Fish--nicknamed for his incredible swimming abilities--is a twelve-year-old boy from a poor farming family. After taking a job as a courier for his uncle, Fish is waylaid by pirates, who steal the package he's carrying. He soon learns that it's the key to locating the Chain of Chuacar, a legendary treasure. As he works to earn the trust of Cobb, the fortune-hunting captain of the Scurvy Mistress, Fish learns of a mutiny headed by a nasty pirate called Scab. With time running out to find the Chain, Fish and some fellow shipmates still loyal to Cobb must thwart Scab's dastardly plan.Fish, a 12-year-old Irish boy from a poor farming family, is sent away to live with his uncle in a port town so he can make money to send back home. Eager to do well, he becomes a courier for his uncle, running packages all over town. One day his uncle asks Fish to deliver a package to a man at the docks, but pirates waylay him before he can pass it off.

It turns out that the package is full of gold coins which are the key to finding a hidden treasure. As Fish makes friends with the kind-hearted pirate captain named Cobb, a helpful young boy named Daniel, and a surprisingly intelligent lummox named Moravius, he falls in love with the sea-going life on board the pirate ship Scurvy Mistress.As they get closer to discovering where the treasure is hidden, Fish becomes aware of a mutiny brewing among certain members of the crew, headed by a particularly nasty pirate who goes by the nickname Scab.When the pirate ship finally reaches the island that is supposed to have the hidden treasure, Scab and some fellow mutineers make their move, but they are thwarted by the quick-thinking Fish and some fellow pirates still loyal to Captain Cobb.