The secret from the title is the lost recipe for shiny Delft pottery, that Jan Vogel is desperately trying to rediscover. For his attempts he needs platinum, but the funds run out because the factory owner Van Haaften has lost his money in stock-speculations. Luckily for Jan, he finds a farewell letter in the park, including a large sum of money. Meanwhile, the owner's son Leo confesses his love for Jan's daughter Annie, but Van Haaften forbids their marriage. Because of Jan's nephew Willem Berg's ongoing spying attempts to gain knowledge of the secret recipe, Jan buys Van Haaften a gun to defend himself against the villain. One night, as Willem has secretly entered the factory, Van Haaften enters his office to find a letter from a friend with a sum of money, that is not nearly large enough to cover his losses. He writes a suicide note and shoots himself, after which Willem enters the office and steals the money and the note. When, on the following morning, Jan has finally succeeded in rediscovering the recipe and goes to show the result to Van Haaften, he finds him dead. With all circumstances against him, he is thrown in prison, but he manages to slip his daughter Lilly the just-discovered recipe. While Willem's attempts to gain hold of the recipe get more and more violent, Lilly tries to gather evidence of her father's innocence. When she has finally succeeded, she sends off a letter to her sister Annie, who by now is guarding the recipe. With the recipe she goes on her way to Lilly, but is chased by Willem...