A sled dog struggles for survival in the wilds of the Yukon. During the late 19th century Gold Rush, Buck, a large, gentle St. Bernard/Scotch Collie, lives contentedly with his master, Judge Miller in Santa Clara, California. One night, Buck is abducted and shipped to the Yukon aboard a freighter. During the voyage, a crew member ill-treats him. After arriving, Buck returns a dropped harmonica to a man named John Thornton, moments before being sold to Perrault and his assistant, Francoise. They use a dog sled to deliver mail across the Yukon. Perrault hopes to make the long trek to the mail depot before the deadline. Buck is introduced to the other dogs, including the vicious pack leader, a Husky named Spitz.