15-year-old Enola Holmes has all the wits and skill of her more famous brothers, Sherlock and Mycroft, and is now living independently in London with a solidly established reputation as a sleuth. When a young professional woman, Miss Letitia Glover, shows up on Sherlock's doorstep, it is Enola who takes the case. It seems her twin sister, the former Felicity Glover, married the Earl of Dunhench and per a curt note from the Earl, has died. But Letitia Glover is convinced this isn't the truth, that she'd know — she'd feel — if her twin had died.
The Earl's note is suspiciously vague and the death certificate is even more dubious, signed by a John H. Watson, M.D. (who denies any knowledge of such). Ignoring her brother's objections, Enola decides to go undercover, and finds out that this is not the first of the Earl's wives to die suddenly and vaguely. The secret to the fate of the missing Felicity is tied to a mysterious black barouche that arrived at the Earl's home in the middle of the night. To uncover the secrets held tightly within the Earl's hall, Enola is going to require a little help from Sherlock and an old friend, the young Viscount Tewkesbury, Marquess of Basilwether — but she is determined to crack the case and find out what really happened to Felicity, no matter what it takes!