Because of the cats.

Bloemendaal aan Zee was a pleasant, slightly featureless seaside town. Modern buildings, high incomes, little crime. So why did the Amsterdam police, of all people, develop an interest in in?

What interested Van der Valk was a teenage gang. At first it was a case of robbery and rape; but things took a more sinister turn, and Van der Valk began to suspect a vicious and corrupting purpose at work. As well, of course, as the cats...