Bud Anderson is just as surprised to find out he had an uncle as he is to find out Uncle Jonas is dead.

Bud's mother insists that her older brother (who stole away from their family home in Sweden at a young age, never to be heard from again) was an bad childa disposition caused primarily by the large purple birthmark covering most of his face. But in a letter left for Bud, Jonas insists that he was driven away by hatred, and entreats his nephew to travel to Amsterdam to discover who he really was, and why he really left all those years ago.

With an old iron key and the mention of a small group of friends as their only clues, Cait and Bud travel to Amsterdam to solve the cryptic message left by Jonas, and honour the dying wish of a long-lost relative.