Ai Miyano is what everyone considers a good woman. She is unaffected, does not form cliques, and carries a delicate, ephemeral quality that draws men to her, though none can truly possess her. The story follows this single woman and the four men whose romantic threads become intertwined with hers. These men include a friends-with-benefits, a childhood friend, a friend she quarrels with, and another whose role is initially more mysterious.

The narrative is set in contemporary Japan and primarily follows Ai, a 26-year-old working at a beer company. At the start, she becomes involved with her company colleague, Sōsuke Suō, in a relationship that begins casually but deepens as he falls genuinely in love with her and considers marriage. Despite this, Ai remains emotionally distant. Her heart is secretly occupied by someone else: her younger brother, Suito Miyano. Suito also struggles with feelings for his sister beyond what is appropriate, and his desire to protect her gradually transforms from a brotherly instinct into the love of a man.

As Ai navigates her relationship with the earnest and passionate Sōsuke, her long-time childhood friend Wataru Tokishima, who has always loved her quietly, and the mysterious bartender Takao Asagi who knows her past, her feelings for her brother become impossible to ignore. The central conflict emerges from this forbidden love. The story explores why Ai keeps people at a distance, which is rooted in a family secret involving her parents that is eventually revealed. Learning this truth solidifies Suito’s resolve and forces Ai to confront what she truly needs.

The narrative arc traces Ai’s journey from superficial relationships to a deep, taboo emotional connection. The story moves through her initial arrangement with Sōsuke, her growing recognition of her feelings for Suito, the revelation of their family's past, and the ultimate confrontation with their forbidden love. After much struggle, Suito confesses his feelings, and Ai accepts them. The manga concludes with Ai choosing her real brother, Suito, and the two decide to live together, overcoming the social taboo for the sake of their bond. The series, which concluded with 11 volumes, explores themes of whether to love is a sin and contrasts the fleeting nature of a moment with the concept of eternal love.