War on the Darnel is the fourth book in the Darnley Mills Adventure series which should be read in the order Colonel Sheperton's Clock (The Mystery of the Colonel's Clock), The Grange at High Force, Sea Peril, War on the Darnel & Skull Island.
War on the Darnel continues where Sea Peril finishes. The heroes of the series, Peter, David and Arthur still have their restored boat Sea Peril II (a bright yellow punt based houseboat, paddle wheel driven, bicycle powered). On their way down river to Powder Quay, which their friend the Admiral has bought and is restoring, they encounter a river toll set up on Ballast Island by George Brody's gang. George is collecting for Christian Aid and hopes to beat the total set by Peter the previous year. Enraged by this extortion Peter attempts to force the passage of Ballast Island but is unsuccessful.
Furious Peter vows a battle and sets to inventing. The book describes the hilarious 'battle' of Sea Peril II versus the 'pirates' on Ballast Island. Further twists are brought in with the restarting of the Darnley Mills railway, mentioned in the other Philip Turner book 'Steam on the Line'. A horrendous storm and flood on the Darnel conclude the story but all ends well.