Buck, a proud and strong St. Bernard mix, lives a princely existence on Judge Miller's estate in Santa Clara. Living an insular life, he has no idea that the discovery of gold in the Klondike has created a demand for dogs like him. Manuel, a gardener on the estate desperate for money, kidnaps Buck, selling him to a dog trader. The man throws Buck into a crate on a train headed north.
Four day's later Buck's crate is unloaded from the train. At first chance, he leaps out, attacking a man in a red sweater. The man stuns Buck with his club, beating him into submission, until Buck learns to obey. Buck spends his days watching other dogs suffer the same treatment, until two Canadian couriers, François and Perrault, purchase him, Curly, Dave, and Spitz for their sled dog team. They sail on The Narwhal to Dyea, Alaska, where Buck encounters snow for the first time.
Within hours of making landfall, Buck sees Curly attacked by a husky, then trampled by the rest of the sled dogs. Her death teaches Buck a valuable lesson about the law of club and fang. He learns to always strive for his survival by never letting his guard down. Buck's lessons continue on the trail, where François harnesses him to the traces for the first time. Under the tutelage of Perrault and the dogs Spitz and Dave, Buck transforms into a working sled dog. He learns not only from their experience, but also from the wild instincts awakening within him.
Mastering his ability to scavenge, steal, and fight, Buck's desire to dominate the sled dog team increases, and his rivalry with the lead dog, Spitz, intensifies. Yet conditions on the trail postpone their imminent showdown. A pack of mad huskies attack the camp, Dolly subsequently goes mad from rabies, and the team struggles crossing treacherous stretches of thin ice. Buck also instigates a mutiny among the weaker dogs. One night, while Buck leads the team on a rabbit hunt, Spitz makes his attack. The rivals spar for supremacy of the pack, but Buck manages to break Spitz's leg, claiming victory.
Buck becomes a masterful leader, making record runs across the Klondike. Yet demands for more mail force François and Perrault to hand over Buck and his team to a Scottish courier, who wears them down with heavy mail loads. On the trail, Dave falls ill, compelling the Scotsman to shoot him out of mercy. Exhausted from this journey, the team is handed over to Hal, Charles, and Mercedes, American fortune seekers, who mistreat Buck and his team terribly. They force them to toil under dense loads, starve them, and beat them incessantly.
Barely alive, Buck's team arrives at White River, where the ice is starting to melt. A local camper, John Thornton warns Hal against crossing, but Hal belligerently beats his animals into moving forward. Buck, sensing danger, refuses to rise. Hal, incensed, beats Buck with his club. Thornton tackles Hal and cuts Buck out of the traces, saving him from the brink of death. Hal, humiliated, proceeds towards the riverbank, while Buck and Thornton watch the sled fade into the distance and suddenly drop beneath the ice.
As Buck recuperates under Thornton's care, he develops a deep affection and loyalty for his "ideal master.” Buck demonstrates his devotion for Thornton many times over, defending him in a bar fight, saving him from drowning, and winning a $1600 bet for him by pulling a sled loaded with one thousand pounds.
With the winnings, Thornton takes Buck deeper into the Klondike in search of a lost mine. While Thornton pans for gold, Buck, haunted by visions of a caveman and the wild's beckoning call, explores the forest. He runs with a timber wolf and hunts prey on his own, but returns to Thornton's campsite when he senses that a catastrophe has occurred.
Seeing the Yeehats dance over the ruins of the camp confirms Buck's suspicions that they have murdered Thornton. Driven by rage, he launches into an attack, killing the chief and overturning the law of club and fang. Buck answers the call when a wolf pack initiates him into their ranks. He runs with them, eventually becoming their leader and a legend.