The First Wives Club (2007)

Bok-soo (Kim Hye Sun) put her husband through medical school by earning a living as a fish seller but her husband does not feel grateful about it and looks down on her instead. She learns that her brother, who is married to her best friend Hwa-shin (Oh Hyun-kyung), is having an affair with another woman but she tries to cover it up. When she learns that her husband, Ki-jeok (Oh Dae-gyoo) is having an affair with a former girlfriend, she finally empathizes with her friend's plight. Her mother, who had to endure an unhappy marriage due to a cheating husband, joins Bok-soo's and Hwa-shin's plan to get revenge on their unfaithful husbands.