Mizue Karasawa, a 50-year-old housewife, lives with her husband and son. Her husband works late every day due to overtime, leaving little time for conversation—their interactions mostly limited to "dinner," "bath," and "sleep." As she finds herself with no one to talk to, her loneliness grows deeper each day. One night, she notices her sleeping son's underwear and can't help but glance beneath it. From that moment on, her relationship with her son gradually intensifies, binding them together in a new, forbidden way.