Buck leads a calm and contented life in the "sun-kissed Santa Clara Valley."
Ah, the life of leisure. Like most initial situations, this is rather boring and unexciting, and begs for Something Fun to happen next.
Buck is stolen by Manuel and sold to the man in the red sweater.
Something Fun happens. Or rather, something illegal and kind of sad. We identify this as a conflict based on the clubbings, illicitness, chains, and all around sinking feeling that a story is about to unfold.
Spitz and Buck clash and compete for alpha dog status.
Spitz stands in the way of Buck completing his transformation, so Buck does the only thing a protagonist can do when faced with a complication: he beats the heck out of it.
Buck’s sled team (minus Buck) crashes through thin ice and drowns.
Sled: crash. Bystanders: oh no. Reader: gasp. Sounds like a climax to us.
John Thornton almost dies. Then he actually does die.
John Thornton adds an element of tension to the end of this story. Once Thornton almost dies we begin to fear that he actually will die. Whether or not Thornton will die constitutes the suspense of Call of the Wild.
Buck leaves the civilized world.
This is told in summary, to make it even less suspenseful and more denouement-y. Buck kissing civilization goodbye for a life in the wild was sort of inevitable anyway, so we can relax once we get here.
Buck answers the Call of the Wild.
Just the kind of conclusion we would expect from a book with such a title as this.