Just before December 5, Sinterklaas mysteriously disappears from the castle. The Club's Pieten don't understand what's going on and become worried. When Sinterklaas reappears a little later, he says unkind things and behaves strangely. What happened to Sinterklaas? And what was that Toy Doctor doing at the castle? It is Mees and his neighbor Katja who discover what is going on with Sinterklaas and what the secret of the peculiar Toy Doctor is. Pakjesavond is in danger, and they must warn the Pieten of the Club van Sinterklaas as soon as possible! Will the Club Pieten and Mees & Katja manage to unmask the Toy Doctor in time? Will they succeed in making sure that Pakjesavond can still go ahead this year?