Thirty days later, the dogs and men arrive back at Skaguay, exhausted and worn down. The drivers expect a long stopover in the town, but they are ordered to deliver more mail right away. The dogs are replaced with a fresh team, and Buck and his mates are sold to two men recently arrived from the States. The new owners, Hal and Charles, are less organized and professional than the previous drivers; Hal carries a knife and a heavy gun, but they are obviously inexperienced and out of place in the Northland.

They load up the sled together with Charles’s wife, Mercedes, a spoiled, pampered woman who is also Hal’s sister. Laden with all their possessions—pots and pans, clothes and tents—the sled is too heavy to be pulled. Hal tries to whip the team, but the dogs still cannot pull the sled, even when Mercedes pleads with them to pull so that her brother will stop whipping them. An onlooker tells them to break out the frozen runners, and this time the sled moves ahead; but as they hit a steep slope, half the load slips off. Angry, Buck keeps running, with the other dogs following his lead.

Friendly townspeople help collect the goods and the dogs and advise Hal to carry less stuff and get more dogs. The load is cut in half but remains heavy. Charles and Hal buy six more dogs, but the new animals are inexperienced. Buck is generally unhappy with these new owners, who are lazy and sloppy. They travel much more slowly than they expected, because of the owners’ disorganization and Mercedes’ demands. To make matters worse, they overfeed the dogs at first, then underfeed them when they realize that they are running out of food. One dog, already injured, dies quickly when the food begins to run out, and the new dogs, weak and unused to the North, all begin to starve. Hal, Charles, and Mercedes squabble among themselves and show little compassion for the animals. Mercedes, in particular, constantly picks fights with the men and insists on riding the sled, increasing the weight and making them travel much more slowly.

At the Five Fingers, a stop along the route to Dawson, the dog food runs out, and the dog owners feed their team horsehide instead of meat. Buck pulls as long as he can and then falls down until the whip or club makes him pull again. He has wasted away from starvation and exhaustion, as have his fellows, who drop quickly. The new dogs die, and so does Billee. Soon only five dogs remain alive in the team, and these five are close to starvation. Meanwhile, springtime has come to the region, and all around them the snow and ice begins to melt.

Eventually, the team reaches John Thornton’s camp, where Thornton, an experienced gold hunter, tells them that the ice is melting and that they cannot push on without risking falling through. Hal ignores him and forces the dogs back into harness by whipping them cruelly. Buck, however, refuses to get up, sensing disaster lurking ahead on the trail, even as blows come from Hal’s whip and club. Near death, he has stopped feeling any pain. Suddenly, Thornton—who has been watching the entire display—leaps up, pushes Hal back, and stands over Buck, threatening to kill Hal if he strikes the dog again. Hal pulls out his knife, but Thornton knocks it from his hand with the handle of an ax. He cuts Buck out of his traces, and the rest of the team staggers on, dragging the sled across the snow.

John looks Buck over, checking for broken bones, but finds him simply exhausted, starved, and bruised. They watch the sled crawl over the ice. A quarter of a mile away, they suddenly see its back end drop down and hear Mercedes scream. Charles turns to run back, but then a section of ice gives way and the whole sled, dogs and humans included, drops down and disappears into the dark water.