The Secret of Platform 13 is a portal fantasy in which characters from an idyllic realm called “The Island,” travel to modern London by means of a “gump,” a portal between worlds that opens for only nine days every nine years. Nine years before the main action of the story, the Queen of The Island gives birth to a baby boy: his English nannies, homesick for their life in London, take him there on a visit. On the ninth day, he is stolen from his pram by a wealthy childless woman, Mrs Trottle. The nannies only discover his loss when they are on their way back to The Island, and it is too late to return.
Nine years later, in the present of the story, a search party sets out from The Island, comprising a young hag named Odge, a wizard named Cornelius, a fey named Gerkintrude, and a giant named Hans. Their task is to find the young prince and rescue him. The ghosts who live near the gump, which is located on Platform 13 of King’s Cross Station, direct them to the Trottles, where they find the son of the house, Raymond, a fat lazy selfish child, and Ben, the gentle household servant, raised by Nanny Brown, Mrs Trottle’s nanny.
Encouraging Raymond to return to The Island is a challenging task. The search party seek to persuade him by staging a set of magical events in a nearby park. While Ben is enchanted, Raymond dislikes the magic, but he likes the idea of being a prince. Odge asks Ben if he would come too, but Nanny Brown is ill and in the hospital, and he refuses to leave her.
Meanwhile, Raymond tells his mother about the plan. Believing they are kidnappers, she takes him to a nearby hotel to hide out. The search party tracks him down but it is difficult to capture him, because of the many bodyguards protecting him. In the end, they take Ben, who is now alone in the world, Nanny Brown having died. It is now very close to the ninth day, and the closing of the gump: fearing that the search party has failed, the King and Queen send a group of Harpies to bring back Raymond, which they succeed in doing.
However, Nanny Brown has left a letter, explaining that Ben is the true child of the King and Queen, and that Raymond is the birth child of Mr and Mrs Trottle, having been born not long after Ben was taken. In the nick of time, Raymond is hastily sent back through the gump.
The people of the Island rejoice. Odge, fearing that her growing friendship with Ben is no longer possible because he is a prince, is delighted when he invites her to come and live with him and his family at the palace.