Following a failed sting operation months prior, Detroit Police Department Sergeant Jake Roenick begins to regularly abuse alcohol and painkillers while clinging to his unambitious assignment as a desk sergeant at Precinct 13. On New Year's Eve, Roenick, along with officers Kevin Capra, Jasper O'Shea, and secretary Iris Ferry are the only people on site, as most of the officers have either transferred or are taking time off for the holidays. Psychiatrist Alexandra Sabian also arrives to evaluate Roenick's fitness for duty. Later, Capra, whose attempts to woo Iris are rebuffed, leaves after his shift ends.

Meanwhile, crime boss Marion Bishop is arrested after killing an undercover policeman in a church, and is set to be transferred to prison with three other criminals: Beck, Anna, and Smiley. When a snowstorm shuts down the roads, the prison transport is directed to Precinct 13. Though Roenick and O'Shea are hesitant, the storm gives them little option, and they place the prisoners in holding cells and let the deputies guarding them inside the precinct.

Soon after, masked gunmen cut off the precinct's communications and electricity, and attack the station, killing the deputies before demanding that Bishop be handed over to them. The lawmen initially believe the attackers to be Bishop's men, but when they kill one of the attackers, they discover he is from a team of undercover policemen led by Captain Marcus Duvall of Precinct 21. Bishop explains that Duvall and his team are corrupt and were formerly his business partners, and that the undercover officer he killed was sent to eliminate him.

Though Roenick is reluctant to trust Bishop, the two forge an uneasy truce as both know they will be killed by Duvall to protect his secret. Heavily outnumbered and outgunned, Roenick releases the prisoners and arms them to help defend the precinct. Their combined efforts repulse several more attacks, eventually leading to a stalemate.

When a vehicle approaches the precinct, Duvall's men open fire on it; Roenick rescues the driver, who turns out to be Capra. Later, Beck and Smiley secretly conspire to escape with Capra's vehicle; simultaneously, the rest of the defenders plan for Anna and Sabian to drive away and get help. When Beck and Smiley sneak out to the vehicle, they are killed by Duvall's men, providing a distraction which allows Anna and Sabian to drive off. However, Duvall has Lieutenant Kahane wait in the vehicle in anticipation; Kahane kills Anna, and Duvall kills Sabian after she refuses to cooperate.

With only five of them left, Roenick and Bishop decide to take action instead of waiting for another attack. When they find the back door unlocked, they suspect Capra to be working for Duvall and handcuff him.

When the snowfall subsides, Duvall calls in a corrupt SWAT team who land on the roof of the precinct. The defenders set fire to the station to cover their escape and flee through a utilities tunnel underneath the building. Emerging from the tunnel, they find themselves surrounded by Duvall's men, and O'Shea confesses that he has been assisting Duvall. As Duvall prepares to execute the others, Bishop secretly plants a flash bang grenade on O'Shea, injuring him. In the confusion, Iris and Capra flee in Duvall's SUV. Kahane shoots out the tires, causing the vehicle to crash and knock Capra unconscious, but Iris manages to kill Kahane after a struggle.

Duvall chases Roenick and Bishop into a nearby forest where they ambush and kill Duvall and his remaining forces. Following this, Bishop, takes Roenick's gun and flees, and Roenick promises to personally arrest him in the future. When Iris arrives with police and firemen, Roenick claims that only he and Duvall's gang are present, effectively granting Bishop a getaway. As the authorities secure the area, Roenick and Iris leave the forest as the sun rises.