Summer. A dark, brooding season of oppressive heat and thunderstorms, and Flotsam yearns for fresh air and a sea breeze. But business is brisk in Baker Street and becomes even brisker when a lady who purports to be a spiritualist brings Sherlock Holmes a mysterious message from the dead. The legendary detective has no time for mediums, yet the message proves remarkably prescient when, later the same day, he is consulted about the Last Will and Testament of the deceased armaments tycoon, Erasmus Belladonna.

Belladonna's only child, a son, had been stolen from his cot twenty years previously and has never been seen since; shortly after the infant's disappearance, Belladonna had blown himself up in an experiment gone wrong, leaving his entire fortune to his son, to pass to him on his 21st birthday. Now, with that date looming, not one but seven young men have come forward. But which, if any, is really the missing child?