Sherlock Holmes uses his amazing brain to unravel another strange occurrence. A mysterious lodger in his own building who remains locked away and is very secretive, causes fear and agitation in his landlady who brings her concern to him. At first reluctant to get involved in such a petty matter, he is persuaded at last and proceeds to unravel a mystery that spans the Atlantic and involves a secret Italian society of murderers and blackmailers. And it has a satisfying conclusion where the killer is actually congratulated.