Junior high school student Ren Homura and his friends are a team in Monster Strike. They combine the forces of different monsters and go to battle against their foes, but for some reason, none of them can remember a certain time in their past. One day, a monster appears from beyond space and time and sends one of them back. Ren and the others start to regain their memory as this incident unfolds. They remember a time when Monster Strike was a highly-kept secret, a time when only they, as children, were allowed to play it. The hidden truth of Monster Strike comes to light. And with it, the lost memories of a childhood adventure.