Andrzej and Krystyna are married and sick of each other. They pick up a hitchhiker and end up offering to take him sailing without bothering to find out what his name is. But his name really does not matter. What matters is that he has a knife.
According to Chekhov, if you make people aware of a gun early on in a story, sooner or later someone in the story will have to shoot the gun. If the gun is not going to be fired, it should not be in the story. Now, knives are more common than guns, and are used for mundane purposes, such as cutting the meat on one’s plate, so the rule that applies to guns cannot automatically be applied to knives. Unless, that is, it is a wicked-looking, gravity-propelled, telescoping knife with a four-inch, locking blade. When you put a knife like that in a story, then Chekhov’s law applies to that weapon as well, and it is required that someone get cut with it.
But no one does. Not only is this knife referred to in the title, but it is introduced early on and emphasized again and again. The tension is built up as the knife is used to play a dangerous game of stabbing between the fingers of a spread out hand. It is used again when it is several times thrown across the cabin and into the wall. And it is used to cut the halyard when the sailboat runs aground. This would be like having a gun in a movie that has the word “gun” in its title, with people showing off their marksmanship or using it for some ordinary practical end. It would not satisfy our need to see the gun used for a more deadly purpose, just as these various employments of the knife do not satisfy our expectation that someone will be stabbed with it. But no one is.
Finally, Andrzej takes the young man’s knife and throws it in the water. The idea is that the young man was very fond of his knife, and Andrzej threw it in the water out of spite. But in that case, the object might just as well have been a harmonica that the young man was fond of. As it is, the fact that no one got stabbed after all the emphasis placed on the knife leaves us disappointed, especially since we put up with a lot of boring nonsense waiting for something to happen. Roman Polanski, who directed this movie, must have eventually figured this out and tried to make amends by having Jack Nicholson get his nose sliced in Chinatown.