Running into your new stepsister in the bathroom is not the best way to make a good first impression, which Basara Toujou learns the hard way. When his father suddenly brings home two beautiful girls and introduces them as his new siblings, he has no choice but to accept into his family the Naruse sisters: busty redhead Mio and petite silver-haired Maria. But when these seemingly normal girls reveal themselves as demons—Mio the former Demon Lord's only daughter and Maria her trusted succubus servant—Basara is forced to reveal himself as a former member of a clan of "Heroes," sworn enemies of the demons. However, having begun to care for his new sisters, Basara instead decides to protect them with his powers and forms a master-servant contract with Mio to keep watch over her. With the Heroes observing his every move and the constant threat of hostile demons, Basara has to do the impossible to protect his new family members.

When a pair of sexy girls, Maria and Mio, come to live with high school student Basara Toujou, his life changes forever. 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!

It centers on Basara Toujou , a seemingly normal boy who encounters his new family members Mio and Maria under the pretense of his father's "rearranged marriage" with their mother. In reality, however, the "sisters" are actually demons who escaped from the Demon Realm after Wilbert 's passing. In order to protect his "stepsisters" who he cherishes, Basara must do whatever it takes to protect them from harm, and the people after Mio's powers. The first season of the series that primarily focuses onto the first three Light Novel Volumes and three major story arcs: First part is about an introduction of the Naruse Sisters into Basara Household as the "new family members" in order to seek asylum after Wilbert's demise.