Howard Brubaker (Lemmon) is married to Phyllis (Sally Kellerman), who doesn’t love him. Catherine (Deneuve) is the stunning wife of an equally uncaring husband, Howard’s philandering boss, Ted Gunther (Peter Lawford).
The evening of the day Ted promotes Howard, Howard attends Ted’s house party where Ted urges him to pick up an available woman there and proceeds to show him how. Howard reluctantly tries it on Catherine, who instantly accepts. The two leave the party and go out for a little adventure on the town. Ted is oblivious, as he is concentrating on other women at the party.
The two find their marriages are loveless as they discover more about each other that night and decide to run away together the next evening. But Ted doesn’t realize the other man is Howard until Howard and Catherine are about to board the plane to Paris.