Treacherous Alliance (2003)

One morning, Marianne tells Paul that she leaves him. She says met the man of her life, a certain Matthieu. Paul accepts the separation. But Laure, Mathieu's ex-girlfriend, comes over to Paul's house looking for Marianne. When she learns that Marianne is not there, she talks to Paul about trying to win Mathieu back. At first, they seek to separate the new lovers, but Paul is convinced that it is hopeless to try to do so. Instead of trying to break up Mathieu and Marianne, they begin to blame each other for having neglected their respective partners. Unlike Paul, who seems willing to accept the new situation, Laure is furious and is determined to win Mathieu back. Consequently, they develop a plan: Paul and Laure will pretend to have fallen madly in love with each other. Thus they will arouse the jealousy of their adulterous halves. Paul, initially reluctant, ends up playing the game. Will Marianne and Matthew fall into the trap?