Shinmai Maou no Testament is an anime series that follows the story of Basara Toujou, a high school student who unexpectedly becomes the step-brother to two demon girls, Mio and Maria. As a member of the Hero Clan, Basara must navigate his new life while protecting his sisters from various supernatural threats. The series combines elements of fantasy, action, and romance, creating an engaging narrative filled with magical battles and complex character dynamics.