Toshio [Kanji Furutachi] and his wife Akie [Mariko Tsutsui] live with their daughter Hotaru [Momone Shinokawa] in a quiet, banal existence, with Toshio running a small machine shop in his garage. Life unfolds with an unremarkable rhythm until an old acquaintance, Yasaka [Tadanobu Asano], who has just been released from prison, arrives to help out and ends up staying as a kind of informal assistant.
Yasaka quickly forms a bond with Hotaru, guiding her through her early attempts at playing the harmonium and becoming a more visible presence around the house and the shop. Over time, Yasaka reveals to Akie that he spent eleven years behind bars for murder, asks for forgiveness for not mentioning this earlier, and his presence gradually shifts from outsider to a central figure in the family’s daily life. There is a growing undercurrent of attraction between Yasaka and Akie, and they share moments of closeness that hint at deeper romantic feelings, punctuated by a kiss on several occasions.
Toshio grows suspicious as the family undertakes a river trip, and the film gradually discloses that Yasaka bitterly bears the responsibility for the crime whose blame Toshio partly shared. A striking visual turn occurs when Toshio removes his white clothing to reveal a red shirt underneath, a mark that signals a darker turn for the remainder of the story.
When Toshio is away, Yasaka oversteps boundaries by attempting to have sex with Akie, who firmly pushes him away. He then goes for a walk and encounters Hotaru. Later, Toshio discovers Hotaru injured and unresponsive, with Yasaka standing over her. Yasaka shouts Toshio’s name repeatedly, but he cannot respond as he is absorbed in his daughter’s condition. Yasaka exits and disappears.
Eight years pass, and Toshio hires a new assistant, Takashi [Taiga], a young man raised by a single mother who shows interest in the now-disabled, non-verbal Hotaru and sketches her. Akie becomes fiercely protective of Hotaru. Takashi reveals to both Toshio and Akie that he is Yasaka’s illegitimate son and that Yasaka was a yakuza member, explaining that he sought employment with Toshio because of letters Yasaka had sent to his mother before his disappearance. After Takashi stands too close to Hotaru, Akie ejects him from the house.
Akie then confronts Toshio about his involvement in the murder, and he admits that he helped restrain the victim while Yasaka strangled him. Later, Toshio receives a tip about Yasaka’s location, and he and Akie, along with Takashi, pursue a plan to kill Yasaka in front of his father. Takashi offers himself willingly, but Toshio and Akie reject the notion. The tip proves false, as the person they confront only vaguely resembles Yasaka.
Having reached her breaking point, Akie takes Hotaru to a bridge. After an image of Yasaka appears on the bridge and seems to smile, Akie and Hotaru leap into the water. Toshio rescues an unconscious Akie, while Takashi appears to pull Hotaru from the river — both of them unconscious along the riverbank. Akie regains consciousness, and Toshio desperately attempts to resuscitate Takashi, then Hotaru. The film ends with Toshio continuing to try to revive Hotaru, leaving both her fate and Takashi’s ambiguous and unresolved.
