Setsuna Kashiwagi, a Japanese high schooler born with a failing heart and worn down by bullying, takes a mediator's offer: ten million yen now, his life within the year. The buyers turn out to be two oni princesses, the loud and volatile Kanna and her younger sister Rena, who speaks by writing and fights with spirit swords. The pair move in as his bodyguards, keeping him alive for exactly as long as the contract runs.