The great hypnotist Professor Montserrat has developed a technique for controlling the minds, and sharing the sensations, of his subjects. He and his wife Estelle test the technique on Mike Roscoe, and enjoy 'being' the younger man. But Estelle soon grows to love the power of controlling Roscoe, and the vicarious pleasures that provides. How far will she go, and can the Professor restrain her in time?
Holmes is curious about a series of attacks on figures of the French Emperor Napoleon Bonaparte around London. One such incident leads to a violent death, and he investigates. Lestrade sees an Italian connection and proposes to focus Scotland Yard's attention on the Italian quarter of London, but Holmes is more interested in Chiswick.