Two young couples come to a secret hot spring inn. In the mixed outdoor bath, the young wives can't take their eyes off the erect penis of another man they happen to see. Even though they know it's wrong, the immorality makes their crotches even wetter, and before they know it, they're sucking another man's penis and going crazy. Even the owner and the landlady of the inn get involved, and their adulterous love flares up!