Stubborn hearts might be the last to fall, but when the hard tumble comes, it'll last forever.

Jonathan Banshire, Viscount Trewellain, has become a bitter, sarcastic lord and against anything remotely smacking of romance. Though engaged to a woman whose hand he'd won in a game of cards years ago, he hasn't thought of her in years. But he's also a king's agent, and as his duty to the Crown, he's being sent to Spain to track down a missing peer. He goes to escape his personal life.

Miss Sophia Wickham is tired of men deciding her future. She's on a mission to live her life on her terms, but can't due to an ambiguous engagement to the Viscount Trewellain. Determined to gain her freedom, she sneaks on board the ship intent to have Jonathan release her from that old promise. When he refuses in a fit of pique, she's stuck as stowaway, bound to the one man she can't stand.

In Madrid, they search for not only the missing lord, but also stolen Spanish crown jewels and an ancient relic. Through crashing a glittering ball to hunting down clues to exploring the hidden catacombs and caves beneath Barcelona, the unlikely pair comes to an understanding. Vulnerable and trapped, truths are revealed as well as the fragile state of their hearts. But machinations of a madman bent on revenge could destroy a budding romance, and when the specter of death makes an appearance, Jonathan must finally decide what it is he desires the most.