Arriving in the chilly North, Buck is amazed by the cruelty he sees around him. As soon as another dog from his ship, Curly, gets off the boat, a pack of huskies violently attacks and kills her. Watching her death, Buck vows never to let the same fate befall him. Buck becomes the property of Francois and Perrault, two mail carriers working for the Canadian government, and begins to adjust to life as a sled dog. He recovers the instincts of his wild ancestors: he learns to fight, scavenge for food, and sleep beneath the snow on winter nights. At the same time, he develops a fierce rivalry with Spitz, the lead dog in the team. One of their fights is broken up when a pack of wild dogs invades the camp, but Buck begins to undercut Spitz’s authority, and eventually the two dogs become involved in a major fight. Buck kills Spitz and takes his place as the lead dog.