It's about an hour into the film before the wife asks her husband the obvious question: why did he bring along the handsome young hitchhiker for a day on their private boat?
That's the question driving the plot of Roman Polanski's Knife in the Water (1962), his feature film debut. Andrzej (Leon Nymczyk), a wealthy journalist, and his wife Krystyna (Jolanta Umecka) are driving to their private boat to spend a day and night out on a lake. From the start, it's obvious this is not a loving marriage. They both stare at the road in silence and only interact when he criticizes her driving until she pulls over to switch spots. They're literally thrown off course by a young hitchhiker (Zygmunt Malanowicz), who stands in the middle of the road to snag a ride and almost causes a crash. Although angered, Andrzej lets (actually orders might be more accurate) the young man to ride along, and when they reach the dock, he further goads him into coming aboard the boat for the trip. That's when this "psychological menage-a-trois" begins.
Knife in the Water is about power and one-upmanship. Andrzej is an experienced sailor, goes about the various tasks with ease, and he has no problem ordering the stranger around. As he puts it, with two men on a boat, one must be captain. The unnamed hitchhiker, still a student, doesn't appreciate the condescending treatment and defies his host every chance he gets. His one advantage is the knife he brought. Meanwhile, Krystyna enjoys playing the men off each other, and it's clear there is some sort of mutual attraction between her and the hitchhiker.
Polanksi keeps everything tight and limited. The action is limited to three character, the dialogue is sparse, and the setting is confined to one boat once they reach it, and as a result, the film manages to be both claustrophobic and isolated. This is reflected with many triangular shots of two characters in close ups while the third is in the sometimes distant background.
While the plot described above could have functioned as a straight up thriller, the tension is laced in the mundane aspects of being on a boat: steering, hoisting the sail, washing the deck, passing time with games, etc. The suspense builds watching the characters trying to control and defy each other. When settling in for the night, the two men race to see who can finish blowing up air mattresses: Andrzej with a pump and the hitchhiker with his breath. Every action carries substantial depth.
Andrzej and the young man also represent different natures. Andrzej embodies dominion and control through his wealth and sailing. The young man defends his love of hiking and walking, which makes a free spirit who isn't tied down by anything. With these two, it's understandable Krystyna would resent her husband and take an interest in the stranger.
While psychologically fascinating, Knife in the Water does not generate much emotional resonance because all three characters are unlikeable. Andrzej is pompous and tyrannical, the hitchhiker prone to childish tantrums, and Krystyna is manipulative. That doesn't make them uninteresting, just unsympathetic. But really, Polanski's direction is the star of this film anyway.