In 1874, after their father is wounded by hard men who have stolen a number of mares and Smokey, a prized stallion, sixteen-year-old Rob McCann and his adopted Native brother, Luke, ride frantically to Fort Ellice, Manitoba, in the vain expectation that the horse rustlers have stopped there. Learning that a new force called the North-West Mounted Police has come west, the brothers continue south to locate them and ask for help.

The NWMP commander, Colonel George French, desperately attempts to have the boys escorted home. However, the brothers manage to tag along on the eight-hundred-mile march of NWMP men and wagons headed for Fort Whoop-Up, in what will one day be Alberta, with the hope that they will catch up with the thieves.

Besides the terrible life-threatening hardships brought on by bad weather and brutal terrain, Rob worries about his mother and wounded father and fears he might even lose Luke and never find Smokey or the other horses.