Although her husband has passed away, Shirayuri, the mother-in-law, lives happily with her daughter and son-in-law. She has a grandchild and lives a comfortable life. One day, when she comes out of the bath, she bumps into her stepson in the dressing room, who is holding Shirayuri's underwear. From that moment on, things start to go wrong... Shirayuri and her stepson end up making love to each other, despite feeling guilty.