High school student Toujo Basara's ordinary life is upended when his father suddenly returns from a trip abroad with two new step-sisters. The girls, Mio and Maria Naruse, are stunningly beautiful, but they harbor a dangerous secret: they are members of a demon clan. Mio is a powerful Demon Lord and the heiress to the throne of the underworld, while Maria is her devoted and mischievous succubus servant.

The fragile peace of this new family is shattered when demonic rivals begin attacking, seeking Mio's inherited power. To protect himself and his new sisters, Basara enters into a magical Master-Servant contract with Mio. However, due to Maria's intervention, the spell backfires, making Basara the master and the powerful demon lord his servant. This contract comes with an unintended and lustful side effect, frequently forcing Basara and Mio into embarrassingly intimate situations that test the limits of their new relationship.

As the series progresses, Basara's household expands beyond the original trio. He is joined by his heroic childhood friends and even a demonic servant named Zest, creating a chaotic but protective harem around the demon princess. The narrative focuses on their daily lives, which are a turbulent mix of ecchi scenarios engineered by Maria, training sessions, and battles against other demons who covet Mio's growing power. The story builds toward a critical climax as Mio's demon lord power becomes so potent that it threatens to force her to live in the underworld permanently, putting her relationship with Basara and their life on Earth to the ultimate test. The Testament of Sister New Devil Storm functions as a mature companion series to the main Testament of Sister New Devil storyline, offering a parallel take focused heavily on passionate antics and fan service.