An outbreak of violent madness at the Explorers Club, the coincidental murders of a recluse scientist in North Kent and a lighthouse keeper on the chalk cliffs below Brighton, and the mysterious disappearance of Alice St. Ives, lead Langdon St. Ives, Jack Owlesby, and their resolute friend Tubby Frobisher into the very heart of danger, where they discover the great secret of the chalk cliffs at Beachy Head and a looming threat to the collective sanity of mankind.