With the money Thornton wins from the wager he sets out eastward with Buck, Skeet, Nig, Hans, and Pete, going deep into the Klondike in search of a lost mine and miner's cabin. Thornton and his team fail to find the site and end up panning for gold in a shallow river valley, but Buck enjoys wandering and living off the land with his master.
Thornton's pursuit of this lost place in the wilderness alludes to his yearning to tame the wild for profit. Finding the mine may lead to riches. But the simple way that Thornton lives proves that he is already a master outdoorsman and a successful miner.
Meanwhile, Buck spends "long hours musing by the fire," dreaming of hunting and gathering with the caveman. One night, Buck awakens from this dream to encounter a real life timber wolf, who beckons him into the forest. Buck senses the summoning of the call as they cavort in the woods, but Buck remembers John Thornton and eventually returns to his camp.
Buck's vivid vision of the caveman and encounter with the timber wolf show that Buck's wild yearnings are strengthening. He cannot yet embrace the call because of his devotion to John Thornton, the one human being who maintains a hold on him.
Buck remains by Thornton's side for two days, but grows restless, returning to the forest in search of the timber wolf. Unable to find him, Buck stalks prey, instead. As he hunts, he feels like a masterful animal, thriving in the wild. After four days of watching a giant, bull moose, he kills the animal, satiating his "blood-longing." Yet he senses that something is amiss back at camp.
The call is irresistible to Buck as his impatience to return to the forest and thirst for blood demonstrate. Killing the moose of his own accord solidifies Buck's feeling of mastery. He is attuned to the wild—he now knows he can thrive in the wild, can dominate—yet his love for Thornton draws him back to the world of humans at camp.
On the trail back to camp, Buck finds one of Thornton's dogs dying and Hans face down in the ground, dead. Seeing the native Yeehats dancing over the camp's wreckage, confirms his suspicions that John Thornton is dead. Buck, overcome by rage and grief, defies the law of club and fang, attacking the Yeehats, killing some of them, and causing the others to flee.
Buck mourns over John Thornton's body but that night hears the call. The wild wolf pack circles him. They lunge and strike at him, but he displays a wolfish agility in fighting back. An old wolf comes forward, sniffs noses with Buck in a friendly manner, and lets out a howl, announcing Buck's initiation into the pack. Buck runs with them, wild and free.
Buck's story morphs into legend as the Yeehats tell of a mythical Ghost Dog, who terrorizes the valley's natives and hunters. But apart from Indian legend, the narrator tells us that there is a handsome wolf that roams, sometimes alone, sometimes at the head of the pack, singing the "song of the younger world," and who fathers many wolf cubs.
Buck's legacy affirms his masterful spirit. Not only does he live on as a wild wolf leader, singing an ageless song, he seems to have cheated death itself by being immortalized in legend. Further, by siring wolf pups he creates a biological legacy—he becomes an ancestor for future generations of wild wolves. He has, in a sense, mastered both life and death.