To Rumi, her son Tatsuya will always remain a child, even if his body grows. So she doesn't mind getting in the bath while her son is bathing and showing him her body. However, when her son's penis gets erect as he looks at Rumi's ass, Rumi realizes that her son has grown into an adult, no, a man. A boy who is fascinated by his mother's ass and a mother who feels her son's manhood. The forbidden feelings between the two will explode in the forbidden bathroom.