Buck's instinct concerning the three amateur adventurers proves to be correct; Charles, Hal, and Mercedes continue on their way across the frozen snow and ice and lose their lives, plus the lives of the rest of the dog team when they try to cross a river of melting ice. All are drowned.
Meanwhile, John Thornton, who is recuperating from frostbitten feet, nurses Buck back to health and wins from Buck a deep devotion and loyalty. Yet, even though Buck is tamed to a certain extent by the kindness of his new master, at times while he sits with John Thornton in the depths of the forest, Buck hears mysterious calls from the wild — calls which awaken long-sleeping instincts within him.
As John Thornton returns to civilization with Buck, a drunken miner attacks John Thornton and threatens to do him harm. Buck immediately reacts and kills the man. Later on, John Thornton is lost in some fast river rapids, and once again Buck saves his master's life by swimming to him with a tow line. On another occasion, Thornton makes a brag that Buck can pull a sled with a thousand pounds loaded atop it. Because of his great love for John Thornton, Buck finally succeeds in moving the heavy sled one hundred yards.
With the money that Thornton wins from his betting feat, sixteen hundred dollars, he goes deep into the wilderness in search of a fabled lost gold mine. There, he works long and hard hours, and while Thornton's men are panning for gold, Buck often goes off by himself in the wilderness in order to stalk wild animals, or catch salmon, or run with the wild wolves; one time, he even spends four days stalking a huge bull moose. Returning to camp, Buck discovers that everyone, including John Thornton, has been killed by Yeehat Indians. Without thinking and without fear, Buck attacks the entire group of Indians, killing several and driving the rest away in such fear that the valley in which Buck revenges his master is from then on considered by the Indians to be a demonic place.
After John Thornton's death, Buck is free of all his attachments to civilization, and so he joins the wild wolves, and as legend has it, he becomes the sire of a new breed of wild dogs which still exists in the wild places of the Great North, loping through the cold nights, with Buck leading them, singing "the song of the pack."
