Mother-in-law in Love With Son-in-law 4: Joon-yeon and So-hyeon are living together with Joon-yeon's young and pretty mother-in-law for a while after his father-in-law remarries. One day, Joon-yeon is left home alone with his mother-in-law due to So-hyeon and her father-in-law's sudden business trip. The mother-in-law seduces her handsome son-in-law on a hot night. Unable to forget the sex he had with his mother-in-law, Joon-yeon even plans to have a threesome with his wife and his mother-in-law.