Fourth Mom (2020)
Eun-mi visits her boyfriend Young-jae's house, meets his uncle Hyuk-soo, who lives with Young-jae, and falls in love. Eun-mi invites Young-jae and Hyuk-soo to her house to meet Hyuk-soo secretly. However, Young-jae finds out that Eun-mi likes his uncle Hyuk-soo, and feels jealous. So, without Hyuk-soo, he goes to Eun-mi's house alone, and at Eun-mi's house, she sees her mother Si-young and feels a strange feeling. Si-young is also attracted to Young-jae, and the two fall in love. While the two are having sex at Eun-mi's house, Eun-mi goes to Young-jae's house while drunk and seduces Hyuk-soo. Hyuk-soo falls for Eun-mi's temptation, and the two have sex. Youngjae and Eunmi learn that they had an affair and try to break up with guilt, but it is difficult to break up because of their relationship. So, they decide to connect Si-young and Hyuk-soo. If you two date, it's not an affair, it's swapping.