In the 1870s, residents of the garrison at the Fort Humboldt frontier outpost of the United States Army are reported to be suffering from a diphtheria epidemic. A special express train is heading up into the remote mountain ranges towards the fort filled with reinforcements and medical supplies. There are also civilian passengers on the train in the rear luxurious private car – Nevada Governor Fairchild (Richard Crenna) and his fiancée Marica (Jill Ireland), the daughter of the fort's commander.