Determined to secure another London season without assistance from her new brother-in-law, Mary Alsworthy accepts a secret assignment from Lord Vaughn on behalf of the Pink Carnation: to infiltrate the ranks of the dreaded French spy, the Black Tulip, before he and his master can stage their planned invasion of England. Every spy has a weakness, and for the Black Tulip that weakness is black-haired women--his "petals" of the Tulip. A natural at the art of seduction, Mary easily catches the attention of the French spy, but Lord Vaughn never anticipates that his own heart will be caught as well.