At the Dankmarsh School for poor orphans, the childen sleep four to a bed and exist on watery porridge. Teachers don't last long, and so when a new one arrives they are intrigued, especially since he has an eyepatch a huge moustach and calls himself the Captain. Miss Scrubshaw interviews him for the post, though he and his cousin 'Mr Mate' seem most interested in the ancient map on her wall. The Captain introduces new lessons- compass use, swordcraft and biscuit identification. On a school trip to visit the Black Barnacle, a ship owned by an 'old pal', the Captain gives Miss Scrubshaw the slip and it dawns on the children that the ship has cast off, and they are the new crew! Bound, they discover, for Doom Island and a perilous quest for hidden treasure, at the mercy of the Captain and his dastardly plan?