It's a year after the Brexit vote. The four housemates and Timmy are on a visit to see their evil genius cousin, Rupert. Rupert owns a chunk of the Jurassic Coast, part of which he has turned into an island and declared independence from Britain. Its fifteenth-century Dorset castle is a tourist attraction, but a computer hack puts the ancient fortifications into lockdown and plunges the Five into peril. Can our intrepid friends escape to the safety of mainland Europe?