Mary is under constant pressure from her mother-in-law to produce a grandchild. When the whole family plans a hot-spring trip, her mother-in-law orders the couple to abstain for a month beforehand to improve their chances of conceiving. Yet on the day of the trip, Mary's husband refuses to have sex with her. Her father-in-law is likewise rejected by his wife. Both left painfully horny, Mary and her father-in-law head to the baths to calm down. Then he happens to see her bathing and can no longer contain his desire...