Oxford in the 1960s :- Shabby but sharp young detective constable Morse comes to work with the experienced D.I. Thursday and is perplexed by the death of twenty-year old Margaret Bell from a supposed heart attack. Then Dr Cartwright,partner of her G.P. Bill Prentice,is shot dead. The widow,Helen,and her father Professor Sloan have another problem with her younger sister,the unstable epileptic Pamela,whose child they mind. Morse also finds a connection between Margaret's boyfriend and his tutor,Sloan,who has been receiving letters terming him a murderer. Furthermore the latest victim of a spate of Post Office robberies seems to have been involved with Cartwright in supplying illegal drugs to the likes of Margaret. Reactionary chief superintendent Bright disapproves of Morse's maverick sleuthing and wants him off the case but Morse uses his love of puzzles and former experience with code breaking at the Royal Signal Corps to draw all the threads together and solve the case.