Fleeing from the police, a plague-carrying terrorist boards the Geneva to Stockholm Express and exposes its passengers to the deadly virus. Colonel MacKenzie is called in to handle the situation and contacts a doctor aboard the train. Jonathon Chamberlain, who finds the sick man to try and stop the plague spreading to the other passengers. Chamberlain tries to evacuate the sick man into a hovering helicopter without success and the terrorist dies. MacKenzie's only choice to stop the plague spreading is to re-route the train via the Cassandra Crossing, where it will plunge into oblivion.

As the train departs after being sealed up and pumped full of oxygen for it's final journey, the passengers begin to show signs of improvement. Chamberlain realises that the enriched air is the antidote to the bacteria, but the train is already on it's way to the Cassandra Crossing. His only hope is to convince Colonel MacKenzie that the train is no longer a threat before it hurtles to it's doom.