Twelve-year-old twins Cleopatra and Alexandra Dodge are reunited with their father and realise that two family heirlooms reveal the location of a treasure that is their birth right. When they set sail to find it 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. 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?