A visit from a young woman with a guilty conscience causes Sherlock Holmes and Doctor Watson to investigate the arrest of her ex-fiance. Imprisoned while awaiting trial for the murder of the lover of his wife-to-be, he seems certain to receive the ultimate punishment.
When it is realised that the sister of the murdered man has also recently died in suspicious circumstances, Holmes turns his attention to another possibility. While in pursuit of information, their informant is killed in his own house. A subsequent interview with a prisoner in Pentonville prison confirms the name of a likely suspect, a criminal known as ‘The Phantom Killer’ because of his ability to change his appearance and evade capture.
Following the trail to Devon, both Holmes and Watson are surprised to find that their suspect lives there under another name, as a prominent and respectable citizen. The local police quickly make it clear that Holmes’ presence is resented, and dismiss his purpose as ridiculous.
Nevertheless, after an initial encounter with the suspect, during which he appears friendly, charming and an unlikely adversary, several ‘accidents’ almost claim the lives of the duo. When they discover that their quarry has suddenly departed for London they follow in haste, to be eventually confronted by his murderous wrath as he boasts of killing ‘for pleasure and for profit’.