A Presumption of Death is a mystery novel by Jill Paton Walsh, based loosely on The Wimsey Papers by Dorothy L. Sayers. These consist of a number or letters written by various Wimseys and published during the war.
The novel is set in 1940 wartime England and features Sayer's famous sleuth, Lord Peter Wimsey, and his wife, detective novelist Harriet Vane. Due to the origins of the novel, the first part of the book is written entirely in the form of letters between various characters, a device also used to introduce Busman's Honeymoon.
Harriet has evacuated her family to Talboys, moving there with her two children, with the three children of her sister-in-law, Mary and with Bunter's wife and son. Meanwhile, Peter and Bunter are on the continent carrying out undercover work.
While Peter is away a murder occurs in the village and the village policeman recruits Harriet to help solve the murder, partly because the police are too busy organizing all the changes necessitated by the war and partly because as the wife of a detective, and as a crime novelist she is the best qualified person to find the murderer.
The murder not only happens in wartime: all details are in one way or another connected with it. The murdered girl had been there in the first place only because she had come from the city to do agricultural work and help the war effort. She was killed in the village street during an air raid drill, while most people were underground, and much of the investigation turns on the issue who had been, or could have been, outside the shelter when the murder was committed. Moreover, many of the witnesses - and some of the possible suspects - are RAF pilots stationed at a nearby air base, who need to be questioned in between going out on dangerous and highly classified missions.
A considerable role is played by Gerald, Lord Peter's favourite nephew, who was first seen a decade earlier as a precocious boy playing a major role in solving "The Learned Adventure of the Dragon's Head"; now, he is a dashing RAF combat pilot, gallantly and only half-humorously affecting to be in love with his aunt-by-marriage.
At least one character introduces the possibility that the murder is directly linked to German espionage - though the suggestion is made in such absurd and paranoid fashion as to make it completely discredited.
While conscientiously conducting the investigation and also riding herd on the five children under her charge, Harriet is consumed with great anxiety about the fate of her husband, whose life is in very concrete danger. She bitterly regrets the lost years when she had put him off before finally agreeing him to marry him, and in which they could have already been happy together.
Eventually, she manages to do something concrete about it and directly save Peter's life. Wimsey, realizing that the Germans had broken the code which he had been using, devises a new one which is unbreakable because of being based on things only he and Harriet knew. His Secret Service colleague brings her Peter's message and she shows considerable skill in decoding it, enabling Peter to come back safely and thus at last "getting even" with him for having saved her from the hangman.
Harriet's anxiety is very palpably conveyed to the reader - even though those who follow the Wimsey canon know from the short story "Talboys" that Lord Peter would be alive and well two years hence, in 1942, by which time they would also have their third child Roger.
Eventually, Bunter comes back from the continent - uncharacteristically dirty, scruffy and so exhausted that for once he actually lets Lady Peter wait upon him and put him to bed - and is followed some days later by Peter, who arrived by a different route. It is never revealed where exactly they had been, though a reference to Bunter coming back via Holland - soon to be occupied by the Nazis - suggests they had been in Germany itself.
To Harriet's relief (and, it seems, his own) Lord Peter is retired from active service and, while still involved in intelligence issues, would not be sent again behind enemy lines. Being presented with Harriet's meticulous record of her investigations - which, as he notes, already solved most of the mystery - he is able to add the last missing pieces. Finding the solution for the mystery does not mean, however, just handing the perpetrator to the police; rather, solving the mystery arouses a complicated new problem involving legal, military, ethical and moral issues - which Lord Peter manages to neatly tie up.
Counting back from "Talboys", which takes place in the summer of 1942 and where Roger is two years old, it seems likely that Roger had already been conceived by the end of the present book - though Peter and Harriet are not yet aware of the fact.
As the Wimsey canon is completely silent about the eventual fate of Gerald, the reader is left wondering whether he would survive the coming Battle of Britain and the five years of gruelling aerial war to follow.