Sae is divorced from her husband and lives with her son. Her son doesn't have a girlfriend, doesn't go out on his days off, and just sleeps all day. She invites him on a trip to a hot spring just for the two of them to get out of the house for once. Her son has always loved his mother... Sae is confused by his sudden confession, but she gives in to her son's enthusiasm and opens the forbidden door with the promise "just once...". It was a one-time promise, but before long, the waves of pleasure awaken Sae's feminine instincts...