"National Velvet" is the story of a 14-year-old girl named Velvet Brown, who rides her horse to victory in the Grand National steeplechase. The horse which Velvet trains and rides in the Grand National is named The Piebald, because it is piebald colour.
The novel focuses on the ability of ordinary persons, particularly women, to accomplish great things. Velvet is a teenager in the late 1920s, living in a small English coastal village in Sussex, dreaming of one day owning many horses. She is a high-strung, shy, nervous child with a delicate stomach. Her mother is a wise, taciturn woman who was once famous for swimming the English Channel; her father is a butcher.
Her best friend is her father's assistant, Mi (Michael) Taylor, whose father– as Mrs. Brown's swimming coach – helped her cross the channel. Mi formerly worked in stables and is familiar with the horse racing world. One day they both watch The Piebald jump over a five-foot-high cobbled fence to get out of a field. Mi says, in passing, that "a horse like that'd win the National". Velvet becomes obsessed with winning the horse in an upcoming raffle and riding him to greatness.
In addition to inheriting several horses from one of her father's customers, a man who left them to her in his will, Velvet actually does win her dream horse. After riding him in a local gymkhana, she and Mi become serious about entering the Grand National steeplechase at Aintree racecourse and train the Piebald accordingly.
Mi uses his connections to the horse training/racing world and obtains a fake clearance document for Velvet in the name of James Tasky, a Russian jockey. Velvet wins, but slides off after the winning-post due to exhaustion, and her sex is discovered in the first-aid station.
The racing world is both dismayed and fascinated by a young girl's winning its toughest race. Velvet and The Pie become instant celebrities, with Velvet and her family nearly drowning in notoriety (echoing her mother's unsought fame after swimming the English Channel), complete with merchandising. Velvet strongly objects to the publicity, saying The Piebald is a creature of glory who shouldn't be cheapened in tabloid trash and newsreels. She insists that she did not win the race, the horse did, and she simply wanted to see him go down in history. The National Hunt Committee finds no evidence of fraud, exonerates all involved, and Velvet and her family return to their ordinary lives; or rather, Velvet goes on "to her next adventures", for clearly she is a person to whom great things happen.