In The Adventures Of Sherlock Holmes, Watson tells us that Holmes always refers to Irene Adler as 'the woman' because she is unique among all women: she is the only woman who has ever outwitted the incomparable Sherlock Holmes. In the story A Scandal In Bohemia, Irene Adler was a one time mistress of the king of Bohemia. However, now that the king is preparing to marry the second daughter of the king of Scandinavia, he is anxious to retrieve some incriminating love letters and a photograph from his former lover.
Irene has threatened to send a photograph of both of them to the king's betrothed. The king is worried sick that the wedding will be called off if Irene is allowed to carry out her threat. Holmes thinks that this case will be a piece of cake. He follows Irene and becomes a witness to her wedding with Godfrey Norton. After soliciting the help of some street loafers to create a commotion when Irene arrives home, Holmes, in the disguise of an elderly priest (and with Watson's help), manages to discern where Irene has hidden the photograph. However, Holmes does not bargain for the fact that Irene uses her own disguise to track him back to his lodgings and to ascertain his true identity as England's master sleuth.
The next day, when Holmes calls at Irene's lodgings, he finds that his quarry has left the country with her new husband. She has taken with her the photograph of her and the king as insurance against any future unpleasant actions the king may see fit to subject her to. Meanwhile, the grateful king tries to reward Holmes with his own emerald ring, but Holmes tells him that he infinitely prefers a more valuable possession of the king's : Irene's photograph. This photograph becomes a physical memento of Holmes' admiration for a woman 'who eclipses and predominates the whole of her sex.'