Mary, a Milf who lost her husband, lives alone with her stepson from her late husband's previous marriage. She begins to develop faint romantic feelings toward him, as he reminds her so much of her deceased spouse. On a hot springs trip together, though legally mother and son, their emotional distance is anything but. Amid this closeness, a forbidden bond begins to grow, and they secretly make love under the cover of night. Overcoming the boundaries of their familial relationship, they become consumed by the thrill of taboo desire. In the quiet night of Joshu, their moans echo through the darkness, their passion impossible to restrain. Their secret affair continues relentlessly until the end of the night.