 Kayrla must convince one of the guards, not all of whom know she's become one of the Duke's spies, to follow her orders. 

Tales of the Left Hand, Book One

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In the tropical-island region known as the Frees, magic is growing stronger, and gunpowder is becoming less reliable. Amid this world of "swashbuckling, intrigue, and a dash of magic," two adventurers literally run into each other and are forced to fight together against pirates, sinister assassins and powerful merchant families, in the process developing a most unlikely friendship. Tesca is the Left Hand, a spy and assassin serving the Duke of Kohaya, ruler of one of the independent realms within the Frees. Kayrla is a sailor and sometime pirate with a few magical tricks up her sleeve. While fleeing the wrath of her former captain, Kayrla collides with Tesca, who’s on a mission of his own, and her captain, assuming them to be partners, sics his crew on both of them. Cutting their way free of the pirates' wrath, the pair come to realize that they make a very effective team, but will their combined talents be enough to stop an assassin seeking the Duke’s life, and who appears to be more than a match for either Tesca’s blades or Kayrla’s magic?

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Tales of the Left Hand, Book One