"Futakoi" is a romantic comedy anime series centered around the life of Rentarou Futami, a high school student who returns to his hometown after many years away. Upon his arrival, he reconnects with his childhood friends, the twin sisters Souju and Souba Shirogane, who run a detective agency. The series explores Rentarou's interactions with the twins and other characters, blending lighthearted humor with moments of emotional depth.

The narrative primarily revolves around Rentarou's relationships with the Shirogane sisters, who are polar opposites in personality. Souju is cheerful, energetic, and often impulsive, while Souba is calm, reserved, and more analytical. Their contrasting traits create a dynamic interplay that drives much of the series' comedic and dramatic tension. As Rentarou becomes involved in their detective work, he finds himself entangled in various cases, ranging from trivial mysteries to more significant personal dilemmas.

Throughout the series, Rentarou also encounters other twins in the town, each with their own unique quirks and challenges. These encounters further complicate his relationships and add layers to the story, as he navigates the complexities of friendship, rivalry, and budding romantic feelings. The series balances episodic detective cases with an overarching exploration of the characters' growth and connections.

Themes of identity, duality, and the bonds between siblings are prominent, as the twins' relationships with each other and with Rentarou are central to the plot. The series maintains a lighthearted tone overall, with occasional moments of introspection and emotional resonance. The interplay between the characters, particularly the twins' contrasting personalities, provides a steady source of humor and drama, making "Futakoi" a blend of romance, comedy, and mystery.