When diphtheria breaks out at Fort Humboldt, a train is dispatched with medical supplies and relief troops. Also on board are Utah's governor (Richard Crenna), his mistress (Jill Ireland), a marshal (Ben Johnson) and his prisoner, outlaw John Deakin (Charles Bronson). As the train passes through the mountains, soldiers go missing, telegraph lines are cut and it's discovered that there is no epidemic. There's a conspiracy afoot, and it's up to Deakin, actually a federal agent, to expose it.