Maurice "Fish" Reidy is an eleven-year-old Irish boy whose life is changed forever by a dead horse, a mildly mysterious uncle, and a stolen pouch of coins. When his family's horse dies, Fish is sent to the city, where he gets a job making deliveries for his uncle Gerry. When an important delivery is stolen out from under Fish's nose, he has no choice but to follow the thief onto a pirate ship, where he gets stranded when the pirates put out to sea.

On the good ship Scurvy Mistress, Fish must swab decks, eat wormy gruel, and use a cabbage for a pillow, as he follows every order of Captain Cobb and his motley crew of characters like Scab, Sammy the Stomach, the Scalawags for Sausages, the Over and Unders, Jumping Jack, and the One-eyed Willies. Will Fish make it back home to his family, or will he end up walking the plank?