Basara Tojo is a reclusive high school student who now has to share his household with an attractive teen girl and her precocious little sister, both of whom were taken in by his father. One morning, Basara discovers that the girls are actually demons intent on killing his father and that he has an unanticipated fate as well.; In order to allow Basara to know where Mio is at all times in order to protect her, Maria proposes casting a supernatural Master-Servant spell between him and Mio. But she casts the spell wrong and Mio ends up being the servant in the pact.; Using powers he didn't know he had, Basara drives Mio and Maria from the house. Later learning that his father was trying to protect them, Basara searches for them and finds them under attack from rival demons.; A childhood friend from the demon hunting clan Basara left shows up at school, looking to rekindle her friendship with Basara, and also hunting Mio.; Threatened by Yuki's closeness to Basara, Maria conspires to use Mio's attractiveness as a tool to seduce Basara back to their side. Meanwhile rival demons argue over who should be the one to destroy Mio and seize her hereditary throne.; Maria can't resist spreading defamatory stories about Basara's sexual habits.; While doing laundry, Maria tries modeling Mio's bra.