"Koikimo" centers on the unconventional romantic dynamic between Ichika Arima, a high school student, and Ryou Amakusa, a 27-year-old businessman. The narrative begins when Ryou, after a drunken night, is cared for by Ichika, the younger sister of his close friend. This encounter sparks Ryou's interest in Ichika, leading him to pursue her romantically despite their significant age gap and societal expectations. Ichika, initially resistant and uncomfortable with Ryou's advances, gradually finds herself drawn into his persistent yet sincere efforts to win her affection.

The series explores the complexities of their relationship, highlighting the challenges posed by their differing life stages and the scrutiny they face from those around them. Ryou's confident and sometimes overbearing personality contrasts with Ichika's reserved and cautious nature, creating a dynamic that evolves as they navigate their feelings. Supporting characters, such as Ichika's protective older sister Rio and Ryou's colleagues, add depth to the story by providing varying perspectives on the relationship.

Throughout the series, themes of maturity, societal judgment, and the boundaries of love are examined. The narrative balances moments of lighthearted humor with more serious reflections on the implications of their unconventional bond. As the story progresses, both characters confront their own insecurities and societal pressures, leading to a gradual development of mutual understanding and emotional connection. The series concludes with an open-ended resolution, leaving the future of their relationship to the audience's interpretation.