In the deep depths of Hell lives a bloodthirsty demon known as "Debby the Corsifa." She loves a good fight, but not a single entity in Hell can match her in combat. Bored, she decides to blow off steam by wiping out the human race! However, after arriving on Earth, Debby meets an equally bored human named Rokurou Sugo just as he audibly wishes for someone to play games with.

Debby immediately challenges Rokurou to a fight that he cannot imagine winning, let alone live through. Panicking for his life, Rokurou dares her to a game of "Old Maid" as a better—and safer—way to kill time. Despite appearances, Debby proves herself to be an airhead who is only meant for fights! Still, no matter how many times she loses, Debby refuses to give up until she beats Rokurou in a game.