In order to get their investigators closer to the population, the Montreal police has moved its investigators away from their headquarters to stations divided in 36 districts. District 31 follows the story of Detective Sergeant Nadine Legrand and her colleague Patrick Bissonette, as well as their other comrades at the station as they deal with day to day crimes that happen in big cities; homicides, kidnappings, break-ins, murders, family violence, drug trafficking, terrorism and more.
This series provides examples of:
- Anyone Can Die: Nadine Legrand and Jeff Morin, both main characters, were murdered.
- Gray-and-Gray Morality
- Oh, Crap!: Christian Phaneuf acts smug as three armed men attack his prison transport and tie up the police officers, only for his smile to fade as the gun is pointed at him next and he understands too late that the three men attacking aren't there to break him out - they're there to kill him.
- Sixth Ranger Traitor: Yanick Dubeau in Season 3.
- Wham Shot: The ending of season 2. Christian Phaneuf is assassinated by none other than Laurent Cloutier, Maxime Vézeau and Daniel Chiasson.