A mad ex-cop, Howard Payne, boobytraps a lift shaft in an LA skyscraper and demands a huge ransom. SWAT team members Jack and Harry arrive and rescue those caught in the lift, and then realise Payne is still in the building. Payne captures Harry and Jack is forced to shoot his partner in the leg to rescue him. The bomb goes off, and it is assumed Payne is killed. Jack learns that he isn't when, sitting in a cafe with a bus driver friend, a bus blows up outside. Payne then calls Jack and tells him he has rigged up another bomb in a bus, which is set to go off if the bus's speed drops below 50mph. Jack races across town and boards the bus, explaining the situation. During the ensuing panic, the bus driver is killed. One of the passengers, Annie, is forced to take the wheel while Jack unsuccessfully tries to defuse the bomb. Payne allows the police to take injured passengers off the bus, while he watches all the drama on TV. But in order not to allow the speed to drop, Annie is forced onto a stretch of motorway which is not yet completed.