Annie and Lilly Vogels are the young daughters of Jan, a specialist in Delft pottery, working as a manager in a porcelain factory.
While father Jan is working on an improved procedure for making porcelain, his daughter Lilly is very busy running the household.
Her older sister Annie does not contribute as much to the household work because she focuses more on her music and her relationship with Leo, Van Haaften's son.
Meanwhile, cousin Willem Berg receives a strong reprimand due to his carelessness while working at the factory.
When the head of the rich family is informed, he wants to prevent a scandal and sends Leo away for an indefinite period to America.
Lilly pays her a visit and reveals to Annie their father is on the verge of discovering the secret to the old Delftware glaze.
At one point, Annie is discovered in the night club by an impresario and is offered a contract for a tour of concerts.
Instead, he takes the remaining 3,000 gulden belonging to the factory which he finds in Van Haaften's pockets and he also pinches his suicide note.
In place of Jan, fraudulent cousin Willem is hired as the new factory manager and is still trying to find out the Delft secret.
In the ensuing trial, Jan is charged with the murder of Van Haaften with a demanded sentence of 20 years imprisonment.
Willem struggles with the experiments, meanwhile Lily's father gives her the task to find the letter written by the person who had committed suicide in the park.
At the end of the movie Jan is cleared of all charges; he gets his job back at the factory and is finally able to complete his experiments to achieve the pottery with a special gleam.