When a senior Russian official, Gen. Malenkov (Robert Shaw), decides to defect to the west, CIA agent Harry Wargrave (Lee Marvin) is sent to lead the team to get him out. Malenkov reveals that the Russians are trying to develop biological weapons. Wargrave decides that Malenkov should travel across europe by train, on the "Atlantic Express", in an attempt to try and lure the Russians into attacking the train so that they can discover who the Russian secret agents in Europe are. During the journey they must survive terrorist attack and an avalanche, all planned by Russian spy-catcher Nikolai Bunin (Maximilian Schell).

During the cold war, a Soviet biological warfare expert plans to defect to the West. A CIA agent gets the man onboard a European train in order to aid his escape, but the Soviets are determined to stop at nothing to prevent this, including starting an avalanche which envelopes the train.