The Yokoyama family reunites annually to honor the memory of their eldest son, Junpei, who tragically drowned 12 years ago while rescuing another boy. This poignant gathering includes Kyohei, Junpei's retired doctor father, and his mother, Toshiko, along with their surviving son Ryota, who has recently married a widow named Yukari and has taken on the role of a stepfather to her young son, Atsushi. They are also accompanied by their daughter Chinami, her spouse, and their children. Ryota grapples with feelings of resentment stemming from the lingering shadow of his brother, who was clearly their parents' favorite. Junpei's belongings remain untouched, a constant reminder of his parents' affection for him, particularly affecting Kyohei, who is still consumed by grief. Disappointed that Ryota chose a path in art restoration instead of embracing the family tradition of medicine, Kyohei’s bitterness adds to the family tension.
As the family gathers, Kyohei and Toshiko are initially frosty towards Yukari, harboring superstitions associated with Ryota marrying a widow. However, they gradually begin to appreciate her kindness and make efforts to connect with the shy Atsushi. Throughout the day, the family reminisces, sharing stories about their lives and careers. Yet, underneath the surface, Ryota privately struggles to reveal his difficulties finding stable work in his chosen field. When Ryota finds out Kyohei is advising Atsushi to pursue medicine, he attempts to confront his father, but Kyohei dismisses his concerns.
An unexpected visitor, Yoshio, the boy whom Junpei saved, arrives later that evening. The family’s reaction is mixed — many feel resentment towards Yoshio, who has grown into a rather aimless adult, believing he should have paid the ultimate price for his actions, while Ryota empathizes, feeling a familiar sting of inferiority. He pleads with Toshiko to cease inviting Yoshio over, arguing that her insistence on blaming him for Junpei's death only perpetuates their shared sorrow. Toshiko, however, insists that holding on to someone to blame helps her cope with her son's loss.
As Chinami and her family depart, the remaining members gather for dinner. Yukari’s inquiries into Toshiko and Kyohei’s marriage surface long-buried resentments and disappointments, yet their bond has endured through the years. A poignant moment arises when Toshiko shares their romantic song, Blue Light Yokohama by Ayumi Ishida, revealing that she was once haunted by jealousy when she heard Kyohei singing it with another woman. This flash of vulnerability illustrates the depth of their relationship struggles.
That night, a butterfly mysteriously enters the home and lands on Junpei's photograph, prompting Toshiko to believe it symbolizes her lost son. Although Ryota approaches this notion with skepticism, he captures the butterfly and releases it outside as a tribute. Toshiko urges Ryota to start a family with Yukari soon, advising that a child could fortify their union against potential divorce.
The next morning, Ryota, Yukari, and Atsushi bid farewell. While Kyohei and Toshiko express eagerness to see them again for the New Year, Ryota and Yukari privately resolve to forgo the visit. As they drive away, Ryota reflects in voiceover that Kyohei passed away a few years later, followed by Toshiko. Their family dynamics shift without Chinami moving in or Ryota fulfilling the promises made during the reunion. The film concludes with Ryota's family, now including a young daughter, paying respects at the Yokoyama family graves, embodying the rituals and traditions he learned from his mother.
