Twelve-year-old twin adventurers Cleopatra and Alexandra 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, for 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, 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?