During World War I, a German U-boat sinks a British ship and takes the survivors on board. After it takes a wrong turn, the submarine takes them to the unknown land of Caprona, where they find dinosaurs and neanderthals.

When his brother disappears, Robert Manning pays a visit to the remote country house he was last heard from. While his host is outwardly welcoming – and his niece more demonstrably so – Manning detects a feeling of menace in the air with the legend of Lavinia Morley, Black Witch of Greymarsh, hanging over everything.