After more than twenty years of marriage, Natsuko had raised her daughter Ryoko with care, and a few years ago, Ryoko got married and left home. Since then, Natsuko has lived a quiet, peaceful life with her husband Kenji. One day, Kenji repeatedly lamented about wanting to see his grandchild, so Natsuko decided to cheer him up by proposing a family trip, inviting Ryoko and her husband Yuji. On the day of the trip, the family arrived at a hot spring inn. They immediately soaked in the bath together, presenting the ordinary, happy image of a close-knit family. However, a few hours later, that peaceful scene took a sudden turn. Yuji's bold and fearless actions triggered an unexpected development.