The setup promises trashy harem chaos, then immediately inverts it: Basara isn't the desperate loser chasing girls, he's the one holding all the actual power, and the show knows this flips the entire energy of the genre. Maria and Mio aren't prizes to be won. They're the ones trying to figure out what having a master actually means when the master refuses to use them. The contract binds them together in ways neither side fully understands, turning every ecchi moment into something that actually costs someone something emotionally. It's still wish-fulfillment nonsense wrapped in demonic lore, but it's wish-fulfillment that lets you watch two girls slowly realize they're not trapped, they're choosing to stay.

Basara Toujou is a high school student whose father suddenly remarries, then heads overseas and leaves him living with two new stepsisters, Mio and Maria Naruse. The girls soon reveal they are demons: Mio is the only daughter of the former Demon Lord and heir to the title, and Maria is her succubus servant. Basara has a secret too, since he once belonged to a clan of Heroes sworn to fight demons. When the girls try to bind him as their servant, the contract flips by accident and Basara becomes Mio's master instead. With demons and Heroes alike pursuing Mio, he chooses to use his powers to protect his new family. Expect action and fantasy battles mixed with romance and plenty of ecchi mishaps.