Dona, Lady St. Columb, makes a sudden visit with her children to Navron, her husband's remote estate in Cornwall, in a fit of disgust with her shallow life in London court society. There she finds that the property, unoccupied for several years, is being used as a base by a notorious French pirate who has been terrorising the Cornish coast. Dona finds that the pirate, Jean-Benoit Aubéry, is not a desperate character at all, but rather a more educated and cultured man than her own doltish husband, and they fall in love.
Dona dresses as a boy and joins the pirate crew on an expedition to cut out and capture a richly laden merchant ship belonging to one of her neighbours. The attack is a success, but the news of it brings Dona's husband Harry and his friend Rockingham to Cornwall, disrupting her idyllic romance. Harry, Rockingham, and the other locals meet at Navron to plot how to capture the pirate, but Aubéry and his crew cleverly manage to capture and rob their would-be captors instead. Rockingham, who has had designs on Dona himself, perceives the relationship between her and Aubéry, and Dona is forced to kill him in self-defence when he attacks her in a jealous rage. Meanwhile, Aubéry is captured while trying to return to his ship, and Dona hatches a plot for his release. In the end, however, she realises that she must remain with her husband and children instead of escaping to France with Aubéry.
