"Romantic Killer" centers on Anzu Hoshino, a high school student who prioritizes video games, chocolate, and her cat over romantic relationships. Her life takes an unexpected turn when a magical being named Riri appears, claiming to be a "romance guide" tasked with helping her find love. Riri confiscates Anzu’s beloved possessions—her games, snacks, and even her cat—forcing her to confront real-life romantic scenarios she has long avoided.

Anzu is thrust into interactions with three potential love interests: Tsukasa Kazuki, a popular and kind-hearted classmate; Hijiri Koganei, a childhood friend with a protective streak; and Junta Hayami, a quiet and mysterious transfer student. Despite her initial resistance, Anzu gradually becomes entangled in their lives, navigating misunderstandings, rivalries, and unexpected feelings. The story balances comedic moments with heartfelt developments as Anzu’s relationships evolve.

Riri’s interventions often lead to chaotic and humorous situations, but they also push Anzu to reflect on her own emotions and the importance of human connections. The narrative explores themes of self-discovery, friendship, and the complexities of young love, blending lighthearted humor with moments of genuine emotional depth. The characters’ personalities and dynamics drive the plot, creating a mix of romantic tension and comedic misadventures as Anzu reluctantly steps into the world of romance.