"Hey, you said you wanted a little sister, right?" High school freshman, Basara Tojo, was suddenly asked this question by his father who tells him that he's getting remarried. He then departed overseas after bringing Basara two beautiful step-sisters; Mio and Maria. But the true forms of the two girl's are actually a newbie Demon Lord and a succubus! Basara was almost forced into a master-servant contract with Mio, but a "reversed" contract was formed by mistake, and Basara is now the master. Moreover, Basara is being hit by ecchi situations one after another due to the contract, but Mio's life is being pursued by other demon tribes and hero clan!! The most powerful contractor's desire action drama begins!