Buck and Spitz are bitter enemies; this is because of the way Spitz acted as Curly was being killed; he seemed to laugh at the sweet dog's death. Buck never forgets Spitz's reaction to Curly's death or the fact that Spitz will steal food from Buck. Despite all of this, Buck is not willing to engage Spitz, instead he avoids confrontations with the lead dog. Spitz, though, tries to provoke Buck at every opportunity, but to no avail.

This lasts until the night the dogs and their humans make camp in an inhospitable spot. Buck makes himself a warm nest to rest in, only to find it inhabited by Spitz when he returns from eating his food. The two dogs fight until the camp is over run by eighty to a hundred starving dogs, who take on the dog team. These starving dogs eat all the food, tear apart the camp, and injure all the dogs. Perrault and FranÒ«ois try to defend the dogs and the camp, unfortunately their success is limited. The next morning the injured dogs, must once again, return to their job of pulling the sled towards Dawson.

The dogs, which attacked the team, are not only starving, but they are also sick. They pass on their sickness to Dolly, one of the team dogs. She, one morning, begins to howl and then she goes mad. She begins to chase Buck, who runs for his life. FranÒ«ois saves Buck by hitting Dolly on the head with an ax, putting the dog out of her misery.

Once at Dawson, the dogs are allowed to recuperate for a week, before starting the trip back to Dyea. Spitz is always trying to goad Buck into a fight, because he knows Buck is his rival for lead dog. Buck will not fight Spitz in front of Perrault and FranÒ«ois, instead he causes trouble by defending the dogs Spitz tries to punish. This causes disunity amongst the dog team, but because they cannot catch him, the humans cannot punish Buck.

Then one day a rabbit comes into camp. The camp is close to a camp of Northwest Police and their fifty dogs, so Buck and the other dogs give chase. Buck means to have this rabbit, but Spitz gets to it first and this starts the fight to the death between the two dogs. The fight does not go well for Buck at first, as Spitz is the more experienced fighter, but Buck is the more intelligent and imaginative dog. He gets to Spitz by faking to hit his shoulder, but instead he breaks Spitz's leg and then he breaks the other leg. In the end Spitz is killed by the other dogs, because he can't defend himself and Buck is left the champion.

FranÒ«ois and Perrault discover Buck has killed Spitz. Buck thinks this means he is the new leader of the dog team, but instead Sol-leks is brought in to the lead dog position. Buck will not allow this to happen and he puts himself into the lead position by driving out Sol-leks. FranÒ«ois takes Buck away and as he turns away Buck once again tries to replace Sol-leks, this behavior continues for a long time. Buck stays far enough away from FranÒ«ois to avoid being hit with the club, but he refuses to be harnessed into his traces. Finally, after chasing him for an hour, the two men give up and put Buck into the lead dog traces.

Buck proves to be an excellent lead dog. He is able to think quickly and is also capable of exerting his will over the other dogs of the team, even those who do not want to follow him are brought into the fold. The team is able to cover the distance to Skaguay in record time, making Perrault and FranÒ«ois proud of Buck and the rest of the team.

One day FranÒ«ois and Perrault are given orders to turn the team over to a Scotch half-breed, who uses them to carry the mail. The work is hard and monotonous, but the team continues to work hard under the leadership of Buck. The dogs are treated well by the men who guide the team, but they also require the dogs to travel eighteen hundred miles to deliver the mail to the gold prospectors.

Dave begins to suffer from the constant hard work. He is in pain, even though he does his job to the fullest of his abilities. He is a topic of conversation amongst the drivers, because even though he has no obvious wounds, it is clear he is in pain. The drivers decide to give him a break from being in the traces, allowing him to run beside the sled. Dave is stubborn though and demands in his own way to be put back into his proper place.

The driver obliges the tough dog, but it soon becomes clear he cannot keep up with the rest of the team. He falls and is even run over by the sled and the next morning he is too weak to walk to the sled. He tries and fails to reach the sled, so he is left behind howling in agony at being left behind and in pain. The Scotch-half breed takes pity on the dog by returning to him to put him out of his pain with a bullet. The dogs all know what fate has befallen their friend.

Buck and Spitz fight to the death. Buck, happily, becomes the new lead dog and excels at his new position. The dogs have a new mission; it is to deliver the mail. They also have new drivers, who treat them well. Dave becomes ill and is put out of his misery by the driver.