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 to coping with the attacks of rival demon contractors, Basara has a big learning curve if he is to survive.