A criminal named Maurice is released from prison after serving a six-year sentence for robbery. Being a criminal, he immediately starts planning his next caper, namely stealing everything inside a rich guy's safe. He stays the night with a girlfriend, Therese, before meeting his partner, Remy, and another crook named Silien (Jean-Paul Belmondo). Silien delivers a bag of burglary and safe-cracking tools, leaves, and places a phone call to Inspector Salignari of the police.
Maurice and Remy leave for the robbery. Silien catches Therese alone in her apartment, and beats her until she is forced to tell him where the robbery is to take place. Maurice and Remy grab the rich guy outside of his mansion, frogmarch him back inside, and are about to start opening the safe when the cops show up. They run, and there's a shootout. Both Remy and Inspector Salignari are killed. Maurice, winged in the shoulder by a bullet, passes out on the street. He winds up at the home of a friend and fellow crook named Jean, without knowing how he got there. Maurice blames Silien for ratting him and Remy out to the cops, and swears vengeance—but all is not as it seems.
- Ask a Stupid Question...: Maurice and Remy, both wearing masks, have jumped the rich guy outside of his mansion and are lugging him back inside, Maurice jabbing a gun (but really it's a Brandishment Bluff) in the rich guy's back.Rich Guy: Is this a robbery?
Maurice: Oh no, it's a surprise party, lots of fun.
- Back-Alley Doctor: Dr. Lequet, a veterinarian, comes to Jean's apartment to remove the bullet and bind the wound in Maurice's shoulder. Maurice knows him, so apparently no-questions-asked surgeries for criminals is Lequet's side business.
- Bound and Gagged: Silien knocks Therese unconscious with a couple punches, then binds and gags her and ties her to the radiator. After he wakes her up by pouring water over her head, he punches her a couple more times to find out the location of the robbery.
- Brandishment Bluff: When Maurice and Remy arrive at the rich guy's house, Maurice gains control of the rich guy by jabbing his fingers in the rich guy's back and pretending they're a gun.
- Chiaroscuro: The French New Wave loved shadowy mood lighting. It's particularly prominent in the opening sequence of this film, when Maurice murders Gilbert the fence. As Gilbert falls over he knocks a hanging light, which swings about wildly, irregularly illuminating what is otherwise a dark and silent apartment.
- Cop Killer: Maurice kills Inspector Salignari. The police emphasize the fact that this murder is more important for them than the murder of Gilbert, who is a criminal.
- Criminal Procedural: The main characters are all criminals.
- Dead Hat Shot: The film ends with a shot of Silien's hat, just after he is killed by Kern. This is also a reference to the title of the film, which is a slang word for "hat".
- Decoy Protagonist: In the beginning, the film focuses on Maurice. He meets Silien when he gets prepared for a robbery. The film starts focusing on Silien only when the cops question him about the murder of Inspector Salignari.
- Doesn't Like Guns: Maurice says that he does not like guns and he does not carry one. He kills Gilbert for revenge with Gilbert's own gun. Then he kills Salignari with the gun that Remy brought.
- Dramatic Gun Cock:
- Silien does this as he waits for the rendezvous which will end in him murdering Nuttheccio and Armand.
- Kern does the same as he conceals himself while lying in wait to kill Silien (or so he thinks).
- Driving a Desk: Some truly awful green-screen desk driving as the cops scour the streets looking for Silien. Particularly noticeable as it goes on for quite a while.
- The Dying Walk: Silien shoots through the wooden screen four times. The screen falls over to reveal Kern, shot multiple times, still holding his gun. He staggers forward a few steps before crumpling to the ground. Unfortunately for Silien he's Only Mostly Dead.
- Epic Tracking Shot: The first shot is a nearly three-minute tracking shot following Maurice as he walks down a pedestrian pathway under the railroad tracks, as the credits roll onscreen.
- Femme Fatale: Therese. She is Maurice's girlfriend. He trusts her and asks her help to prepare a robbery, but she is the long-term girlfriend of Inspector Salignari and she rats Maurice out to the police.
- A series of flashbacks accompanied by Silien's narration illustrate how he realized that Therese was an informant, then raced to the scene of the robbery too late to stop it, but instead rescued Maurice and then went back and killed Therese.
- Another flashback reveals how Maurice, who thought Silien was the informant, hired Kern to kill Silien.
- Frame-Up: Silien concocts an elaborate frame for the Gilbert murder, arranging to fake a Mutual Kill for Nuttheccio and Armand and then planting the evidence on their corpses, all to get Maurice off the hook.
- In Love with the Gangster's Girl: Silien's love interest is Fabienne, the wife of Nuttheccio, a mob boss.
- The Informant: The opening titles explain that le doulos, while literally referring to a kind of hat, is slang for police informant. Someone informs the police about the robbery; the question is who.
- Just Got Out of Jail: Maurice is released from prison after serving a six-year sentence for robbery. He immediately starts planning his next caper.
- Last Breath Bullet: Silien makes the major error of turning his back after Kern, shot several times, falls to the ground. Kern has enough life left in him to fatally shoot Silien In the Back.
- Make It Look Like an Accident: Therese's murder. She is put into a car and the car is driven into a chasm. However, the police immediately suspects it is a settling of accounts.
- The Oner: An eight-minute take during the police interrogation of Silien, with the camera in a central location, slowly turning as the cop paces around the room, firing questions at a calm Silien.
- Outlaw Couple: Maurice work with his girlfriend Therese to organize capers, but Therese secretly work for the police.
- Plot-Triggering Death: The plot starts moving because Maurice kills Gilbert in the first minutes of the movie.
- The Reveal: Eventually it's revealed that Silien did not rat out Maurice and Remy to the police. Therese did. In fact, she was the long-term girlfriend of Inspector Salignari. When Silien was beating the location of the robbery out of her, it wasn't to betray the others, it was an attempt to save them. Silien was the one who showed up and took an unconcious Maurice to safety at Jean's house.
- Revenge: It turns out that this is why Maurice murdered Gilbert the fence in the opening scene—five years earlier, while Maurice was in jail, Gilbert killed Maurice's old girlfriend Arlette because he was afraid she would inform to the cops.
- She's Got Legs: When Therese is reading a magazine before Silien's arrival, Melville could have framed her sitting on the couch normally, reading. No, far better to frame her lying on her back on the couch as she reads, her stocking-clad calves prominently shown in the frame.
- Smoking Hot Sex: A naked Fabienne is shown lounging in bed and lighting a cigarette, Silien having already gotten up and gotten dressed after their sex.
- Would Hit a Girl: Silien has no qualms in hitting Therese so that she tells him where the robbery is to take place.
- You Wake Up in a Room: After the failed robbery, Maurice wakes up in Jean's apartment and he does not know how he ended up there.