Henri Verdoux had been a bank teller for thirty years before being laid off. To support his wheelchair-bound wife and child, he turns to the business of marrying and murdering wealthy widows. The Couvais family becomes suspicious when Thelma Couvais withdraws all her money and disappears, only two weeks after marrying a man named “Varnay”, whom they only know through a photograph.
As Verdoux prepares to sell Thelma Couvais’s home, the widowed Marie Grosnay visits. Verdoux sees her as another “business” opportunity and attempts to charm her, but she refuses. Over the following weeks, Verdoux has a flower girl repeatedly send Grosnay flowers. In need of money to invest, Verdoux, as M. Floray, visits Lydia Floray and convinces her he is her absent husband. She complains that his engineering job has kept him away too long. That night, Verdoux murders her for her money.
At a dinner party with his real wife and their friend the local chemist, Verdoux asks the chemist about the drug he developed to exterminate animals painlessly. The chemist explains the formula and that he had to stop working on it after the local pharmaceutical board banned it. Verdoux says he could test the drug by using it on a tramp off the street, then laughs it off as a morbid joke. Later at his furniture office he attempts to recreate the drug.
Shortly thereafter, Verdoux finds The Girl taking shelter from the rain in a doorway and takes her in. When he finds she was just released from prison and has nowhere to go, he prepares dinner for her with wine laced with his newly-developed poison. Before drinking the wine, she thanks him for his kindness, and starts to talk about her husband who died while she was in jail. After she says her husband was a helpless invalid and that made her all the more devoted to him, Verdoux says he thinks there’s cork in her wine and replaces it with a glass of unpoisoned wine. She leaves without knowing of his cynical intentions.
Verdoux makes several attempts to murder Annabella Bonheur, who believes Verdoux to be Bonheur, a sea captain who is frequently away, including by strangulation while boating, and by poisoned wine, but she is impervious, repeatedly escaping death without even realizing while, at the same time, putting Verdoux himself in danger or near death. Meanwhile, Grosnay eventually softens and relents from the continual flowers from Verdoux and invites him to her residence. He convinces her to marry him, and Grosnay’s friends hold a large public wedding to Verdoux’s disapproval. Unexpectedly, Bonheur shows up to the wedding. Panicking, Verdoux fakes a cramp to avoid being seen and eventually deserts the wedding.
Before the Second World War breaks out, the European markets collapse, and Verdoux loses his assets. The Girl, now well-dressed and chic, once again finds Verdoux on the street. She invites him to an elegant dinner at a high-end restaurant as a gesture of gratitude for his actions earlier. The girl has married a man she doesn’t love to be well-off. Verdoux reveals that he has lost his family. At the restaurant, members of the Couvais family recognize Verdoux and attempt a pursuit. Verdoux delays them long enough to bid the unnamed girl farewell before letting himself be captured by the investigators.
Verdoux is exposed and convicted of murder. When he is sentenced in the courtroom, rather than expressing remorse he takes the opportunity to say that the world encourages mass killers, and that compared to the makers of modern weapons he is but an amateur. Later, before being led from his cell to the guillotine, a journalist asks him for a story with a moral, but he answers evasively, dismissing his killing of a few, for which he has been condemned, as not worse than the killing of many in war, for which others are honored, “Wars, conflict – it’s all business. One murder makes a villain; millions, a hero. Numbers sanctify, my good fellow!” His last visitor before being taken to be executed is a priest. When guards come to take him to the guillotine he is offered a cigarette, which he refuses, and a glass of rum, which he also refuses before changing his mind. He says “I’ve never tasted rum”, downs the glass, and the priest begins reciting a prayer in Latin as the guards lead him away and the film ends.