A woman's naked body is found floating in the weeds of a lake near Bath by an elderly woman walking her Siamese cats. No one comes forward to identify her, and no murder weapon is found, but sleuthing is Superintendent Peter Diamond's speciality. A genuine gumshoe practising door stopping and deduction: he is the last detective.

Struggling with office politics and a bizarre cast of suspects, Diamond strikes out on his own, even when Forensics think they have the culprit. Eventually, despite disastrous personal consequences, and amongst Bath's rambling buildings and formidable history, the last detective exposes the uncomfortable truth....