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To escape exposure as former Jacobites, Robin and his sister Prudence have exchanged identities and assumed new names. The tall Prudence appears to be a youth by the name of Peter Merriot, who is escorting the slighter Robin to London in the guise of his younger sister, Kate. They are to meet their eccentric and larger-than-life father, who they call "the Old Gentleman", with whom they have been living a chancy and peripatetic existence on the European continent since birth, despite their English nationality.
On their way to London, Prudence and Robin encounter the villainous Gregory Markham eloping with a beautiful heiress named Letitia Grayson, who is now having second thoughts. The pair decide to rescue Letitia; Prudence knocks out Markham with her sword hilt, and Robin, who quickly becomes fascinated with the romantic 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 gentlemen in highest circles.
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'inconnu" 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 insinuate himself into high society, despite the fact that his claim is, as yet, unproved. Robin and Prudence, long used to their scheming father's delusions of grandeur, are sceptical of his claims, which they have never heard before. As tensions mount over who is the legitimate heir, Rensley, overhearing Prudence disparage his manners, challenges her to a duel. Sir Anthony undertakes to forestall this meeting by insulting Rensley in order to force him into an earlier duel and wound him. Startled by this unaccountable intervention, Prudence begins to wonder if Sir Anthony suspects her masquerade.
Prudence is invited to dine with Sir Anthony who, despite his oblivious air, has guessed that "Peter" is actually a woman and has fallen in love with her. Prudence refuses his proposal, asking him to wait until her father's claim is proved, which would elevate 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 asks for money, but the cunning Old Gentleman persuades him to instead exchange the document for a letter that could expose Letitia's wealthy father as a traitor. As the Old Gentleman expects, Markham threatens Letitia with the letter and induces her to run away with him again.
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 describes "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.