"Shinmai Maou no Testament" centers on Basara Toujou, a high school student whose life takes a dramatic turn when his father suddenly remarries, introducing him to two step-sisters, Mio and Maria Naruse. Shortly after their arrival, Basara discovers that Mio is the daughter of the Demon Lord, and Maria is her succubus servant. The two are on the run from the Demon Realm, as Mio is being targeted by powerful factions seeking to exploit her latent powers. To protect her, Basara forms a master-servant contract with Mio, granting him enhanced abilities but also binding them in a relationship with significant consequences.

As the story progresses, Basara becomes entangled in a conflict between the human world, the Demon Realm, and the Hero Clan, a group of humans dedicated to combating demons. Basara, who is revealed to be the son of a former Hero Clan leader, possesses unique abilities that make him a key figure in the struggle. The series explores the growing bond between Basara, Mio, and Maria, as well as their interactions with other characters, including Yuki Nonaka, a childhood friend and skilled swordswoman, and Zest, a demon warrior who becomes an ally.

The narrative delves into themes of loyalty, trust, and the complexities of relationships, as Basara and his companions navigate a world where alliances are fragile, and enemies are not always who they seem. The series balances action, supernatural elements, and romantic tension, with a focus on the evolving dynamics between the characters. As the stakes escalate, Basara must confront his own heritage and the responsibilities that come with his powers, while Mio grapples with her identity and the legacy of her father. The story builds toward a climactic confrontation that tests the limits of their bonds and resolves key conflicts within the Demon Realm and the Hero Clan.