St. Louis, Missouri in 1859. The wealthy owner of the Pony Express Michael H. Arnesen partners with gambler and frontiersman Sam Colton to protect his mail service from attacks from Native American warriors and bandits working for his competitor, stagecoach owner Peter Marquette. Things are complicated by the fact that Arnesen's wife Cathy is in love with Marquette. Colton himself, falls for Arnesen's daigjter Ann.