It is 1942, and struggling up the hill to the new military hospital, Heron's Park, Kent, postman Higgins has no idea that the sender of one of the seven letters of application he is delivering will turn out to be a murderer in a year's time. When Higgins is brought in following injuries from a bombing raid in 1943, his inexplicable death from asphyxiation at the operating table casts four nurses and three doctors under suspicion, and a second death in quick succession invites the presence of the irascible - yet uncommonly shrewd - Inspector Cockrill to the scene. As the prospect of driving back across Kent amid falling bombs detains the inspector for the night, a tense and claustrophobic investigation begins to determine who committed the foul deeds, and how it was possible to kill with no evidence left behind.