Strapped for cash, Luke Sunday takes a job as a scout for the army's war against the Sioux. Raised by the Cheyenne and Crow, he runs afoul of the army when they attack a peaceful Cheyenne village, believing it to be Sioux leader Sitting Bull's camp. When he accuses them of wrongdoing, the outlaw Bill Bogart leads the charge to oust him from the campaign.

Set adrift, he happens upon the Freemans, who need a guide to the Gallatin Valley. When they meet the sinister-looking Sunday, they're hesitant to hire him. But when Mr. Freeman is killed in a Sioux attack and the reckless Bogart shows up, Mrs. Freeman must put her trust in the man called Sunday.