outside the game, Yuuto, along with his classmates try to clear the game, however... the common point among all 30 participants, relays the terrible truth to Yuuto and co.