Roman Polanksi's Oscar nominated feature debut. A couple driving along a deserted road to a lake almost run over a young man, a student, who flags down the car almost too closely. The driver is a self-absorbed husband, the woman his pretty, irritated wife. They take him along and the husband, out of the sheer patronising will, invites him to come sailing on their boat. During the boat ride there is clash of personalities.

The boy has a habit of playing with a switchblade knife. The husband is a self-made man and delights in showing up the boy. In an argument he knocks the boy off the boat. The couple thinks he has drowned and the husband swims to shore for help. But the boy has hid behind a buoy and comes aboard to seduce the wife. She finally brings in the boat and assures her husband the boy is alive and that she has cheated on him.