When his oddball father suddenly decides to remarry, high school student Toujo Basara is about to acquire some very peculiar new family members. Enter his new seductive step-sisters, Mio and Maria. 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 and coping with the attacks of rival demon contractors, Basara has a big learning curve if he is to survive.