"Nisekoi" centers on Raku Ichijou, the son of a yakuza boss, and Chitoge Kirisaki, the daughter of a rival gang leader. To prevent a gang war, the two are forced into a fake romantic relationship, pretending to be a couple despite their mutual dislike for each other. Raku carries a pendant with a key, a memento from a childhood promise to a girl he vowed to marry. Chitoge, on the other hand, possesses a locket that cannot be opened, hinting at a possible connection to Raku's past.

As the story progresses, other characters are introduced, each with their own ties to Raku's childhood promise. Onodera Kosaki, Raku's classmate and crush, also holds a key that might fit his pendant, suggesting she could be the girl from his past. Tsugumi Seishirou, Chitoge's bodyguard, and Marika Tachibana, a police officer's daughter with feelings for Raku, further complicate the romantic entanglements. Each character's backstory and interactions add layers to the central mystery of who Raku made the promise to years ago.

The narrative blends comedy, romance, and drama, with the fake relationship between Raku and Chitoge serving as the foundation for much of the humor and tension. Over time, their initial animosity evolves into a more complex dynamic, as they navigate their feelings while maintaining the façade of their relationship. The series explores themes of love, identity, and the impact of past promises on present relationships, all while keeping the mystery of the pendant and locket unresolved until later in the story.

The plot is driven by a mix of episodic comedic scenarios and longer arcs that delve into the characters' histories and relationships. The series balances lighthearted moments with emotional depth, as the characters confront their feelings and the implications of their connections to Raku's childhood promise. The unresolved tension surrounding the pendant and locket serves as a central narrative thread, keeping the audience engaged as the story unfolds.