"I Fell for My Friend's Older Sister" centers on the romantic entanglements of a high school student, Haruto, who develops feelings for his best friend's older sister, Aoi. Haruto has known Aoi for years, often visiting his friend's home, where he gradually becomes captivated by her kindness, maturity, and beauty. Aoi, a college student, is initially unaware of Haruto's feelings, viewing him more as a younger brother figure. The story explores the complexities of their relationship as Haruto navigates his emotions while trying to maintain his friendship with Aoi's younger brother, who is unaware of his feelings.

The narrative delves into themes of unrequited love, personal growth, and the challenges of pursuing a relationship with someone who is both older and closely tied to a close friend. Haruto's internal struggle is a focal point, as he grapples with whether to confess his feelings or keep them hidden to avoid disrupting the dynamics of his friendships. Aoi, on the other hand, begins to notice Haruto's attentiveness and starts to question her own feelings, leading to moments of introspection and emotional tension.

Supporting characters, including Haruto's classmates and Aoi's peers, add depth to the story by providing contrasting perspectives on love and relationships. The manga balances lighthearted moments with more serious emotional beats, creating a relatable and engaging exploration of young love. The pacing allows for gradual character development, with both Haruto and Aoi evolving as they confront their feelings and the potential consequences of acting on them. The story avoids overly dramatic tropes, instead focusing on the realistic challenges and emotional nuances of its central relationship.