"To Love-Ru -Trouble- Darkness" is a romantic comedy and harem anime that serves as a sequel to the original "To Love-Ru" series. The story centers on Rito Yuuki, a high school student whose life becomes increasingly chaotic due to his interactions with various alien girls. The narrative expands on the relationships and conflicts introduced in the earlier series, with a particular focus on the character Momo Belia Deviluke, the younger sister of Lala Deviluke, Rito’s primary love interest.

Momo, who harbors feelings for Rito, devises a plan to create a harem for him, believing it to be the best solution to the romantic tension surrounding him. Her scheme involves encouraging other girls, both human and alien, to pursue Rito, leading to numerous comedic and dramatic situations. The series introduces new characters, such as Nemesis, a shapeshifting assassin with a mysterious past, and Tearju Lunatique, a scientist who shares a connection with Nemesis. These additions deepen the lore and introduce new conflicts, particularly involving the Devilukean royal family and their enemies.

The plot intertwines lighthearted romantic comedy with moments of action and intrigue. Rito’s accidental encounters with the girls, often involving physical comedy and misunderstandings, remain a recurring theme. However, the series also explores more serious elements, such as the political struggles within the Devilukean empire and the moral dilemmas faced by characters like Nemesis. The balance between humor and drama is a key aspect of the narrative, with the harem dynamic serving as both a source of comedy and a framework for character development.

The main characters include Rito Yuuki, whose kind-hearted but indecisive nature drives much of the story’s conflict; Lala Deviluke, the cheerful and inventive alien princess; Momo Belia Deviluke, the scheming yet caring younger sister; and Nana Astar Deviluke, Lala and Momo’s fiery and protective sibling. Other significant characters include Haruna Sairenji, Rito’s childhood crush, and Yui Kotegawa, a classmate who struggles with her feelings for Rito. The interactions between these characters, along with the introduction of new allies and adversaries, create a dynamic and engaging storyline.

"To Love-Ru -Trouble- Darkness" builds on the foundation of its predecessor while introducing new layers of complexity to its characters and plot. The series maintains a focus on romantic entanglements and comedic situations but also incorporates elements of science fiction and action, making it a multifaceted addition to the "To Love-Ru" franchise.