What seems like a tragic accident when a young novice nun is discovered dead in her locked smoke-filled bedroom to DI Poole seems like murder when he believes the room was locked from the inside by the murderer. Suspects include Father John (Kenneth Cranham), the mother superior sister Anne (Gemma Jones), other nuns, as well as a mysterious 'phantom' nun who nobody can identify. The convent has holy springs on the grounds which is being promoted by an unscrupulous PR man (Patrick Baladi). Poole's childhood experiences of nuns at his boarding school taint his views.