After divorcing her husband, Sae lives with her son. Her son doesn't have a girlfriend and never goes out on holidays; he just sleeps all day. Occasionally, she tries to take him out, inviting him on a hot spring trip for the two of them. Her son, who has always loved his mother... suddenly confesses his feelings to Sae, leaving her bewildered. However, overcome by her son's passion, she agrees 'just this once...' opening the door to forbidden desires. Although it was supposed to be just once... gradually, waves of pleasure awaken Sae's feminine instincts.
