When Jan Servaas, a world famous Dutch violinist, makes a rare concert appearance in his home country it is an eagerly awaited event. In the weeks leading up to the concert van der Valk receives a series of mysterious notes, written on blue paper, warning that Servaas will be ""executed"" if he comes to Amsterdam. Van der Valk ignores Kroon's pleas to have the concert canceled until the case is solved, perhaps in part because he and Arlette have tickets and are looking forward to it! It is a decision that will have tragic consequences as a drama which began with betrayal in wartime Limburg plays out to a fatal conclusion in 1970s Amsterdam.

A young German is murdered in his bed. The apartment is littered with clues. For Van der Valk there is a clue too many.