Nicholas Nosh is preparing for the Pirate Games when his aunt, uncle and cousin arrive. His outspoken little cousin, Primrose, is rude to Nicholas about his short stature, and mocks him until he is determined to show her that he may be small, but he's a real pirate. He'll take her out on his ship The Golden Pudding and attack somebody and steal their treasure!
When they find a ship to attack, the Jolly Dodger, the tables are turned. Captain Manners of the Jolly Dodger asks them to surrender, and to everyone's relief he also takes Primrose hostage. Nicholas realises (reluctantly) that it's up to him to save Primrose, and he does so in a night-time raid, only to discover that Captain Manners has been expecting him: a note 'Take her - please!' is pinned to Primrose's dress.
Poor Primrose is mortified at this and bursts into tears. It turns out that all she wants is to be a pirate herself, so to keep her quiet, Nicholas promises to teach her. But it will be a long trip home.