Saei divorced her husband and lives with her son. My son doesn't have a girlfriend, he doesn't go out on holidays, he just sleeps all day. Once in a while, I invited her to a hot spring trip just for the two of us to take her outside. A son who has always liked his mother ... Sae was confused by the sudden confession, but she lost her son's enthusiasm and opened the forbidden door with the promise of "If only once ...". It was a one-time promise, but... Eventually, a swell of pleasure awakens Saei's female instinct ...