Alexander Ramsay Jr is travelling back to New York by sea after spending a fun two months in India with his Uncle Ralph. He is enjoying the voyage and proud of the fact he has not been seasick at all. As they are preparing to leave port in Arabia, Alec sees four men attempting to lead a wild black stallion up the gangplank onto the ship. Eventually, after a long struggle, they manage to wrestle him into a stall but the stallion hates being contained and every night the passengers can hear him kicking and trying to escape.
The Drake sails through Arabia and North Africa and is heading for England when a storm hits; it is a wild, severe storm that rocks the ship. Alec puts on his lifebelt and makes his way onto deck. The ship seems to survive the storm but as it is subsiding, lightning strikes the hull and the engines go dead. The Drake seems to be cut in half and is sinking. Captain Wilson piles passengers into a lifeboat but a huge wave turns it over and it disappears into the ocean. Soon the ship begins sinking; Alec sees the black stallion break free of his stall and begin to swim so Alec grabs hold of the rope hanging from his halter and the Black pulls him along. He ploughs on through the water all night until morning when they finally reach an island. It is completely deserted and Alec realizes that he and the Black are the only survivors of the shipwreck.
Alec follows the Black to a small spring water pool and drinks thirstily. He finds a berry bush and eats hungrily. Then he turns his attention to building a shelter for himself . The rest of the day is devoted to trying to catch fish with no success. He continues to try day after day but catches only one or two. He remembers how to start a fire, cooks the fish and tries to come up with a plan for alternative foodstuffs. He hits upon a edible moss called carragheen, which both he and the Black are able to eat.
The Black seems to trust Alec, and their friendship grows. Alec is determined to ride the horse and one night throws himself onto the stallion's back but the stallion tears and Alec is thrown off.He continues to try until the horse begins to race down the beach, jumping a gulley and ending up on the sand again.
Alec's fire is caught by the wind and ends up burning his shelter down, but this turns out to be a positive thing as a passing ship sees the flames and Alec finds himself explaining his odyssey to the sailors. He refuses to leave the island without the Black. Alec is exhausted and injured and spends a few days in bed cheered by a wire from his parents who have been contacted by the sailors. When they arrive in New York, there is fanfare and scores of people which frightens the Black. To avoid over stimulating him they blindfold him which seems to help. Alec's parents are delighted to see him although his mother is a little scared of the stallion. They are approached by a young reporter who wants to interview Alec as he is the only survivor of the shipwreck off the coast of Portugal; Joe Russo procures a van for them to drive the Black home in in exchange for an interview. As they near home, in Flushing, they come to the Dailey's home. They have a stable that they do not use so Alec begs Mrs Dailey to let him use the empty stall for the Black in exchange for him doing it up. Mrs Dailey explains that there is already a horse in the barn but if the two get along then the Black can stay. Alec is concerned but the Black loves old Napoleon immediately and is greatly calmed by him.
Henry Dailey is very impressed by the Black; Henry used to be a championship jockey and really knows horses. He wants Alec to race the Black in the future but try as they might they cannot find any paperwork or lineage for him which means he cannot participate in any races. Undeterred, they begin to train him anyway, under cover of darkness, at the racetrack. He is lightning fast; one night they have Alec ride him for a time trial and find him to be much faster than the two quickest champions of the day. Sun Raider is the west coast champion and Cyclone the east, but the two have never raced each other, much to the disappointment of the public who are captivated by their rivalry. Famed sportswriter Jim Neville challenges both owners to race the horses against each other in the middle, and a race is set up for June 26. Joe Russo arrives one day to follow up on their story and Alec tells him about the Black's speed, and the fact they cannot enter him in a race. Acquainted with Jim Neville, Joe decides to invite the writer to watch another speed trial. Jim is fascinated, especially when he realizes that Henry is the same Heny Dailey who rode Chang to victory all those years ago. He is astounded by the Black's speed and decides to try to add him to the big showdown in Chicago; since it is not an official championship race, his lack of papers and history won't matter. He begins to write cryptically about him in his column, building up the "mystery horse" who is faster than both Cyclone and Sun Raider. The owners accede to having the "mystery horse" added to the ticket; Alec is off to Chicago to ride him in the race! Although his father is not travelling with them he surprises Alec by flying to Chicago to watch him. Coincidentally his mother is also in attendance although she is unaware of her son's role in the big event; she is visiting her sister, who has bought tickets to the event of the year for them both. The Black has a new racing blanket with his name embroidered onto it and Ec wears Henry's old jockey colors. Sun Raider and the Black immediately dislike each other and Sun Raider kicks at the Black as they begin. It looks like Alec and the Black are out of it before they start, but the Black starts to run and they start to claw back the ground between them and the other two horses. Sun Raider takes the lead as Cyclone drops back; with only a hundred yards to go, the Black pulls alongside Sun Raider and speeds towards the finish. His leg is bleeding badly and the veterinarian takes a look at it before Alec and the Black go to the winner's circle. The governor of the state gives him the trophy that symbolizes his supremacy on the track and then Alec learns that they have broken the world record. Alec leads the Black back through the crowd, with Napoleon at his side, before giving him his victory oats.