When his oddball father suddenly decides to remarry, high school student Toujo Basara is about to acquire some very peculiar new family members. When Basara discovers that one of his step-sisters is a succubus, and his other a recently crowned demon lord, all hell breaks loose! Basara is almost forced into a contract with his demonic sisters where he would act as their eternal servant - but something goes wrong. Instead, a reverse contract is formed, and Basara becomes the master. Between fending off their seductive advances to coping with the attacks of rival demon contractors, Basara has a big learning curve if he is to survive.