On a summer's evening, Robert and Greta Gerdner are shot dead at their isolated home in the Devon countryside. DI Wesley Peterson suspects the execution-style murders might be linked to Robert's past career in the police - until Robert's name is found on a list of people who've been sent tickets anonymously for a tour of Darkhole Grange, a former asylum on Dartmoor. When his friend, archaeologist Neil Watson, finds the skeleton of a woman buried in a sealed chamber dating back to the 15th century at his nearby dig, Wesley wonders whether there might be a connection between the mysterious stone cell and the tragic events at Darkhole Grange. With the clock ticking, Wesley must solve the puzzle before the next person on the list meets a terrible end . . .