The book sees Dick and Dorothea Callum visiting the Norfolk Broads during the Easter holidays, eager to learn to sail and thus impress the Swallows and Amazons when they return to the Lake District later that year.

The Callum children, Dick and Dorothea, spend their Easter holidays on The Broads with their good-natured family friend, Mrs Barrable, who is staying on the small yacht Teasel, moored near the village of Horning.

There they encounter the Coot Club, a gang of local children comprising Tom Dudgeon, twin girls 'Port' and 'Starboard' (Bess and Nell Farland; their nicknames come from one of them being left-handed and the other right-handed), and three younger boys — Joe, Bill and Pete (the "Death and Glories").

Dick shares the Coot Club's keen interest in local bird life, and Dorothea uses the voyage as fodder for her new story, "Outlaw Of The Broads", based on the Hullabaloos' vow to catch Tom.

William the pug is encouraged to make a heroic journey across the mud towing a thread, by which a rope and heaving-line are hauled across to rig a makeshift zip-line trolley to transport food between the two vessels, without which some of the party would have had to go unfed for 12 hours.

It turns out that the Hullabaloos were alerted to Tom's whereabouts by George Owdon, a late-teens Horning youth who selfishly makes money by selling birds' eggs to collectors, and who therefore has no love for the Coot Club.