After divorcing, Sae lives alone with her son. He rarely eats the meals she prepares, never goes out on weekends, and spends almost the entire day sleeping. Wanting to expose him to the outside world, even just a little, she plans a hot spring trip for the two of them. Her son has always been considerate of her, and his feelings gradually begin to affect Sae's heart. Though initially shocked by his sudden confession, she is moved by his passion and agrees—on the condition that it happens only once. But that single promise quickly spirals into a chain of intense pleasure, awakening Sae's long-suppressed desires. What began as a one-time agreement leads their relationship into uncharted territory...