The series centers around the titular Debby the Corsifa, the strongest devil in history. Realizing that no demon can possibly match her in a fight, she decides to alleviate her boredom by traveling to Earth and killing some humans. Coming across high school student Sugo Rokurou, she explains that she wants nothing more but a challenge. Rokurou manages to persuade her into playing old maid, and Debby accepts - with the rule that if she wins, she will kill him. However, while playing Rokurou realizes something astounding - while Debby is incredibly strong, she is a borderline idiot when it comes to anything else!