The Traitor's Blade (The Blackthorn Key, Bk. 5)

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.