Archibaldo de la Cruz (Alonso) is a wealthy Mexican man. As a privileged child during the Mexican Revolution, he witnessed the death of his governess, who died as she told him a fable about a music box that his mother had just given him. Because of the contents of the story and the coincidental timing of the governess's death, a young Archibaldo concludes that he had killed the woman using the music box. It is from there that his desire to kill begins.
As an adult, Archibaldo relates this story to a nun, and threatens to kill her with a straight razor. The terrified woman runs from Archibaldo, eventually running into an empty elevator shaft to her death. Archibaldo is called in by a judge investigating the incident. He confesses that he is responsible for her death, and for many others.
Patricia Terrazas (Macedo), is rather loud and constantly at conflict with her lover. After witnessing a fight between her and her lover, Archibaldo follows her and offers to drive her home, which she accepts. On arriving, he fantasizes of killing her with his straight razor, but is interrupted when her lover arrives home. The next morning a police officer arrives at Archibaldo's home, with news that Patricia had committed suicide.
Next, Archibaldo turns his attention to Carlota Cervantes (Welter), a woman who outwardly appears very religious, and is purported to be a virgin. However, she is having an affair with a married man. Archibaldo approaches her saying that he is very conflicted between good and evil. He thinks that Carlota can fix his problems, and proposes to marry her.
In the meantime, Archibaldo pursues another woman, Lavinia (Miroslava), whom he met in an antique shop, and later a bar. Lavinia works as a model for mannequins, and also shows American tourists around town. Archibaldo invites Lavinia to his home under false pretenses. He tells her that when they met, she reminded him of Joan of Arc being consumed by flames. He plots to strangle her and burn her in a kiln, but unexpectedly, the doorbell rings. It is Lavinia's tourist friends, the gringuitos. Lavinia has a disappointed Archibaldo take them on a tour of his home. When they leave, Lavinia says that she must go as well, and that she cannot see Archibaldo again because she is getting married.
Just as this is happening, Carlota comes in with her mother to accept Archibaldo's proposal. But Archibaldo soon learns of her adulterous relationship, and is displeased. He fantasizes about making her kneel and pray on her wedding night, and shooting her while she is praying. The wedding goes through, however, her jilted ex-lover ends up shooting her instead.
After having heard all of this, the judge says that Archibaldo has committed no crimes, and that merely thinking of killing people is not a crime. Archibaldo, despite his intense feelings of guilt, is deemed innocent. As if to be processing this unexpected reaction, Archibaldo takes a walk in the park. He then throws his music box into a lake, and moves on. He runs into Lavinia, whose marriage did not work out. The two walk together and the film ends.Miroslava (actress) - Lavinia
Ernesto Alonso - Archibaldo de la Cruz
Rita Macedo - Patricia Terrazas
Ariadna Welter - Carlota Cervantes
Andrea Palma - Mrs. Cervantes
Rodolfo Landa - Alejandro Rivas
José María Linares-Rivas - Willy Corduran
Leonor Llausás - The Governess
Eva Calvo - Señora de la Cruz, Archibaldo's mother
Enrique Díaz - Señor de la Cruz, Archibaldo's father
Carlos Riquelme - The Commissioner
Chabela Durán - Sister Trinidad
Carlos Martínez Baena - Priest
Manuel Dondé - Colonel at wedding
The film has been analyzed and studied by international academics such as Victor Fuentes and Gerardo T. Cummings. Fuentes, a retired professor from University of California, Santa Barbara, has included chapters dedicated to the detailed analytical study of Ensayo de un crimen in his last book, La mirada de Buñuel (Spain: Tabla Rasa Libros y Ediciones, 2006). Furthermore, he has found correlations between Ensayo de un crimen and Él.
Gerardo T. Cummings, on the other hand, published an article—by the Puerto Rican journal Revista Horizontes in 2004—on the intertextual connections between the cinematic version and the original literary source, the novel by Rodolfo Usigli. The article was part of a larger dissertation chapter on the echoes of the novel in the film. Other critics who have dissected the film version of Ensayo de un crimen have been Marsha Kinder and Ilan Stavans.
Buñuel briefly references the production of Ensayo de un crimen in his 1983 autobiography Mon dernier soupir (My Last Breath). In it, he recounts how he was syndicated into orchestrating an original composition for the movie, and that when the entirety of the members of the orchestra disrobed due to heat, he saw that "at least 3/4ths of them carried holstered guns". Later in the book, he sadly recalls the ironic parallel between a scene where the protagonist cremates a wax mannequin based on Miroslava, and Miroslava's actual cremation following her suicide shortly after production wrapped up.