Hello-Goodbye (1970)

A car salesman journeys to France and encounters an apparently lonely woman. He immediately begins to successfully woo her only to learn that she is actually a baron’s wife. Fortunately, the baron believes in open marriages and winds up hiring the Englishman to teach his son (from an earlier marriage) everything about automobiles. Meanwhile the car salesman finds himself falling seriously in love with the wife. The baron really doesn’t mind as he himself is involved with another.