When a Mexican heiress defies Victorian society to protect her country a British war hero makes it his new mission to protect her.

Isabel Luna Valdés has long since resigned herself to being the "forgotten" Luna sister. But thanks to familial connections to the Mexican ambassador in London, wallflower Isabel is poised to unearth any British intelligence hidden by the ton that might aid Mexico during the French Occupation. Though she slips easily from crowded ballrooms into libraries and private studies, Isabel's search is hampered by trysting couples and prowling rogues—including the rakish Captain Sirius Dawson.

As a covert agent for the British Home Office, Sirius makes a game of earning the aristacy's confidence. He spends his days befriending foolish politicians and seducing well-born ladies in order to learn their secrets. But after he spies a certain sharp-tongued Luna sister lurking in the shadows where no proper debutante should venture, it's clear Sirius is outmatched, outwitted, and soon to be outmaneuvered by the one woman he can't resist.

Their mutual attraction is undeniable, but when Isabel discovers private correspondence that could turn the tide of political turmoil in Mexico, she's willing to do whatever it takes to protect her country—even if this means ignoring her heart and courting danger...