Four years ago, we crossed that line once... Since losing her husband eight years ago, Kiyoka has lived alone with her son. As her son prepares to leave for a job far away, memories of four years ago resurface. For her son's college entrance celebration, she was asked for sex, and they had sex once. At the hot spring inn, she let go of her usual reason, and in this unusual situation, Kiyoka released her primal lust. And today, she repeats the same mistake...