London, 1814. The entire city has turned out for the biggest Frost Fair in decades. But beneath the frozen Thames, revellers are disappearing, snatched through the ice and pulled into the depths where a terrifying monster lurks. Will the Doctor and Bill stop the slaughter before they too are dragged into the icy waters?

Why do floorboards creak? When a sinister landlord shows Bill and her friends the perfect houseshare, they have no idea what lies ahead... knock knock, who's there?