**あらすじ** 魔王な妹が交わした“逆契約”。それは兄の言うことを何でも聞く事!? 「お前、妹が欲しいって言ってたよな」高校生の東城刃更に再婚宣言した父は、義理の妹になる女の子二人を連れて同居させた。と思ったら自分は海外出張へ行ってしまう。澪と万理亜、二人の少女は父が家を離れるや態度を一変させ、刃更を圧倒しようとする。二人の正体は、新米な魔王とサキュバスだった！　だが刃更と主従契約を結ぼうとして誤って逆契約、刃更がマスターの立場になってしまい!?　──最強契約者のディザイア・アクション！ **Synopsis**: “Hey, you said you wanted a little sister, right?” First year high school student, Toujo Basara, was suddenly asked this question by his father, and he fell into a state of panic. On top of that, the eccentric father told him that he will be getting remarried. He then departed overseas after bringing Basara two beautiful step-sisters. But Mio and Maria's true forms are actually a newbie Demon Lord and a succubus!? Basara was almost forced into a master and 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 tribes!! The most powerful contractor's desire action drama begins!