os Angeles SWAT officers Jack Traven and Harry Temple thwart an attempt by a bomber to hold an elevator full of people for ransom. They corner the bomber, but he grabs Harry. Jack shoots Harry in the leg, forcing the bomber to release him. He turns and runs around a corner, apparently dying in an explosion. Jack and Harry are praised by their superior, Lieutenant "Mac" McMahon.
Later, Jack sees a city bus explode. The bomber contacts him, explaining that a similar bomb is rigged on another bus, and will get activated if it goes over 50 miles per hour (80 km/h) and blow up when the speed drops below 50. The bomber demands a larger ransom and says he will set off the bomb if passengers are offloaded. Jack races through traffic and manages to board the moving bus. However, it is already going over 50: the bomb is armed. Jack explains the situation to the driver, Sam, but then a panicked small-time criminal on the bus, fearing Jack is going to arrest him, fires his gun, wounding Sam. Passenger Annie Porter takes Sam's place behind the wheel while Jack explains the situation to everyone aboard. Jack examines the bomb under the bus and phones Harry, who uses clues to identify the bomber as a former police officer.
The police clear a route for the bus to a closed-down freeway. Mac insists they offload the passengers onto a pacing flatbed truck, but Jack warns about the bomber's demands. The bomber allows the officers to offload the injured Sam, but detonates an explosive under the bus's front stairs when another passenger attempts to escape; she falls out and is run over. Jack learns that part of the freeway ahead is incomplete, but gets Annie to accelerate the bus and jump the gap. Jack directs Annie to nearby Los Angeles International Airport to drive on the runway. Meanwhile, Harry has identified the bomber as Howard Payne, a retired Atlanta bomb squad officer with a local address, and takes a SWAT team there. Too late they find the house is rigged with explosives, and Harry and the team are killed.
Jack attempts to defuse the bomb while riding on a towed sledge under the bus, but it hits debris on the runway and he can only hold on by puncturing the fuel tank. He is pulled back aboard by the passengers, and learns that Harry has been killed. Enraged, Jack smashes his cell phone, but Annie helps him recover. Jack finds the hidden camera Howard is monitoring them with. Mac gets a local news crew to record the transmission, then rebroadcast it in a continuous loop to fool Howard while the passengers are unloaded before all the fuel leaks out. The rescue is interrupted by a flat tire, stranding Jack and Annie. They then fix the positions of the gas pedal and the wheel, setting the bus on its course, and escape via a floor access panel. The empty bus rams an empty 707 cargo plane and explodes.
Jack and Annie go to Pershing Square, the drop point for the ransom. Howard, realizing he has been fooled, poses as a police officer and seizes Annie. When the drop is made into a waste can, no one shows up to take it. Jack discovers a hole under the can leading into the Metro Red Line subway. Jack finds Howard and Annie, but she is wearing a vest covered with explosives and rigged to a pressure-release detonator. Howard hijacks a subway train, handcuffs Annie to a pole, and has the train set in motion; but Jack manages to board at the last second. Howard then kills the train driver. Howard attempts a bribe with the ransom money, but is enraged when a paint bomb in the money bag goes off. He and Jack battle on the train's roof. Jack forces Howard's head into an oncoming signal, decapitating him. Jack removes the vest from Annie, but she is still handcuffed to the pole. Since they cannot stop the train, Jack instead sets it for full speed, causing it to derail through a construction site and onto Hollywood Boulevard before coming to a stop. Jack and Annie both survive and kiss passionately as onlookers take pictures.