Pia isn't really what you'd call houseproud, but once a year the place does need to be cleaned properly! The children aren't very enthusiastic about this task, though Stefan decides to do his bit by polishing his beloved motorbike in the shed. Kalle just about manages to rescue his squeaking rubber duck from Pia's vacuum cleaner. He decides to hide it in the shed, and as he does so he comes across a painting entitled Nude on the Beach. Pia never rated the picture very highly and thought it had been thrown out long ago. But Stefan hands it over to his friend Kostas, who seems to be able to make money out of just about anything, after all.
Meanwhile, a spectacular art robbery comes to light in Flensburg. 15 pictures are stolen from the studio of a painter called Niehoff, who died not long ago. In the painter's obituary in the Flensburg Evening News Stefan described him as "the Picasso of the North German shore" - without suspecting for a moment that he himself possessed a genuine Niehoff.
Pia and Olli initially suspect that the theft is some kind of insurance trick, since the works of art were insured for €100,000 even though none of them sold for much while the painter was still alive.
In contrast, Lorenz suspects this is an organised art robbery.
Meanwhile, Kostas opens a shop of his own - the Appolo Art Shop - where the Nude on the Beach is on sale for €10,000. A potential buyer appears, who has already seen some Niehoff paintings in a souvenir shop; he says he would like to buy the painting but can't afford it.
The following day Kostas reports to the police that the painting has been stolen. With Kostas’s help, Pia and Olli track down the thief, who turns out to have a very personal motive. But this doesn't mean that the case is solved, not by a long chalk. And it is Kalle - again searching for his squeaking rubber duck - who comes up with the crucial clue.