Yuma is a smart uni student whose grandfather is a magician. One day, he gets cornered by gangs, and he calls up the devils by using his grandfather's ring. The devil who appeared is called Lieselotte. She gets rid of the gangs quickly, saving Yuma, and says to him, "Give me your life in return." If he proves that he is a real summoner in ten days, he will be saved. Lieselotte treats him roughly, but there is also one surprise: she needs his semen to refill her magic power.