Ruka just lives a boring life. He is an ordinary student, but thinks that he wants to change the world. One day, a devil appears in front of him, "I'll make your wish come true." Her name is Mephisto. Though she has a big power, a half of it has been confiscated by the authorities. To get it back, Ruka makes a contract with her. At the same time, the authorities decide to send seven battle maidens to eliminate Mephisto...