The woman introduced as his new mother, Nanako. The three of them go on a trip to Izu. While the father is away, he mischievously approaches Nanako, who is sleeping on the train—groping her breasts, lifting her skirt to peek at her panties—his innocent teasing continues. Upon arriving in Izu, they visit a museum together, but while the father is absorbed in viewing the exhibits, he seizes the chance to act again. At the inn, the son offers to massage Nanako's stiff shoulders. Yet his hands gradually slide into more intimate areas. Eventually, despite being his stepmother, Nanako surrenders to pleasure and becomes intimately involved with her stepson. While hiding their actions from her husband, their forbidden relationship grows deeper.