The story opens with Buck, a large and powerful St. Bernard-Scotch Shepherd, living happily in California's Santa Clara Valley as the pampered pet of rich Judge Miller and his family. However, he is stolen by the gardener's assistant, Manuel, and sold to finance his gambling addiction. He is shipped to Seattle. Put in a crate, he is starved and ill-treated. When released, he attacks his overseer, known only as of the "man in the red sweater" but this man teaches the "law of the club", hitting Buck until he is sufficiently cowed (but the man shows some kindness after Buck stops). Buck is then sold to a pair of French-Canadian dispatchers from the Canadian government, François and Perrault, who take him with them to the Klondike region of Canada. There, they train him as a sled dog. From his teammates, he quickly learns to survive cold winter nights and the pack society. A rivalry develops between Buck and the vicious, quarrelsome lead dog, Spitz. Buck eventually beats Spitz in a fight. Spitz is killed by the pack after his defeat by Buck, and Buck eventually becomes the leader of the team.
When Francois and Perrault reach Dawson with their dispatches and are given new orders from the Canadian government, the team is then sold to a "Scottish half-breed" man, who is also working the mail service. The dogs must carry heavy loads to the mining areas, and the journeys they make are tiresome and long. One of the team, a morose husky named Dave, becomes sick and is eventually shot
Joe Bronson, instead of studying for a school exam, goes out kite-flying with his school friends; on their way back he gets involved in fights with gang members in a poor part of the city. After he fails the exam the next day, he walks out of school and takes a ferry across the bay to Oakland. Looking at the boats on the wharf, he imagines the exciting life on a boat.
His father, a businessman, has a liberal attitude to his son; but, critical of his recent behavior and poor school report, tells him that he might send him to a military academy. Joe later leaves a farewell note for his family; returning to Oakland, he joins the crew of a sloop, the Dazzler. The captain Pete Le Maire is known as "French Pete", and the one other crew member is 'Frisco Kid, a boy of about Joe's age.
He soon realizes that French Pete is involved in criminal activity. They take scrap iron from a factory; the job is abandoned when shots are fired. Later, they work as oyster pirates.
Joe, not wanting to be involved in crime, tries to escape, but each time is thwarted. French Pete tolerates Joe's opinion of him that he is a criminal. 'Frisco Kid tells Joe that he hates his life at sea; he had no family, and once worked for Red Nelson on another sloop, the Reindeer, but ran away. Arrested as a tramp, he was sent to a "boy's refuge", where conditions were intolerable; he escaped and joined French Pete. Joe resolves to leave and take 'Frisco Kid with him.
French Pete and his associate Red Nelson steal a safe. Joe sees that it belongs to his father's company. The Dazzler and Reindeer sail into the Pacific, pursued for a time by a yacht; they intend to sail to Mexico. There is soon a storm and the Dazzler's mast breaks. The Reindeer gets close enough for French Pete to jump onto it but, before the boys can follow, the Reindeer disappears under the waves.
The Dazzler drifts ashore at Santa Cruz, finding a small wharf on the lower San Lorenzo River. Joe goes to his father's office. His father makes him "feel at once as if not the slightest thing uncommon had occurred. It seemed as if he had just returned from a vacation, or, man-grown, had come back from some business trip." His father, after hearing his story, says that the $5000 reward for the return of the safe would be shared, 'Frisco Kid's half being held in trust for his future.
