Gruff American gangster Dickey pushes his broken-down car along a causeway through rising seawater while his eccentric companion Albie lies inside, bleeding from a gunshot wound after a bungled robbery. Cut off by the unexpected rising tide, they are on the only road to a bleak and remote tidal island, where, in a dark castle on a hilltop, a deeply neurotic and effeminate middle-aged Englishman named George lives with his promiscuous young French wife Teresa. Dickey disconnects the phone lines and proceeds to hold the two hostage while awaiting further instructions from his underworld boss, the mysterious Katelbach. When Albie dies from his injuries, Dickey decides to take over the castle. George briefly entertains some of his obnoxious friends who show up at the castle unannounced, leading Dickey to pose as a servant while Teresa begins to flirt with one of the guests, Cecil. Dickey eventually gets word that his boss Katelbach is not going to come, so he demands George drive him to the mainland by causeway. George, who has had enough of Dickey's bullying, suddenly goes berserk and shoots him dead with his own gun. Before dying, Dickey manages to retrieve his tommy gun from his broken-down car, which he had hidden away in the chicken house. Too weak to fire the gun at George, Dickey collapses to the ground and the automatic discharge from the weapon causes the car to explode in flames inside the chicken house. Fearful of being implicated in the killing, Teresa frantically insists that she and George abandon the castle together. But George is in a state of shock and seems unable to leave. Desperate and afraid, Teresa runs off by herself and hides in a closet. She is later rescued by Cecil, who had returned to retrieve his rifle. Now utterly alone, George runs along the beach at daybreak. He finally sits down on a rock in the fetal position and weeps hysterically as the early morning tide rises around him.
Like Polanski's previous film Repulsion, released the year before, it explores themes of horror, frustrated sexuality and alienation, which have become characteristic of many of the director's films, especially Rosemary's Baby and The Tenant. Cul-de-Sac has been compared in tone and theme with the works of Samuel Beckett and Harold Pinter, and these similarities are underscored by the casting of two principal roles in the film: Jack MacGowran was renowned for his stage performances of Beckett's plays and Donald Pleasence originated the role of Davies in Pinter's The Caretaker. The film's German title is Wenn Katelbach kommt. Christopher Weedman also notes the film's similarities with "such hard-edged Humphrey Bogart hostage thrillers as The Petrified Forest, Key Largo, and The Desperate Hours."
