Ben Taylor, an old prospector who has bragged extensively about his exploits apprehending criminals with the Mounties, loses the respect of his grandson when he turns over his hard-earned gold to a band of crooks. Sergeant Preston learns of the robbery and determines to capture the thieves with the assistance of Ben and his dog, Yukon King.
Sergeant Preston goes to Skagway, Alaska on a mail run and is asked to help solve a series of eight murders. With no official authority to arrest, he becomes Forty Mile Bill, a sourdough, and begins his investigation.
In the Beaufort Sea, the Snowbird, a ship rented by the Royal Geographic Society, is locked in the ice. Sir Philip and Coleville stay aboard while the Captain and crew abandon the ship. They are found by natives where Sergeant Preston finds them and the adventure begins.
Bart Larson, a police-hating killer jailed by Sgt. Preston, has broken out at Selkirk and plans revenge, before he returns to the U.S. with his partners. Unaware of the partners, Preston asks his superiors for permission to track Larson alone.