After a sudden outbreak of diphtheria, the authorities dispatch an express train carrying reinforcement soldiers and medical supplies through the frozen West across the Rocky Mountains to aid the remote garrison of Fort Humboldt. Along with the troops, there are also aboard Governor Richard Fairchild; the outpost commander's daughter, Marica; the squad's captain, Major Claremont; the U.S. Marshal, Pearce, and his mysterious prisoner, John Deakin. Indeed, this was supposed to be a routine operation; however, before long, in this confined but well-guarded place, missing officers and unexplained deaths start to shroud the mission with mystery and suspicion, as an unstoppable unknown killer is picking them off one by one. Who is the murderer in their midst?