An undercover agent tries to solve a series of murders aboard a train transporting US Cavalry troops and medical supplies to an isolated and disease-stricken military outpost. Murder mystery Western, adapted by Alistair MacLean from his own novel, starring Charles Bronson, Ben Johnson, Jill Ireland, Richard Crenna and Charles Durning.

At the height of the frontier era, a train races through the Rocky Mountains on a classified mission to a remote army post. But one by one the passengers are being murdered, and their only hope is the mysterious John Deakin, who's being transported to face trial for murder.