Lord Charles Sheridan and his wife, Kate, visit Dartmoor prison. Charles plans to implement a fingerprinting program and arrange a meeting with one of its most infamous inmates, Samuel Spencer, who admitted to killing his wife. He`s convinced that Spencer is innocent and he believes he has evidence to prove it. But Spencer continues to maintain his own guilt. Lord Charles and his acquaintance Arthur Conan Doyle are most perplexed. When a body turns up on the moor they, along with clever Kate, must find out if the convict is related to Spencer`s case.