High school first-year Mana was six years old when she lost her parents, and the man who took her in and raised her ever since is Kazuki, a man with no blood relation to her. The two have always lived alone together, and Mana has grown up viewing Kazuki simply as her father. But as she enters adolescence, her feelings for him begin to shift from filial affection to romantic attraction, and she starts to see the man behind the parent. The story follows Mana as she wrestles with these new emotions, unsure whether Kazuki could ever see her as anything other than a daughter, while Kazuki must confront his own changing perception of the girl he raised.

The narrative tracks the gradual transformation of their relationship, with Mana eventually confessing her love. Kazuki accepts her feelings but sets a boundary: he wants to remain a father figure until she turns twenty, insisting that she have time to grow and be certain of her own heart. The pair continue living together under this tense, unspoken promise, struggling to balance their lingering parental dynamic with the undeniable romantic tension that now exists between them.

Later story arcs, collected under the sequel Papa ga Otoko ni Kawaru Toki II, follow Mana now eighteen and nearing the agreed-upon age. As she navigates college life, a brief stint as a local celebrity from a television commercial complicates matters, and a fellow student named Shirai enters the picture, stirring jealousy in Kazuki. The sequel delves deeper into the push and pull between Kazuki’s protective instincts and his growing desire to treat Mana as a woman, culminating in Mana’s suggestion that they spend a day acting like a genuine couple to ease his confusion. The series explores the delicate line between familial duty and romantic intimacy, set against the everyday backdrop of home life, school, and social obligations.