Upon returning to Kaminohara, Ren Homura struggles to recollect his old home. As he picks up his repaired phone, an app named "Monster Strike" captures his attention - even though he didn't install it. Despite his confusion, he contemplates opening it. His life takes a perilous turn when he's attacked by an unknown aggressor and cornered on his school's converted rooftop, which has become a "stage." Astonishingly, the assaulter summons a life-like monster from the app that can physically harm humans. Ren's only hope for survival is by summoning Oragon, a dragon that vows to vanquish the aggressive monster. Lamentably, Oragon, who's minute, unimportant, and completely useless.