The Truth About Love (movie, 2005)

For years, Archie has been pining for Alice, the wife of his law partner, Sam. When Archie impulsively sends Alice an anonymous love note, she assumes it to be a playful gesture from her husband, and replies with her own unsigned letter. After Sam hides the note, Alice's sister, Felicity, convinces her that he must be having an affair. As the extent of Sam's infidelities becomes clear, Alice turns to Archie for support.