A waste disposal company has a Russian nuclear bomb to transport, and an employee decides to save money by concealing it on a freight train. The train, also loaded with hazardous and flammable chemicals, suffers a brake failure and becomes a runaway heading for Denver. A wreck would be bad enough, but much, much worse if the bomb goes off. A heroic NTSB investigator boards the train; he and the railwaymen try various ways to stop the train, but nothing works. Meanwhile Denver residents are struggling to collect their families and then leave town, despite rioters and gridlock. Will the train have to be derailed to stop it? If it does derail, will the bomb explode, and if it does, what then?