The "rather risque battle action" story centers around Basara Tojo, a first-year high school student who is flustered by a sudden question by his eccentric father, "Hey, didn't you used to say you wanted a little sister?" Plus, his father announces that he is getting remarried. His father brings over two beautiful step-sisters, but then embarks on an overseas trip. However, the two sisters Mio and Maria Naruse are actually a novice devil and succubus. Basara almost ends up forming a dangerous servant and master contract with them, but due to an error, the contract becomes reversed, and Basara becomes the master instead. To add to things, erotic situations continuously start to happen in Basara's life. However, it turns out Mio is having her life targeted by other monsters and heroes. And so, the "desire/action" tale of the strongest contractor begins.