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.

Aboard the Almira... Twelve-year-old twin adventurers Cleopatra and Alexander Dodge are reunited with their father and realize that two family heirlooms reveal the location of a treasure that is their birthright. When they set sail with Captain Tarboro on the Almira, they know they're heading into danger—the ocean is filled with new and old enemies, including their nemesis, the infamous pirate Felix Worley. But like a coral reef that lurks below the surface of the waves, trouble is brewing between the siblings. Alex is determined to become a sailor and is happy with his role aboard the Almira, but Cleo—the only girl on the ship—is tired of washing dishes in the galley. In an effort to find her own purpose, she begins studying sword fighting with Tarboro, but neither Alex nor her father approves. Can the twins remain close as they pursue different goals and dreams, or will their growing differences tear the family apart before the treasure can be found?