Yuri lives with her son. One day, she asked him what he wanted to celebrate his university admission. He then demanded to have sex with her. Yuri blurted out something she didn't mean: "Just this once... Can you promise?" And so they had intercourse. It was the only time ever. Then one day, Masaki invited Yuri on a hot spring trip as a last mother-son memory. Yuri readily agreed, feeling that something was about to begin...
