In New York City, four men separately board the southbound Lexington-Pelham subway beginning at 51st Street, carrying concealed weapons and dressed similarly in hats, trench coats, glasses and false mustaches. Referring to one another by the names Mr. Grey, for Giuseppe Benvenuto, Mr. Brown for George Steiver, Mr. Green for Harold Longman and Mr. Blue for Bernard Bryer, the men strategically align themselves in the first car where the leader, Blue, orders driver Denny Doyle at gunpoint to surrender control to Green, a former subway employee. After Green halts the train between stations, he disconnects the rear nine cars then Blue orders Doyle to walk the passengers back to the last station as the remaining seventeen passengers in the lead car look on in disbelief. At the Grand Central control center, supervisor Caz Dolowicz is puzzled, then angered, by a report that train Pelham One Two Three is motionless and not answering hails from the transit command center. In the meantime, Blue orders the lead engine car to start, while in the command center supervisor Frank Correll watches the lighted subway map, which shows the movement of all trains, and realizes that the train has been split in two. While a shocked Correll radios Dolowicz to demand an explanation for the erratic movements of Pelham One Two Three, Blue tells the passengers that they have been taken hostage and Green stops the car again just beyond the 28th Street station. Furious that the blocked tunnel is now backing up the entire subway system, Dolowicz sets off for the 28th Street station. At the command center, Correll receives a radio hail from Blue, just as transit police Lt. Zachary Garber arrives with a tour of Japanese VIPs. Although Correll is incredulous at Blue’s hijacking announcement, Garber accepts it as authentic and, after sending the VIPs away, radios Lt. Rico Patrone at the transit police office to inform the uptown police of the situation. Arriving at the tunnel, Dolowicz runs into Doyle and the released passengers and, hearing of the hostage situation, berates Doyle for not remaining with the engine, ignoring the engineer’s explanation that the four hijackers are heavily armed. Although also affronted by the hijackers’ actions, Correll radios the Pelham train for further clarification. Blue answers and advises the command center that seventeen hostages, including a conductor trainee, are being held for a million dollar ransom that must be delivered in one hour, or a passenger will be executed every minute. Garber then gets on the line to inform Blue that the request is impossible, but after demanding that the power be cut throughout the transit system, Blue breaks off radio contact. Conferring with Patrone, who has listened in on Blue’s communication, Garber suggests that the uptown police will take too long to respond and asks Patrone to bring in any transit police at the station nearest the halted train. When Patrone contacts Patrolman James, he reveals that Dolowicz is in the tunnel headed toward the train, prompting Patrone to order James to bring the supervisor back. Meanwhile, in upper Manhattan at Gracie Mansion, the mayor receives a call from deputy-mayor Warren LaSalle and is stunned to hear of the train hijacking. On the train, an anxious Green, who is battling a head cold, sneezes continually, but joins Blue in the driver’s booth. Blue, however, orders him back into the car, revealing that he does not trust the hot-tempered Grey. Just then, Dolowicz comes within sight of the stopped car and shouts that it is illegal to interfere with the transit authority. When Dolowicz ignores Grey’s command to halt, Grey sprays the tunnel with machine-gun fire, killing the supervisor. James, following some distance behind Dolowicz, takes cover at Grey’s initial warning and escapes harm, then radios Patrone and, in a whisper, reports the killing. Garber then receives contact from transit police Capt. Costello, who reports that a plainclothes police officer is among the hostages. Back at Gracie Mansion, the mayor reluctantly meets with the police commissioner, the head of the transit authority and the city comptroller and, on the advice of his wife, agrees to authorize the ransom. Meanwhile, the frustrated city police commissioner orders snipers posted throughout the darkened subway tunnel around the train. Blue radios Garber to inquire about the ransom status and when the nearby Green sneezes, Garber dryly offers a "Gesundheit." Moments later, when Garber relays the ransom approval to Blue, the hijacker responds by announcing the number of minutes left to deliver the money. Despite Garber’s protests, Blue remains unyielding and proceeds with specific instructions, which are interrupted by another of Green's sneezes and Garber's automatic "Gesundheit." Concluding that one of the hijackers knows how to drive a subway train, Garber asks Patrone to identify former transit employees fired within the last decade. Meanwhile, after frantic bank efforts, the ransom money finally is provided to a patrol car, which then speeds across the city towards 28th Street, but crashes trying to avoid a pedestrian. Police Chief Daniels then radios Garber to request more time as another escort for the money is hurriedly arranged. When Garber relates the activity to Blue, he concedes that the time limit can be met when the money arrives at 28th Street. Knowing that they still will run out of time, Garber suddenly realizes the hijackers are dependent on his information and falsely informs Blue that the money has arrived. Although skeptical, Blue allows five minutes to deliver the bag to the car by two unarmed policemen swinging a flashlight. When the money finally arrives at the station, Daniels orders two hapless patrolmen down into the tunnel to make the delivery. After an over eager sniper sets off a volley of gunfire in the tunnel that wounds Brown, Blue kills the conductor trainee in response. Dismayed, Garber nevertheless radios James to order the delivery cops to continue to the train car with the ransom. After the patrolmen successfully turn over the money to the hijackers, Blue contacts Garber to demand that power be restored, the tracks cleared of all police and all signal lights switched to green down to the South Ferry station. After Garber agrees and reports to Daniels, he joins the chief when he decides to follow the train in a car on the streets above. When Blue orders the engine to move before the tracks are cleared, there is panic in the command center. Daniels asks Garber if the hijackers could have jumped off the train, but Garber says there is a safety element called the dead man’s switch that requires the force of a man’s hand on the throttle to keep the train in motion. Green stops the engine at 18th Street and sets up an elaborate device made from pipe that allows him to restart the car from outside and lock pressure on the throttle. After the hijackers disembark, as Green starts the train, the armed plainclothesman, dressed as a hippie, jumps off the moving car. On board the car, the passengers note the continual green lights and, as the car picks up speed, begin to panic. Above ground, Garber finally convinces Daniels the hijackers must have disembarked and heads down into the subway. The hijackers congregate at a stairwell to divide the money and dispose of their disguises, but when Grey refuses to surrender his gun and threatens Blue, the leader kills him. Just then, the dazed plainclothesman revives and, seeing Brown leaning out of the stairwell, shoots him. Ordering Green to meet him later, Blue goes down the tracks to confront the plainclothesman. Blue wounds the plainclothesman, but as he is about to finish him off, Garber arrives. Realizing that he is trapped, Blue drops his gun, then purposely steps on the electrified third rail, electrocuting himself. Meanwhile, the train car’s excessive speed triggers an emergency safety brake, enabling the passengers to survive unharmed. That evening, Patrone joins Garber to question the nine fired former transit employees. After several fruitless interviews, the policemen arrive at Green’s apartment, where he is inside celebrating with the piles of ransom money. Alarmed when the police identify themselves, Green hides the money in the oven, then opens his door for Garber and Patrone. Because their questions seem to lead nowhere, the men are about to depart when Green sneezes. As Garber automatically says “Gesundheit,” he realizes that he has his man.