In 1946, off the coast of North Africa, young Alec Ramsey and his father travel aboard the H.M.S. Drake, a passenger ship from Liverpool, England. One afternoon, Alec sees a wild black stallion on deck. The animal rears and thrashes as a team of men whip the horse, restrain him with ropes, and quarantine him in a makeshift stall. Alec later brings the stallion sugar cubes, but is chased away by a Middle Eastern passenger wearing a headscarf. That night, Alec’s father shows him winnings from a poker game and gives the boy a pocketknife and a statuette of Bucephalus, a magnificent black horse. Alec’s father recites his version of the ancient legend of Bucephalus, an untamable animal whose owner, King Philip II, planned to kill it in an arena. However, the king’s son protested, and Philip declared the boy could keep the stallion if he could ride it. The young man succeeded, and later became Alexander the Great. As Alec and his father prepare to sleep, a fire breaks out on the ship and they race to the deck. Separated from his father, Alec tries to help the stallion escape, but the horse’s Middle Eastern keeper interferes and cuts off Alec’s life preserver. The boy’s father attacks the man and tries to toss the preserver back to his son. As the horse jumps overboard, Alec is thrown from the ship without a life vest and plunges dangerously close to the propeller. He sees the stallion’s ropes caught in the blades and cuts him loose with his new pocketknife. While the vessel sinks, Alec ties the horse’s rope around his waist and the stallion swims away. Alec awakens on a desert island with only his knife and the Bucephalus statuette. After days of solitude, he finds the stallion, tangled in his ropes. He creeps toward the rearing beast and sets him free yet again. One afternoon, the boy awakens to a cobra within striking distance of his face, but the stallion stomps the snake to death and gallops away. Over time, Alec learns to make fire and eat seaweed. He eventually lures the horse to eat dried leaves from his hand. Alec leads his new animal friend, “The Black,” into the ocean, where he mounts the horse and learns to ride. The boy and The Black become inseparable. When fishermen discover Alec, the stallion refuses to stay behind on the island and amazes the men by swimming after them. They haul The Black onto their boat, and Alec is returned home to his mother in Flushing, New York. However, the stallion is displeased by his containment in a suburban backyard, and Alec finds it impossible to readjust to domestic life. One day, The Black is startled by trash collectors wearing headscarves and gallops away through the city’s streets. Alec gives chase to no avail. Early the next morning, he awakens to the sound of a horse-drawn carriage and meets an elderly black gentleman named Snoe, who claims that his horse, “Napoleon,” knows The Black’s whereabouts. Snoe tells the boy to follow the morning star to a remote barn. There, Alec sneaks inside and falls though the floor into an underground shelter. An elderly farmer named Henry Dailey appears, insisting the horse belongs to him. However, he changes his mind when he sees Alec’s mastery over the wild animal. Henry feeds the boy breakfast and explains that he has failed at making a living. He agrees to give The Black a home on condition that Alec cleans the stable. Alec returns home but continues to work at the stable and learns that Snoe’s “Napoleon” also lives on Henry’s farm. One day, Alec discovers an abandoned office, filled with old racing trophies, and realizes that Henry Dailey was once a jockey trainer. He asks Henry to help him become The Black’s jockey, and the old man obliges. However, The Black remains wild and is resistant to saddle and reins. Henry teaches Alec to ride on a bale of hay and warns him to always hold The Black’s mane. Early one morning, Henry secretly begins training Alec and The Black on a real racetrack. He invites his former partner, Jake, to time The Black’s speed and they agree the horse has enormous potential. As training continues, Snoe tells Alec that he should keep his animal wild, but the boy is intent on proving The Black is the fastest horse alive. On another night, Henry covertly invites horseracing’s famous radio announcer, Jim Neville, and a host of investors, to the racetrack. During a thunderstorm, The Black surpasses record time, but Alec finishes the lap unconscious, clinging to the horse’s mane. The boy comes to with a handful of hair and hears Jim Neville agree to promote The Black as a “mystery horse,” keeping the identity of Henry and Alec a secret. Sometime later, Neville publically challenges the two top racehorses in the world to compete against The Black, and arranges a tournament. He encourages his radio audience to bet on the newcomer. At home, Alec tells his mother that he will ride the “mystery horse,” and Henry Dailey makes an impassioned plea on the boy’s behalf, but she is unwilling to put her son in danger. However, she changes her mind when Alec shows her the Bucephalus statuette and tearfully explains how The Black saved his life. At the Santa Anita Racetrack in Arcadia, California, a sold-out crowd anticipates Neville’s contest. Dressed in a masked uniform, Alec tries to contain The Black at the starting gate, but the stallion fights a contender and injures his leg. The race begins, but The Black falls behind, and Alec loses balance. However, the boy and his horse gallop closer toward the lead as his mother, Henry, Snoe, and “Napoleon” watch. When Alec and The Black overtake the world-champion horses and win the race, the crowd goes wild, and Alec throws his arms into the air, remembering his days on the island. A veterinarian promises Alec that The Black’s leg will heal, and the boy shows the Bucephalus figurine to his beloved stallion.