Charles Masson (Bouquet) has an affair with Laura, the wife of his best friend François Tellier. They practice BDSM and one day he strangles her during a sexual intercourse. The police investigates the case, but they have no clue who the murderer is.
Juste avant la nuit provides examples of:
- Animal Motifs: Just after Charles killed Laura, his son tells that he heard a rat in the house. Hélène suggests to buy a rat trap. In the end, Charles sees the rat as he gets caught in the trap, just before he decides to turn himself in and Hélène decides to kill him to avoid a scandal.
- Best Friend: François is Charles's best friend.
- Casual Kink: Charles and Laura practice BDSM and this is how he killed her. When Charles tells his wife, she is not shocked.
- Chekhov's Gun: The laudanum that Charles regularly takes to get some sleep. In the end, Hélène poisons him with it.
- A Deadly Affair: Charles kills his lover, Laura, during BDSM practice.
- Dies Wide Open: Laura is shown with her eyes staring blankly immediately after the murder. Charles later recounts that Laura had instructed him to strangle her as part of their sex games, telling him she would signal him to stop by closing her eyes. Because she never closed her eyes, he didn't realize she was dead until it was too late.
- Drop Dead Gorgeous: Laura's nude body is shown lying on the bed after she is strangled.
- Erotic Asphyxiation: Laura instructs Charles to strangle her as part of their sex games. It winds up going too far.
- The Film of the Book: The script is based on The Thin Line by Edward Atiyah (1951).
- Karma Houdini: Decontructed. The people who have clues that Charles might be involved in the crime (François Tellier and Gina Maillardi) do not tell the police, so Charles seems to get away scott free. Yet, he feels guilty, so he tells his wife, then his friend, but they do not denounce him. So he decides eventually to turn himself in, but his wife prefers to avoid a scandal and poisons him.
- Never Suicide: Charles's death looks like a suicide, but actually Hélène poisoned him.
- Plot-Triggering Death: The film starts with Laura's murder.
- Posthumous Character: Laura. She is killed very early in the film and she plays an important role in the plot.
- Stealing from the Till: Bardin steals from the till of Charles's company.
- Sympathetic Murderer: Charles, Laura's murderer, gets a sympathetic point of view. He explains that Laura made him suffer.
- Villain Protagonist: Charles, the protagonist, is Laura's murderer.
- Vomit Discretion Shot: After the murder, Charles goes to a bar and vomits due to his guilt and stress. The scene fades to black as he bends over the toilet, then fades back in to show him washing his face afterwards.