Sherlock Holmes has just uncovered the truth about the theft of a priceless ruby. The wealthy Lady Damury staged the theft and tried to frame her husband - but just as Holmes reveals the truth, Lady Damury is found murdered. Holmes deduces that this is no crime of passion, but the work of a ruthless killer with no connection to the jewel. With reports of a man in a strange, trance-like state, Holmes finds himself entangled in a dangerous game of cat and mouse with the sinister Dr Caligari...