A woman is found strangled in the back of a car, left parked in a field. Dr Lorrimer of the Hoggits Laboratory is at loggerheads with D.I. Doyle for the slowness of his results. Lorrimer singles out Clifford Bradley to take the blame.

Dr. Maxim Howarth is promoted as director the Hoggits Laboratory a head of Dr. Edwin Lorrimer. There is no love lost between the pair due to his wife Domenica. Commander Dalglish takes over the murder investigation of the strangled girl.

Dr. Edwin Lorrimer is found murdered in his laboratory having argued with Paul Middlemass over his bullying of staff. Dalgleish is suspicious that cleaner Mrs Bidwell was told not go to the laboratory making it a premeditated murder.

Clifford Bradley cracks up under questioning about Dr Lorrimer's murder. A mysterious woman telephones Lorrimer's father quoting several numbers, to him, including 1840. He believes Stella Mawson has stolen documents from him.

Domenica confesses to Dalgleish that she had an affair with Dr Lorrimer. Stella Mawson confides to Angela Foley that she knows who the murderer is and plans to write it in a book believing it will pay off their mortgage.

Stella Mawson's body is found hanging in the chapel. Dalgliesh informs Angela that Stella was once married to Dr Lorrimer. He is also convinced that she was blackmailing the murderer but Angela denies she would even if it was a woman.

Dalgliesh and Massingham spot that the numbers on the hymn board match those given by Mr Lorimmer Snr. Angela confronts D.I. Doyle when she finds a note that he destroyed evidence. Nell Kerrison holds vital information.