The Doctor and Rose travel to the 47th century, Earth's 'Second Dark Age', but when they arrive in the floating box city of High High Wycombe, it's more like a paradise. The residents live an existence of endless leisure, and at night sleep a blissful, dreamless sleep. But cracks soon begin to show on its perfect, polished surface, and something terrible is brewing in the mind of teenage Elsa - something ancient and hungry - something that sounds like a nightmare.