Ken Blake is approached by an old friend, Dean Halliday, who tells the story of his family estate, Plague Court. Halliday explains that the house is haunted by the ghost of the original owner, Louis Plague, a hangman by profession. Halliday invites Blake and Chief-Inspector Humphrey Masters to Plague Court to take part in a seance, run by psychic Roger Darworth and his medium Joseph. However, Darworth is a fake, being monitored by the police. The night of the seance, Darworth locks himself in a small stone house, behind Plague Court, while the seance proceeds. When Masters and Blake go to get him, he has been stabbed to death, with the dagger of Louis Plague. But all the doors and windows are bolted and locked, and thirty feet of mud surrounds the house, unbroken -- and all the suspects have been holding hands in the seance. The only one who can solve the crime is locked room expert Sir Henry Merrivale.