After killing a demon lord in another world after getting summoned. Kazuhira Miller finds himself back to his own world only to be mistreated, and mentally broken again. One day he snaps, and the world suddenly changes. Suddenly the skies show a timer, and after the timer starts the great catastrophe begins. The world suddenly changes, and some humans start mutating into humanoids with animal like features. Monsters start to appear from the ground, and the world changes into something unrecognizable.