Luzena the mercenary has been searching for her sister but to no avail. After breaking into a castle and stealing a secret box, Luzena discovers that her client has paid her in counterfeit coins, leaving Luzena empty-handed. Then a prince offers her a job as an undercover guard. All she has to do is marry him. Luzena agrees and vows to protect the prince's title. But when a mysterious woman lays claim to the title, what will it mean for everyone's future?