On New Years Eve, Detroit's oldest police station, Precinct 13, is about to close forever. Only a skeleton crew of officers are on duty for New Year's Eve, including Sergeant Jake Roenick (Ethan Hawke), veteran officer Jasper (Brian Dennehy), secretary Iris (Drea de Matteo), and psychiatrist Alex (Maria Bello). Roenick is haunted by a botched undercover operation from eight months before that resulted in the death of the other two members of his team.Meanwhile, crime lord Marion Bishop (Laurence Fishburne) is arrested after a confrontation results in the death of an undercover officer. Bishop is put on a prison bus with three other criminals: meth addict Beck (John Leguizamo), petty crook Anna (Aisha Hinds), and counterfeiter Smiley (Ja Rule). A raging snowstorm and treacherous road conditions force the bus driver to make a detour to Precinct 13, where the prisoners are to be held until the storm is over.New Year's celebrations at the precinct are soon cut short, however, as an intrusion by two masked gunmen results in the death of one of the prison guards, and fatally injures the other. The officers soon find they are surrounded by unknown assailants, preparing to lay siege to the precinct. The electricity in the station then goes off and all the land-line phone lines are cut. Cellular phone reception has also been jammed, as well as any radio communication. After one of the attackers throws a brick into the precinct with "Bishop" written on it, Jasper claims they are Bishop's men, here to extract him.This is proven to be false, however, after Roenick kills one of the attackers in another assault and reclaims the man's wallet, which contained his police badge. Bishop then reveals the attackers are actually policemen, led by corrupt Captain Marcus Duvall (Gabriel Byrne). Previously, Bishop and Duvall were partners in crime, but Duvall got greedy. Now he and his men, including Sgt. Mike Kahane (Currie Graham), are there to kill Bishop before he can testify at trial, saving all officers involved a hefty prison sentence. However, now that everyone inside Precinct 13 knows of the true plot instead of just Bishop, they are all a liability and must be eliminated.Heavily outnumbered by a growing police force made up of dirty street cops and SWAT officers, Roenick makes the decision to free and arm the prisoners to help them hold out until dawn. Although the officers do not trust the criminals, and vice-versa, Roenick and Bishop reach an agreement to cooperate in order to stand a chance to survive the night. With their combined efforts, they manage to repel further attacks by the police forces, and reclaim their weapons.Soon Precinct 13 is joined by another officer, off-duty cop Capra (Matt Craven). After driving erratically enough to evade the snipers from the buildings across the street, he scurries into the building completely confused and panicked. The attending officers welcome him with open arms, but Beck accuses him of being a plant. Fearing he may start a shootout, Bishop subdues Beck to calm him down and settles the conflict.Smiley also thinks that Capra is a plant, so he and Beck decide to risk the police force outside for a shot at freedom, and plan an escape over a nearby fence.As the prison bus explodes and they notice Smiley and Beck escaping, they decide to use this as a distraction and also plan an escape, making use of Capra's car in order for a couple people to flee the precinct and call for help while the police are busy with Beck and Smiley.Anna, who can hotwire a car, volunteers to go. Alex also goes along. Beck and Smiley make it to the perimeter fence before they are killed by snipers. Anna and Alex appear to have made it, before a cop hidden in the backseat shoots and kills Anna, causing the car to crash and Alex to be captured.Alex is brought before Duvall, who asks her how many are in the precinct. Defiant, she replies "a hundred", and she is executed by Duvall after complimenting her on her bravery.With only Roenick, Bishop, Jasper, Capra, and Iris left, the precinct stands little chance of withstanding further assaults. Bishop discovers the handcuffs that were holding the back door locked are removed. He concludes that someone inside is helping the police and accuses Capra, but before anything can be done, the police begin to lay siege to the precinct with SWAT reinforcements from the rooftop when a police helicopter arrives and begins dropping men onto the roof.Jasper miraculously remembers an old sewage entrance beneath the precinct, and Roenick and Bishop set the precinct on fire to slow their pursuers. The survivors escape along an abandoned sewer passageway, but even as they find themselves on a deserted street they are suddenly surrounded by the corrupt policemen. Jasper reveals himself to have been the traitor all along, and the officers led by Duvall prepare to execute the remaining survivors.Bishop sneaks a flash grenade into Jasper's jacket, and in the ensuing distraction the survivors disarm the officers and escape. Iris and Capra attempt to escape in the officers' car, while Roenick and Bishop flee into the forest. Duvall and another officer pursue Roenick, while Kahane goes after the car. The car flips and Capra is heavily injured, but Iris manages to stab Kahane with his own knife.In the nearby forest, Roenick is suddenly confronted by Bishop, who offers to cooperate in order to kill Duvall. As the corrupt officers have night vision and Roenick and Bishop do not, Roenick volunteers to draw their fire so Bishop can see the muzzle flashes from their weapons, and dispose of them while they are distracted. The plan works, and Bishop kills the accompanying officer, but he is then shot in the stomach by Duvall who orders Bishop to lure Roenick into a trap. Roenick arrives and guns down Duvall, but is shot in the side and injured also.After considering their cooperation throughout the night, Bishop decides to spare Roenick, who vows that he will find and arrest him someday. After Bishop limps away, officers led by Iris arrive. An officer asks Roenick if anyone else is in the forest, and after looking around for a moment he replies "no". The last shot is of an injured Roenick and Iris making their way to the ambulance, where Iris states she did not even recognize the battle-hardened Roenick of the night, and he replies, "get used to it."
Output: Question: Why were prisioners set free?, Answer: To help repel the invasion of the Anderson police precinct station.