In 1668, in London, Dona St. Columb, who is bored with city life, leaves her husband Harry when he refuses to acknowledge, and protect her from, the lecherous advances of his friend, Lord Rockingham.  Dona takes her two children to Harry's ancestral home on the coast of Cornwall, where she is surprised to find only William, a servant unfamiliar to her.

Dona laughs at her neighbor, Lord Godolphin, when he warns her of the treacherous French pirates who have been raiding the homes along the coast, but she is then kidnapped and taken to a pirate ship, La Mouette, that has dropped anchor in her own cove.  Dona is thrilled by the presence of the pirate captain, who is known to her only as "The Frenchman," and invites him to her home for dinner.  Although she is aware that Godolphin is forming a vigilante group to capture the pirates, Dona accepts The Frenchman's invitation to sail with him, and she leaves her children in the care of William.  Wearing men's clothing, Dona narrowly escapes capture when she helps The Frenchman steal a French schooner laden with goods from Godolphin's cousin.  Dona and The Frenchman fall in love, and when she finally returns home, she discovers that Harry and Rockingham have arrived to help Godolphin capture the pirates.

William has covered for Dona's absence by telling her husband that she was sick in bed and could allow no visitors, and on Dona's urging, William goes to warn The Frenchman to set sail immediately.  William delivers his message, but is shot in the arm by vigilantes, and returns to Dona to tell her that La Mouette will not be able to sail until midnight.

That night, vigilantes gather at Dona's house for dinner, and she tries to detain them past midnight.  The Frenchman and his men unexpectedly take the party hostage, and Rockingham becomes suspicious when he sees the loosely concealed familiarity between Dona and The Frenchman.  The Frenchman bids farewell to his lady and, after the pirates make their escape, Rockingham jealously tries to force himself on Dona, who kills him in self-defense.

The vigilantes, meanwhile, engage in battle with the pirates.  The Frenchman allows himself to be captured in order to save his ship, but the ship stays nearby and Dona helps him escape from prison.  Dona is tempted to leave her dull life to join The Frenchman, but chooses instead to remain with her husband and children, and The Frenchman sails away.