Buck, a physically impressive (read: jacked) dog, is living the good life in California when he gets stolen and put into dog slavery. For him, this means pulling a ridiculously heavy sled through miles and miles of frozen ice with little or nothing to eat and frequent beatings—yeah, dog slavery is no joke.
Because he's basically the definition of a domestic dog, Buck's out of his element until he begins to adapt to his surroundings and learn from the other dogs. Buck also starts having strange dreams about the primitive days of dogs and men, before the advent of cities or houses or culture. There are no rules or morality here (interesting, since Buck’s first owner was a judge), save for what is called "the law of club and fang," a kill-or-be-killed, ruthless way of thinking.
Buck becomes involved in a struggle for power with another dog, Spitz. They end up fighting and Buck wins, taking over as leader of the sled dog team. The team changes human management (new drivers) and the new people don't seem to be very competent. They’re bad drivers and end up killing everyone, including themselves. Fortunately, Buck's saved by a kind man named John Thornton moments before the group death in an icy river.
Buck becomes attached to Thornton and even saves his life several times. Buck sets off on a journey with his new master and several other men. He's loving his new life—except that he's racked with the primal urge to run off and kill things in the woods every once in a while.
Buck fights with a conundrum: should he stay with Thornton...or kill things? Be civilized...or be wild? We're guessing, since you picked up a book called The Call of the Wild, that you already know which side is more seductive for our buckaroo Buck.
At the end of Call of the Wild, Thornton is killed by the Yeehat tribe, and Buck gets a heaping helping of revenge on the people that murder his master. But there's a silver lining—Buck's now free to run with the wild dog packs...but only on the condition that he is leader, natch.