College students Shinya Otani and Miyako Tachibana have been inseparable since high school and now share a small rented apartment in the city. They keep their romantic relationship a secret to avoid unnecessary complications, living together in a state of quiet happiness. Their delicate balance is disrupted when they meet Yukiko Mizusawa, an old friend from high school. While Tachibana welcomes her warmly, Otani grows uneasy, haunted by old rumors that Tachibana and Mizusawa once dated. His jealousy intensifies as he perceives Mizusawa as perfect and becomes convinced she is in love with Tachibana. The situation reaches a breaking point when Mizusawa gives Tachibana a perfume called Jealousy for his birthday, which is the same scent she wears. Consumed by doubt, Otani becomes cold and hostile toward his lover. A series of misunderstandings leads to a heated argument, and the couple decides to separate temporarily.

Otani seeks refuge with a college friend, Shougo Sunochi, who is navigating his own long-distance relationship with an American named Alfred. Meanwhile, Tachibana attempts to reconcile and sends Otani a text message asking to talk. Otani never receives the message because his phone stops working after getting wet. Believing he has been ignored, Tachibana sinks into depression. Observing the growing rift, Mizusawa finally gathers the courage to confess her true feelings to Otani. She reveals that it was always him she loved, not Tachibana. In that moment, she realizes the truth: Otani and Tachibana are a couple. With the misunderstanding cleared, Otani and Tachibana reconcile. Feeling remorseful for his behavior, Otani invites Mizusawa to join them for fireworks at their university. She accepts that her feelings for Otani will never be returned but finds peace in knowing she has always enjoyed seeing them happy together. The story concludes with Otani and Tachibana reunited as a couple, with Otani hoping that one day his words of devotion will truly reach Tachibana.

A sequel, Ai no Kotodama: Sekai no Hate Made, shifts focus to new characters Hirofumi Sasaki and Shinji Tamura, with Yasuka Sait reprising his role as Miyako Tachibana in a supporting capacity. The sequel follows Sasaki and Tamura as they navigate their relationship, with Tachibana appearing as an acquaintance who offers guidance to the younger couple.