Mary Morstan has received a pearl in the post every year since her father’s disappearance; this leads Holmes and Watson to the truth about a secret pact between four convicts during the Indian Rebellion of 1857.

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?