Holmes and Watson journey to the Lake District at the bidding of Squire Foley, who claims to be tormented by a man he believes he has killed in a duel. After an unsatisfactory conclusion they return to London, where they investigate the abduction of a woman in most peculiar circumstances.

Next, Inspector Lestrade brings them news of queer goings-on in the National Gallery, before a perplexed priest requests their help with the curious behavior of his colleagues. Early on, Watson realizes that a common thread runs through these events, and Holmes sets out to bring to justice "probably the most evil woman I ever hope to encounter".