Philip and Margaret Waverton and their friend Roger Penderel are driving through the mountains of Wales when a torrential downpour washes away the road and forces them to seek shelter for the night. They take refuge in an ancient, crumbling mansion inhabited by the strange and sinister Femm family and their brutish servant Morgan. Determined to make the best of the circumstances, the benighted travellers drink, talk, and play games to pass the time while the storm rages outside. But as the night progresses and tensions rise, dangerous and unexpected secrets emerge. On the house's top floor are two locked doors; behind one of them lies the mysterious, unseen Sir Roderick Femm, and behind the other lurks an unspeakable terror. Which is more deadly: the apocalyptic storm outside the house or the unknown horrors that await within? And will any of them survive the night?