A Papal legate, on a secret mission to England from Rome, is discovered dead. How did he die? And why? Mycroft Holmes, master of many of the secrets of the realm, calls in his younger brother Sherlock to discover the answer to these questions.

When he arrives at the scene of the crime, Sherlock Holmes discovers that the Cardinal's corpse has been moved – inside a locked room. A search for a missing document which may prove to be the clue to this riddle – but Holmes and Watson find themselves caught in a tangled web of mystery and political intrigue as they investigate The Death of Cardinal Tosca.