"To Love-Ru: Trouble - Darkness OVA" is a continuation of the "To Love-Ru" series, specifically expanding on the events of the "Darkness" storyline. The OVA focuses on the romantic and comedic misadventures surrounding Rito Yuuki, a high school student whose life becomes increasingly chaotic due to his interactions with various alien girls. The central narrative revolves around Rito’s accidental engagement to Lala Satalin Deviluke, a princess from the planet Deviluke, and the subsequent complications that arise as other girls, both human and alien, develop feelings for him.

The OVA delves deeper into the dynamics between Rito and the female characters, particularly Momo Belia Deviluke, Lala’s younger sister, who actively schemes to create a harem for Rito. Momo’s efforts to bring Rito closer to multiple girls, including Yami, Nana, and others, drive much of the comedic and romantic tension. Yami, a former assassin, struggles with her growing affection for Rito, while Nana, Lala’s other sister, remains fiercely protective of her family and wary of Rito’s intentions. The OVA also explores the relationships with human characters such as Haruna Sairenji, Rito’s long-time crush, and Run Elise Jewelria, a childhood friend who harbors her own feelings for him.

The plot is episodic, with each segment focusing on humorous or romantic scenarios involving Rito and the girls. These scenarios often involve misunderstandings, accidental encounters, and fantastical elements, such as alien technology or magical transformations, which heighten the comedic and romantic stakes. The OVA maintains a lighthearted tone while occasionally touching on themes of love, loyalty, and self-discovery. The interactions between the characters are central to the narrative, with Rito’s indecisiveness and the girls’ competing affections creating a recurring source of conflict and humor.

The OVA also introduces new elements to the "Darkness" arc, such as the ongoing mystery surrounding the Darkness transformation, which affects Yami and other characters. This adds a layer of intrigue to the otherwise comedic and romantic focus of the series. The OVA serves as a bridge between the main "Darkness" storyline and its subsequent developments, providing additional context and character interactions that enrich the overarching narrative. Overall, "To Love-Ru: Trouble - Darkness OVA" continues the series’ tradition of blending romantic comedy with sci-fi elements, offering fans further insight into the relationships and dynamics of its central characters.