The ordinary coastal town Sandsborough is turned upside down when the Blunderbuss family arrives, a sword-fighting, rum-drinking bunch of pirates. While the respectable residents are appalled by the family's ship-turned-home, guard shark and pet octopus, Michael (12) becomes friends with the new boy next door, pirate Billy (13).

However, Michael might lose his new friend when the Blunderbusses’ archenemy Crooked Cornelius suddenly turns up, determined to lure the pirates back to sea. Together with Elizabeth (12), a girl from the neighborhood, Michael and Billy go into battle against the most dangerous pirate of the seas…