In the film
A man calling himself Ryder hijacks Pelham 123, a New York City Subway 6 train that departed from Pelham Bay Park station at 1:23 p.m. Ryder is accompanied by three other heavily armed men; Bashkim, Emri, and former train operator Phil Ramos. They uncouple the front car from the rest of the train and take the passengers hostage. MTA employee Walter Garber is working the Rail Control Center as a train dispatcher and receives a ransom call from Ryder, who demands $10 million in cash be paid within 60 minutes. Ryder warns that every minute past the deadline they force him to wait, he will execute a hostage.
Bashkim kills a plainclothes Transit Police officer who attempts to arrest him. He and Ramos then allow all the passengers not in the front car to be released except for the motorman. Garber reluctantly negotiates with Ryder and develops a rapport, while Ramos and Emri set up internet access in the tunnel. Ryder uses his laptop to watch the Dow Jones Industrial Average plunge nearly 1,000 points during the next hour in response to the taking. One of the passenger's laptops also connects to the internet, and the computer's webcam is activated. The webcam allows the people in the control center to monitor the train, which they use to observe Ryder and Ramos. Lieutenant Camonetti of the NYPD Emergency Service Unit enters the RCC to take over negotiations and Garber is ordered to leave the premises. The change infuriates Ryder, who shoots and kills the train's motorman in order to force Camonetti to bring Garber back.
Camonetti learns that Garber is being investigated for allegedly accepting a $35,000 bribe over a contract for new subway cars. Ryder also discovers the allegations through online news reports and forces Garber to confess by threatening to kill a passenger. Garber explains why he took the bribe and tells Ryder that he used the money to pay for his child's college tuition. Ryder expresses his admiration for Garber's willingness to risk himself to save a stranger. Meanwhile, the Mayor agrees to deliver the ransom money to Ryder and orders the NYPD to deliver it. On the way to the delivery point, the police are involved in an accident and fail to get the money there in time. Garber attempts to bluff Ryder by telling him the money has been dropped off, unaware that Ryder has been monitoring events on his laptop and knows he's being lied to. An enraged Ryder threatens to execute one of the children hostages and the child's mother. Another hostage, a former soldier, sacrifices himself to save the mother and child and is killed. A short gunfight erupts after an NYPD sniper is bitten by a rat and accidentally discharges his weapon, killing Ramos.
Based on clues that Garber receives during his conversations with Ryder, the NYPD discover that Ryder's real name is Dennis Ford. He was a manager at a private equity firm before being sentenced to prison for investment fraud. Ford had agreed to a plea bargain for a three year sentence, but was instead sentenced to ten years by the judge. One of the Mayor's aides mentions the extreme drop in the major stock indexes in response to the train hijacking, and the Mayor deduces that Ryder is actually attempting to manipulate the market via put options. Ryder demands that Garber himself deliver the ransom money, and Garber is given a pistol and flown to the terminal to make the drop. Ryder brings Garber aboard and orders him to operate the train to the next station, where he and the hijackers exit the train during a brief stop. Ryder then uses a device to rig the train to go on without them. Garber manages to separate himself from Ryder at a railway crossing and then follows him as he escapes to the Waldorf-Astoria Hotel. Ryder parts from Bashkim and Emri, who are later shot dead after being surrounded by police.
The train comes to a halt safely, and police learn that Ryder is no longer on board. Ryder hails a taxi while Garber follows him in a truck. Ryder checks his laptop and finds that his scheme has amassed a $307 million profit. He leaves the cab on the Manhattan Bridge and takes the bridge's pedestrian walkway but Garber catches up to him. Garber holds Ryder at gunpoint, and Ryder gives him a 10 second ultimatum to pull the trigger. In the final seconds of the countdown, Ryder pulls out his gun and forces Garber to shoot him. As he lies dying, Ryder tells Garber that he considers him a hero. Afterward, the mayor thanks Garber for saving the hostages and reassures him about his bribery charges. The film concludes with Garber walking into his home carrying a bag of groceries, including a half-gallon of milk he promised his wife, Theresa, he'd bring home earlier in the film.