During the late 19th century, 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 beats him as a means of discipline. After arriving, Buck returns a dropped harmonica to a man named John Thornton, moments before being sold to Perrault and his assistant Francoise for their dog sled to deliver mail across the Yukon. Perrault hopes that with Buck, he can 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.
Throughout their travels, Buck gains the loyalty and trust of Perrault, Francoise and the other sled dogs, after proving himself along the way and even rescuing Francoise, antagonizing Spitz. Buck begins experiencing ancestral spiritual visions of a black wolf that acts as his guide throughout their travels. One night, Buck catches then releases a rabbit. Spitz kills it before attacking Buck to assert his dominance. Spitz seems to win, until the rest of the pack encourage Buck. Buck pins Spitz down, displacing him as pack leader; Spitz then disappears into the wild. Perrault grudgingly makes Buck the lead when no other dog assumes the position. Buck’s speed and strength allow the sled to arrive with the mail on time. There, Thornton hands over a letter he has written to his former wife expressing his feelings about their dead son. When Perrault returns, he learns the mail route is being replaced by the telegraph, forcing him to sell the dogs. Hal, a mean-spirited and inexperienced gold prospector, buys the pack and gradually works them to exhaustion carrying a heavy load in weather unsuitable for sledding.
The exhausted dogs stop to rest before Hal can force them to cross an unstable frozen lake. When Buck refuses to move, Hal threatens to shoot him. Thornton appears and rescues Buck while Hal forces the other sled dogs to cross the lake. Under Thornton’s care, Buck recovers. Later, at a saloon, Thornton is attacked by Hal, who reveals the dogs managed to run off leaving him with nothing. Witnessing the scene, Buck attacks Hal who is subsequently thrown out. Buck and Thornton then travel beyond the Yukon map where they can freely live in the wild. They come across an abandoned cabin in an open valley and settle in. Meanwhile, Hal relentlessly hunts them, believing Thornton is hiding gold.
In the open wilderness, Thornton and Buck bond over their daily activities, primarily fishing and gold panning. Throughout their time together, Buck is drawn to a female white wolf. Going back and forth between Thornton and the white wolf, Buck is conflicted by his domesticated life with Thornton and his place with the wolf pack that the female belongs to. Thornton believes it is time to return home and tells Buck he is leaving in the morning, and to come and say good-bye. Buck heads into the forest and sleeps beside the white wolf, clearly conflicted. Hal subsequently finds and shoots Thornton. Buck returns and kills Hal by pushing him into the cabin, which has caught on fire. Thornton wants Buck to live for himself. He reassures him with his final words, “It’s okay, boy. You’re home.”
The next morning, Buck returns to the wilderness. There, he mates and has offspring with the white wolf and becomes the pack leader, fully embracing the call of the wild.