Christopher, Tom, and Sally are back in London at last. Everyone is relieved at their return--the plague has ended, and the king, pleased with their service, offers a surprising reward. But trouble has followed them home...

First, an old friend is ambushed and left for dead. Then an anonymous letter arrives at Blackthorn--a mysterious warning hidden inside a riddle and secret code. As Christopher and his friends begin to investigate, they soon discover they've stumbled upon a plot to kill the king...and anyone else who dares stand in the traitors' way.