Ren Homura has recently returned to his hometown of Kaminohara, yet he struggles to recall much about his time there, aside from vague flashbacks. Upon retrieving his cell phone from a repair shop, he discovers that a mysterious game titled "Monster Strike" has been installed on it. Unaware of its purpose or how it arrived there, Ren decides to open it, an action that will change his life forever as he finds himself suddenly attacked by a strange man. Now trapped on the roof of his school, which has strangely been designated as a "stage," Ren is shocked to see the man summon a realistic monster from the game, one more than capable of inflicting damage on a human. His only chance of surviving this situation is by summoning the dragon Oragon, who tells Ren that he will crush the opposing monster for him. Unfortunately, it turns out that Oragon is a tiny dragon that is completely useless!