Tokyo, 1998. Sisters Keiko and Sumiko learn of their father's death through a message on an answering machine. The weather is abnormal and the winter days continue. Keiko's mental state deteriorates in response, and she is finally hospitalised. Her sister Sumiko, lonely and anxious, begins to use sleeping pills regularly. Yuko, a female friend who had an abortion, left for the south after the man she had been having an affair with committed suicide. In addition, her sister, who has escaped from the hospital, jumps into a train right in front of Sumiko. On television, they report the abnormal breeding of fireflies. In desperation, Sumiko takes the train to the home of the fireflies.