Fish is a twelve-year-old boy from a poor farming family who loves to swim and hates to fight. After taking a job as a courier for his uncle, Fish is waylaid by pirates, who steal a precious purse of golden coins. He follows the thieves onto their ship, the Scurvy Mistress, and soon learns that the coins are the key to locating a legendary treasure. As he works to earn the trust of Cobb, the captain of the Scurvy Mistress, Fish struggles to survive among the band of wild, often violent pirates. Eventually Fish and his new friends uncover a planned mutiny headed by a nasty, greedy pirate known as Scab, and they're left to thwart his dastardly plan and hunt down the treasure on their own.