Julie Walters narrates as we visit rugged Northumberland, the county with England's cleanest rivers, darkest night skies – and more castles than anywhere else in the country. Robson Green tries out a very testing sleepover on the Farne islands, while Denise Welch and Carol McGiffin are walking the Roman Wall. Ade Edmondson heads to sea in search of the treasures of the deep, and Harry Potter star David Bradley returns to Alnwick castle – better known as Hogwarts.