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To escape exposure as former Jacobites, Robin and his sister have exchanged clothes, and Prudence appears to be a tall youth by the name of Peter Merriot escorting his younger, beautiful sister Kate. This scheme was conceived by their father, whom they call "The Old Gentleman."
On their way to London, Prudence and Robin encounter Gregory Markham kidnapping a beautiful heiress named Letitia, who intrigues Robin. Prudence knocks out Markham with her sword hilt, and a friend of Letitia's father, Sir Anthony Fanshawe, arrives on scene to discover that the elopement has already been frustrated.
Prudence finds herself oddly drawn to Sir Anthony, whom her brother dubs "the Mountain" owing to his massive frame. She slowly realises that Sir Anthony is far more observant and quick witted than his exterior would suggest.
"Peter" and "Kate" take London's society by storm – Robin becomes the celebrated beauty of town and Prudence, under the patronage of Sir Anthony, is inducted into the high circles of the London Gentlemen.
The pair is invited to a masked ball, and while “Kate” lies ill at home, Robin disguises himself and attends the ball, introducing himself to Letitia as "The Unknown" to woo her. Promising to return in her hour of need, he vanishes.
The Old Gentleman, Prudence and Robin's father, appears on the London scene as the long lost Viscount Barham and proceeds rapidly to ingratiate himself into high society, despite the fact that his claim is, as yet, unproved. Overhearing Prudence disparage his manners, the much frustrated Mr. Rensley, the current Lord Barham who stands to be ousted from his title by the Old Gentleman's claim, challenges her to a duel. This is foiled by Sir Anthony who manages to wound Mr Rensley before the duel takes place. Startled by his unaccountable intervention, Prudence begins to wonder if Sir Anthony suspects her masquerade.
Prudence is invited to dine with Sir Anthony alone, and it is revealed that, despite his air of oblivion, the observant Sir Anthony has guessed that "Peter" is actually a woman. Having fallen in love with her, he asks her to marry him. Prudence refuses to marry him until her father's claim is proved, therein elevating her to a status worthy of his hand. Sir Anthony agrees to wait but informs Robin, the Old Gentleman and Prudence that, whatever the outcome, he will carry her off and marry her when that time comes.
Markham, meanwhile, has obtained a document that could send the Old Gentleman to the gallows by proving he is a Jacobite. In an attempt to blackmail him, Markham exchanges the document for a letter that could expose Letitia's father as a traitor. He threatens Letitia, who is an heiress, with the letter and induces her to run away with him again. This event, despite Markham's belief, was orchestrated by the subtlety of the Old Gentleman, who is known for his great intelligence and cunning.
The Old Gentleman dispatches Robin, disguised as a highwayman, to kill Markham and thwart the elopement, thereby disposing of the nuisance Markham and inspiring Letitia to fall deeper in love with her Unknown rescuer. Robin tells Letitia that the next time she sees him, he will claim her as his bride. When questioned by the authorities, Letitia gives a false description of the "highwayman" to protect her love. Unfortunately, she unwittingly provides an exact description of Peter Merriot.
Prudence is arrested by officers of the law, and reflects that any deviation from the exact plans of the Old Gentleman, such as her presence on the night of the elopement, results in disaster. Sir Anthony, informed of her arrest, rescues her from the officers of the law and they gallop cross-country to the residence of Sir Anthony's sister. There, “Peter” dons a gown and becomes the dazzling Miss Prudence Tremaine of Barham. Having spent so long alone in his company following the escape, Prudence must now marry Sir Anthony and, happily, she consents to wed the man she loves.
Following “Peter’s” disappearance, suspicion is cast over both the Marriots, and so "Kate" flees to France until the battle over his father's inheritance is resolved. The Old Gentleman proves conclusively that he is Tremaine of Barham and Robin returns from France, causing a sensation as Mr. Robin Tremaine, heir to the Viscounty of Barham. Calling on Letitia's father, the future Viscount is readily accepted as a future son-in-law. Robin reveals himself to Letitia as the Unknown of her dreams, and she consents immediately to be his bride.
Watching Robin, Letitia, Prudence and Sir Anthony together on his newly acquired estates, the Old Gentleman reflects that he had exactly planned everything to this end, and remarks, quite truthfully, that he is a Great Man.