New York City, 1860.

When their father goes missing, twelve-year-old twins Alexander and Cleopatra Dodge join the Black Hook Gang. Soon after, Alex is arrested for burglary, but the twins cut a deal with the police chief and leave town to start a new life. They assume different identities, but upon their arrival in New Orleans, they find even more trouble. Alex is separated from his sister and pressed into service on a ship bound for San Francisco, and Cleo, hoping to meet him at the end of his journey, stows away on a steamer ship.

What neither twin realizes is that they hold the keys to a lost pirate treasure, and the pirates want their treasure back. Pursued by the bloodthirsty Captain Felix Worley and Luther, the vengeful leader of the Black Hook Gang, Alex and Cleo must survive a series of harrowing adventures if they're ever going to see each other—or their father—again.