A married woman named Aoi, who has been married for three years, feels lonely due to her husband's distant attitude compared to his initial kindness. While shopping for dinner, burdened with her troubles, she unexpectedly reunites with Hosoda, a classmate from her student days. Hosoda, now working at a swimming school, invites Aoi to visit, and she decides to attend the classes. As Aoi continues to go to the swimming school, she begins to be drawn to Hosoda's kindness, which encourages her worries about her husband. On the day her dissatisfaction with her husband explodes, Aoi crosses the line with Hosoda...
