Sachi works as a physician at the Amamiya clinic, living a quiet professional life until his childhood friend Kengo, a yakuza, suddenly appears with an unexpected request. Kengo asks Sachi to help raise a child named Mushi, whose birth had been kept a secret. For the child's sake, Sachi agrees to a startling arrangement: pretending to be Kengo's lover and living together as a family. Despite his stated hatred for Kengo's yakuza lifestyle, Sachi finds himself participating in domestic activities like shared baths and trips to the amusement park, gradually strengthening the bond between the three of them. This creates a complicated living situation where Sachi, a beautiful and tsundere physician who dotes on the child, navigates his feelings for the egotistical yakuza.

The story continues as Sachi and Kengo's feelings become mutual, leading them to start a new life together with the child, now named Fumi. The family faces external challenges when Fumi gets into a quarrel at nursery school. Further tension arises with the appearance of a beautiful woman named Misuzu, who is close to Kengo. Her presence forces Sachi to question his role in the family and confront his own deep-seated insecurities about what he means to both Kengo and Fumi. The stakes escalate dramatically when a Sicilian mafia man named Gino appears, claiming to be Fumi's uncle and demanding to take the child away. Sachi fiercely opposes this, even as he worries about the fact that Kengo is not Fumi's biological father, forcing the trio to consider what they must do to remain a family forever. Later, a beautiful person in traditional Japanese clothing named Tsukasa of the Sanohara-gumi arrives at their house seeking refuge, bringing a baby with her. The household is thrown into further chaos when Inukai, a lawyer obsessed with Tsukasa, appears, leading to the baby being caught in the commotion. Through these events, the family continues to grapple with their unconventional dynamics and what it truly means to be together.