It is May 1902 in Baker Street and Watson receives a distressing letter from his Aunt Rachel in Lymington. One of her dearest friends has, seemingly, been killed by an unearthly presence. Together, Holmes and Watson visit the scene of the death and discover that a fiendish, premeditated murder has been committed.
As a result, Watson must endure a séance in St. John's Wood where The Emerald Spirit is conjured but in a display designed to titillate rather than comfort those present.
Holmes has to take the law into his own hands and Lestrade is invited to join Holmes and Watson in one final, frightening séance. Holmes's skills and a little Shakespearean theater forces the killer to confess.