Toujou Basara is a high school student whose father has suddenly just remarried. His father then departs overseas leaving Basara with two new beautiful step-sisters. Little does he know, his new sisters, Mio and Maria are actually the new Demon Lord and a succubus!? Almost trapped into a life of servitude, Basara forms a reverse contract by accident and ends up becoming Mio's master. Hijinks ensue as Basara finds himself in one ecchi situation after another. However, Mio's life is in danger as she is pursued by demons and heroes.