Fearing that his father, the Duke of St. Pancras, is about to marry a "pseudo-Russian countess," Lord Geoffry Troon seeks the help of female detective Shirley Travis against the advice of his fiancée, Barbara Blythe. Shirley agrees to take the case and arranges to meet the duke that evening, when she will be disguised as the famous hunter, Lady Diana Drummond. At his castle, the duke excitedly prepares for Lady Diana's visit, while Countess Helenka tries to flirt with him. Later that evening, Shirley entertains the duke by telling hunting stories. Meanwhile, Helenka and her cohort in crime, von Dittersdoff, plan to trick "Lady Diana," when they suspect that she is an impostor. The real Lady Diana is reputed to be an expert horseman, unlike Shirley, who they know has never been on a horse. When they suggest that she join their hunting party the next morning, Shirley, in order to preserve her disguise, is forced to go along. While the others are asleep, Shirley tries to get the stableboy, Robert, to show her how to ride. Realizing that the duke is smitten with "Lady Diana," Helenka decides to make advances toward Geoffry, and she and von Dittersdoff finalize their plans for holding up the guests at the gala dinner which is to follow the hunt. The next morning, Shirley is given a mount, Bullet, that is too strong for her. The horse tears off, drags her through brush and throws her. The fox being pursued then lands in her lap, as the rest of the party looks on in amazement. When the duke learns that Lady Diana is married, he plans to propose to Helenka and announce their engagement at the party, but he is unable to do so because Helenka feels too ill to have company, as she suffered a fall during the hunt. At the same time, Shirley and Geoffry plan to collect evidence against Helenka; while Shirley, disguised as a nurse, busies herself in Helenka's bath, Geoffry will make love to Helenka, and Shirley will be able to overhear them. Von Dittersdoff and his gang, however, have plans of their own, and they kidnap Shirley and hide her in a house so that Helenka can compromise Geoffry. A page, who noticed Shirley being spirited away, mentions the event to Robert, and he rushes to save her. When she returns to the party, Shirley bungles the thieves' robbery plans by performing an Oriental dance number and then goes on to expose Helenka as a Russian dancer from the Folies Bergère. When the duke finds out that Shirley is a detective and not Lady Diana, he proposes to her. After accepting, she almost sits down on a tiara that was left in the seat of her chair, and the duke gallantly places it on her head while the guests applaud.