A twenty-eight-year-old virgin named Atsushi Ueda is at a low point in his life when a phone call changes everything. A middle-school friend has been arrested for murder and asks Atsushi to take custody of his infant daughter, Suzu. Atsushi accepts and raises the girl as his own. He works as a delivery driver, and Suzu grows into a cheerful middle-school student who affectionately calls him Tatan. He never tells her that they are not biologically related.

Fifteen years later, Atsushi still struggles with the secret. His quiet life is disrupted by two female coworkers who both show romantic interest in him, and by Suzu’s natural teenage curiosity about her own origins. Believing her mother might still be alive, Suzu and a friend run away to Osaka, the city where Atsushi grew up, hoping to find answers. During the bus ride home, Suzu encounters a mysterious older man and feels an immediate attraction. That man is in fact her biological father, Seiji Kuzuki, who has recently been released from prison. He too is drawn to Suzu, at first unaware she is his daughter. The story interweaves Atsushi’s fear of losing Suzu, Suzu’s innocent infatuation, and the biological father’s attempts to reconnect.

As Suzu’s feelings deepen, Atsushi grows increasingly anxious. He resolves to tell her the truth before she discovers it on her own. The confession comes on a visit to an amusement park on Suzu’s birthday, where both the biological father and Atsushi are present. Overwhelmed, Suzu runs away and stays with one of Atsushi’s coworkers. After a period of reflection, she confronts her biological father and tells him she cannot forgive him and asks him to stay away. She returns to Atsushi, reaffirming her bond with the man who raised her.

Two years later, Atsushi and Suzu continue their life together, and Atsushi eventually forms a relationship with one of the women from his workplace. The story balances warm family comedy with the painful emotions of secrecy, identity, and unconditional love.