Renko, Tomoko Tabata, is an 11-year-old girl caught in a family storm as her parents edge toward divorce. Her mother Nazuna, Junko Sakurada, is an upfront woman who longs to break free from family obligations, while her father Ken'ichi, Kiichi Nakai, is an affectionate but often distracted office worker; both frequently neglect Renko and wrestle with alcohol. Ken'ichi's brother Yukio, Taro Tanaka, and his wife Wakako, Mariko Sudo, occasionally try to steady the fragile household.
After Ken'ichi completes a move to a new apartment, Nazuna takes Renko out to eat and lets herself go, a rare moment of loosened control. To restore a sense of normalcy, Nazuna drafts multiple pacts detailing how daily chores should be shared, but these agreements mostly drift into oblivion thanks to Nazuna's carelessness.
One evening, Renko discovers a divorce paper her mother has yet to file, a finding that unsettles her and sparks a cascade of misfortunes. Renko tries to run away but is blocked by extended family, and a confrontation between Nazuna and Ken'ichi erupts. She ends up in trouble at school after striking a classmate during a quarrel with a girl whose parents are also separated. She later makes up with both parents, yet the pattern of misbehavior persists, including a blaze in the school lab.
To coax the family toward a moment of togetherness, the parents are coaxed to attend the August fireworks by Lake Biwa. Renko runs away again and spends the day with an elderly couple who take her to the festival. From a hill, Ken'ichi watches the fireworks, tempted to drink from his flask, while Nazuna finally weaves through a packed bridge to locate her daughter.
Just when reconciliation seems possible, Renko heads toward the mainland, where festival-goers burn effigies by lifting and lighting hay bales. By morning the festival is over and Renko wanders the woods, by a river, howling at the moon, sliding through fallen leaves. In a dream-like sequence, she wades through the lake and encounters doppelgängers of herself and her family, who tow a ceremonial boat with help from some young men. A fire erupts on the boat, distressing Renko's double and causing her parents to fall silent. The men vanish and her parents dive underwater as the boats multiply and catch fire. The doppelgängers cling to Renko, hug her, and take her place as they push forward in the water, continuously shouting, > "Congratulations!"
Returning to the riverbank to warm by the fire, Nazuna arrives with an unusually cheerful demeanor. The trio boards a train home, and Renko finally hands back the crumpled divorce certificate to her mother. The Lake Biwa holiday becomes the central subject of her school essay, which she reads aloud, ending with the notion that her parents have a new contract. Renko then leaves class to join the street celebration outside the school, greeting Wakako's family and newborn, offering a flower to her mother, and setting down her father's wine glass. As a passerby asks where she’s heading, Renko answers simply: “the future.”
