In their Baker Street lodgings, Sherlock Holmes and Doctor Watson meet a mysterious new client, the King of Bohemia. The nobleman explains that he once enjoyed an affair with an American, Irene Adler, who holds a personal photograph depicting them together. He has become desperate to retrieve the photograph as he is shortly to marry a princess, and details of his past dalliance would scupper the union. He fears Adler will send his fiancée the photograph when the engagement is announced, and urges Holmes to retrieve the evidence.

Disguised as a groom, Holmes inveigles his way into the confidence of Adler's staff at her London home. He gains information regarding her routine and reputation, but after following her to a church he witnesses her marriage to a lawyer, Godfrey Norton. Suspecting the wedding will prompt her to leave England, he resolves to act swiftly.

Disguised as an elderly cleric, he intervenes in a fight he has orchestrated outside Adler's home. When he is ostensibly injured, she takes him inside to recover and Watson throws a smoke bomb into her chambers. Anticipating a fire, she unwittingly reveals the secret location of the photograph which, of course, she has been eager to protect.

The following morning Holmes, Watson and the King call on her, but she has departed England with her husband. She has left a photograph of herself for her ex-lover and a note for Holmes, which explains she retained the incriminating photograph to ensure the King does not bother her in her new life. She gives her word she will not use it, and the delighted King offers Holmes a valuable ring as a reward. He declines, asking instead for the photograph of Adler, the lady whom Watson reveals, was thereafter referred to by his friend as the woman.