After experiencing a real-life pirate adventure, nine-year-old Mattie and his friend Peter are ready to leave the 1880s and return to their world. They helped twelve-year-old Ben find a treasure that only Ben's uncle and a ruthless pirate knew the whereabouts. But first, Ben asks for Mattie's help in locating his missing uncle. Clues in a journal suggest his uncle may have sailed to the San Juan Islands.
Before Mattie and Peter can leave, Peter is kidnapped by pirates. Mattie and Ben must devise a plan to rescue Peter without getting caught. If they succeed, there is still the problem of reaching the San Juan Islands with many dangers at sea and a pirate ship that seems to appear out of nowhere. The discovery of strange markings along the way is a foreboding sign that worries them.
Will Mattie and Ben find a way to outwit the pirates and locate Ben's uncle?