Erasmus Septimus Knight, fresh from his previous success, is facing the New Year and new problems. Unexpectedly chosen for the lead role in the school musical, he reluctantly accepts the challenge to play Rex, a former climbing boy looking for shelter. However he has no idea what's in store. He becomes immersed in the story of nineteenth century London and in his hero, Sherlock Holmes. And before long he doesn't really know where he ends and Rex begins.