Wealthy landowner Sir Ralph Skelkton is due to marry Caroline, the orphan who has grown up as his ward, when he is seduced by the latter's oldest school friend, Barbara. The unlikely pair are married, and one guest at their wedding is the attractive architect Kit Locksby, who instantly falls in love with Barbara.

Married life quickly bores Barbara, which is not lost on Caroline, who has stayed to keep house. When Ralph's sister and brother-in-law, Lady and Lord Kingsclere, arrive from London, Barbara sits up late with them at cards and gambles away all her possessions, including a ruby brooch given her by her dying mother. Regretting her action, and provoked by Lady Kingsclere wearing the brooch, Barbara disguises herself as a highwayman (inspired by stories of the legendary Captain Jackson) and steals it back.

But Barbara is now thrilled to have discovered a new pastime; one night she encounters the real Captain Jackson, and after escaping pursuit together they become lovers. Subsequently learning from one of her husband's ex-tenants of the passage of a bullion coach, Barbara persuades Jackson to ambush it, and in the affray shoots one of the guards. This causes wide-spread alarm, and Ralph is particularly anxious to do something to protect his people.

Barbara takes no interest in these proceedings and is chided by Caroline who, having realised that she is in fact deeply in love with Ralph, departs for London when she discovers that Ralph feels the same way. Barbara now overhears a plan to apprehend Jackson and sets off to warn him. She discovers him in bed with a dozy in an inn and, still disguised as a highwayman, throws a note revealing his whereabouts into Ralph's drawing room.

Jackson is caught and taken to London for trial. But Barbara's secret has been discovered by Ralph's old servant Hogarth, a Puritan fanatic, and after undergoing a rapid conversion to gain time, she determines to dispose of him by slow poisoning. Caroline is summoned back to restore order to the house, but is too late to save Hogarth who dies before he can reveal Barbara's secret.

Realising that she has no hope of living with Ralph, Caroline returns to London having decided to marry Kit Locksby in the knowledge that she does not love him, nor he her. On the gallows, Jackson sees Barbara in her coach and understands that she has betrayed him. A fight breaks out which allows the highwayman to be cut down before he dies. Back in the country, he turns up one night in Barbara's room determined to strangle her, and in the ensuing struggle rapes her.

She now realises that she loves Kit but despairs of ever being able to marry him. She again disguises herself as a highwayman, bent this time not on robbery but murder. She encounters Jackson, and shoots him dead when he tries to stop her, but her attempt on Ralph's life goes wrong and she is herself fatally wounded. Confessing all her misdeeds, she dies in Kit's arms, which leaves Caroline and Ralph free to marry at last.

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