Yuri lives with her son. One day, he asked her for one thing to celebrate his acceptance into college. He asked for sex with Yuri. Yuri blurted out something she didn't Kokoro. "Just once... Can you promise?" And the two of them made love. It was a one-off. Then one day, Masaaki invited her to go on a trip to a hot spring to make a final memory for mother and son. Yuri readily accepted. She had a premonition that something was about to happen...