Admiral Westfield sucked in his breath and released it in a tremendous gust. "My dear," he said, "let me be clear: You are a young lady. You will not tell silly tales and you will never be a pirate."

Hilary Westfield has always dreamed of being a pirate. She can tread water for thirty-seven minutes. She can tie a knot faster than a fleet of sailors. She particularly enjoys defying authority, and she already owns a rather pointy sword. There's only one problem: The Very Nearly Honorable League of Pirates refuses to let any girl join their ranks of scourges and scallywags.

Girls belong at Miss Pimm's Finishing School for Delicate Ladies, learning to waltz, faint, and curtsy. But Hilary and her dearest friend, the gargoyle, have no use for such frivolous lessons—they are pirates! (Or very nearly.)

To escape from a life of petticoats and politeness, Hilary answers a curious advertisement for a pirate crew and suddenly finds herself swept up in a seafaring adventure that may or may not involve a map without an X, a magical treasure that likely doesn't exist, a rogue governess who insists on propriety, a crew of misfit scallywags, and the most treacherous—and unexpected—villain on the High Seas.

Will Hilary find the treasure in time? Will she become a true pirate after all? And what will become of the gargoyle?