When a pair of sexy girls, Maria and Mio, come to live with high school student Basara Toujou, his life changes forever. The two girls have a secret that they don't keep for long--Maria is a succubus, and Mio is a future Demon Lord. But Basara has a secret of his own, he's the last descendant from a clan of demon-fighting heroes. To make things even more complicated, the girls force Basara into a master-servant contract, and Basara ends up becoming their master! What happens when you put a succubus, a demoness, and a demon-fighting hero under the same roof. They form a bond that will help them to take on anything.