A married couple pick up a student hitch-hiker whom they have almost run over and tensions develop when the student is invited to join the couple on their yacht. The older man wishes to show off before the student. More secretly, it is suggested he wishes to enliven a boring weekend routine and a fractious marriage. His exhibitionism and self-assurance crumple under events which make him uncertain whether the student, whom he has caused to fall off the yacht, has drowned or whether he has returned and seduced his wife.
The eponymous knife is owned by the youth and coveted by the older man. Knives recur in Polanski’s films as a favourite weapon when the protective psychological layers of his characters are pared away. Many future fingerprints are here: the hints of sexual frustration; the tense struggle for supremacy between three characters in a confined setting; an enigmatic finale that resonates long after the close. Nominated for a best Foreign Film Oscar, this is one of the cinema’s most satisfying directing debuts, bristling with talent and potential.