The film opens with a title card reading "Once upon a time". A middle-aged man (Luis Buñuel) sharpens his razor at his balcony door and tests the razor on his thumb. He then opens the door, and idly fingers the razor while gazing at the moon, about to be engulfed by a thin cloud, from his balcony. There is a cut to a close-up of a young woman (Simone Mareuil) being held by the man as she calmly stares straight ahead. Another cut occurs to the moon being overcome by the cloud as the man slits the eye of a calf with the razor, and the vitreous humour spills out from it. Visually, the suggestion seems to be that it's the woman's eye that's been cut.
The subsequent title card reads "eight years later". A slim young man (Pierre Batcheff) bicycles down a calm urban street wearing what appears to be a nun's habit and a striped box with a strap around his neck. A cut occurs to the young woman from the first scene, who has been reading in a sparingly furnished upstairs apartment. She hears the young man approaching on his bicycle and casts aside the book she was reading (revealing a reproduction of Vermeer's The Lacemaker). She goes to the window and sees the young man lying on the curb, his bicycle on the ground. She emerges from the building and attempts to revive the young man.
Later, the young woman assembles pieces of the young man's clothing on a bed in the upstairs room, and concentrates upon the clothing. The young man appears near the door. The young man and the young woman stare at his hand, which has a hole in the palm from which ants emerge. A slow transition occurs focusing on the armpit hair of the young woman as she lies on the beach and a sea urchin at a sandy location. There is a cut to an androgynous young woman in the street below the apartment, poking at a severed hand with a cane while surrounded by a large crowd and a policeman.
The crowd clears when the policeman places the hand in the box previously carried by the young man and gives it to the young woman. The androgynous young woman contemplates something happily while standing in the middle of the now busy street clutching the box. She is then run over by a car and a few bystanders gather around her. The young man and the young woman watch these events unfold from the apartment window. The young man seems to take sadistic pleasure in the androgynous young woman's danger and subsequent death, and as he gestures at the shocked young woman in the room with him, he leers at her and grasps her breasts.
The young woman resists him at first, but then allows him to touch her as he imagines her nude from the front and the rear. The young woman pushes him away as he drifts off and she attempts to escape by running to the other side of the room. The young man corners her as she reaches for a racquet in self-defense, but he suddenly picks up two ropes and drags two grand pianos containing dead and rotting donkeys, stone tablets containing the Ten Commandments, and two rather bewildered priests (played by Jaime Miravilles and Salvador Dalí) who are attached by ropes. As he is unable to move, the young woman escapes the room. The young man chases after her, but she traps his hand, which is infested with ants, in the door. She finds the young man in the next room, dressed in his nun's garb in the bed.