With the American Civil War looming, construction of the Pacific railroad is being held up in Kansas, sabotaged by Confederate sympathizers. The government dispatches an Army engineer, John Nelson (Sterling Hayden), to oversee construction, and he initially rubs the wrong way both his proud boss (Barton MacLane) and his boss's feisty daughter (Eve Miller). Eventually, though, both fall for Nelson's charms, and the hero seeks to uncover the Southern rat so he can lead the Union men due west.