When a letter arrives at the police station addressed to a chief inspector Norbert, it causes quite a stir. Though Norbert has long since moved on, the letter claims to have evidence that the man convicted of murdering the Reverend Shipbourne during the course of a robbery in 1991 is innocent. Despite having a full case load—including investigating a series of vicious attacks on a local supermarket chain—detective Wesley Peterson is forced to follow up on the letter writer's claims. Meanwhile archaeologist Neil Watson is excavating a site in Pest Field when he discovers a mass grave that leads him to conclude that the site—earmarked for development—houses an ancient medieval plague pit. More disturbing is the discovery that the grave is home to a more recent resident.