"The Testament of Sister New Devil Departures" is a two-episode OVA series that serves as a continuation of the original anime adaptation of the light novel series by Tetsuo Uesu. The story picks up after the events of the second season, "The Testament of Sister New Devil BURST," and delves deeper into the relationships and conflicts surrounding the main characters.

Basara Toujou, a high school student, continues to navigate his complicated life as a human living with supernatural beings. His household includes Mio Naruse, a half-demon girl who is the daughter of the previous Demon Lord, and Maria Naruse, her succubus maid. Basara is bound to Mio and Maria through a master-servant contract, which grants him control over their powers but also creates a deep emotional connection between them. The OVA explores the dynamics of their relationships, particularly the growing romantic tension between Basara and Mio, as well as his protective instincts toward both girls.

The plot revolves around a new threat that emerges, forcing Basara and his companions to confront their pasts and the lingering consequences of their actions. The story introduces additional challenges, including the reappearance of old enemies and the revelation of hidden truths about the demon world. As Basara and his allies prepare for battle, the narrative emphasizes themes of loyalty, sacrifice, and the bonds that tie them together.

Key characters include Basara, whose determination to protect those he cares for drives much of the story; Mio, who struggles with her identity as a half-demon and her feelings for Basara; and Maria, whose playful demeanor masks a deep sense of responsibility toward Mio. Other supporting characters, such as Yuki Nonaka, a childhood friend and skilled swordswoman, and Kurumi Nonaka, her younger sister, also play significant roles in the unfolding events.

The OVA maintains the blend of action, fantasy, and ecchi elements present in the original series, while further developing the characters and their relationships. It provides closure to certain story arcs while leaving room for potential future developments, making it a fitting addition to the franchise for fans of the series.