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To escape exposure as former Jacobites, Robin and his sister Prudence have exchanged clothes and assumed new identities. Prudence appears to be a tall youth by the name of Peter Merriot escorting his petite younger sister, Kate. This scheme was conceived by their father, whom they call "The Old Gentleman." Robin and Prudence have spent many years living a peripatetic existence with their eccentric and larger-than-life father on the European continent, so their trip to London is one of the first times they have been in England, despite English being their mother tongue.
On their way to London, Prudence and Robin encounter Gregory Markham kidnapping a beautiful heiress named Letitia. The pair decide to rescue Letitia; Prudence knocks out Markham with her sword hilt, and Robin, who quickly becomes fascinated with the beautiful and naive Letitia, befriends her in his guise as Kate. Shortly afterwards, 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. Sir Anthony projects an air of bored detachment, but Prudence realises that "the sleepy gentleman" is far more observant and quick witted than his exterior would suggest.
"Peter" and "Kate" quickly rise to prominence in London society – Robin is a new beauty in town and Prudence, under the patronage of Sir Anthony, begins socializing with high circles of young gentlemen of London.
The pair is invited to a masked ball, and while “Kate” lies ill at home, Robin puts on a mask and attends the ball as a man, introducing himself to Letitia as "L'inconnue" to woo her. Before the unmasking, he departs, but promises Letitia that he will return in her hour of need.
The Old Gentleman, Prudence and Robin's father, abruptly reappears and claims to be the younger brother and legal heir of the recently deceased Viscount Barham, much to the consternation of the man, Rensley, who had long believed himself to be the heir, and who had already installed himself as the new Viscount. The Old Gentleman, now referring to himself as "Tremayne of Barham," proceeds rapidly to install himself into high society, despite the fact that his claim is, as yet, unproved. Robin and Prudence, long used to their father's delusions of grandeur and eccentric claims to greatness, are skeptical of his claims, never having heard them before. As tensions mount over who is the legitimate heir, Rensley, overhearing Prudence disparage his manners, challenges her to a duel. This is foiled by Sir Anthony who deliberately seeks to offend Rensley before the duel takes place, so that he may duel and wound Rensley before the appointed date of the duel with Prudence. 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 Merriots, 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.