A Test of Wills is one of the many historical mystery novels written by mother and son Caroline and Charles Todd, published under the pen name Charles Todd. Like many of their books, A Test of Wills is set in England just after World War I and features Scotland Yard's Inspector Ian Rutledge, a WWI vet hoping to move past his shell shock after serving for four years in the trenches. When he is appointed to solve a murder case in the small English village of Upper Streetham in Warwickshire, he hopes he can prove he still has valuable skills as a detective but fears failure and being sent to an asylum.
The victim is Colonel Charles Harris, and evidence found by local police points to shell-shocked veteran Captain Mark Wilton, who is a local war hero. Naturally, working on a case with such close associations to the war is going to prove to be very trying for our own shell-shocked protagonist, Inspector Rutledge. Moreover, one reason for his own post-traumatic stress syndrome is that, during the war, he was driven to execute a Scottish soldier named Hamish McLeod for disobeying an order; Hamish lives inside Rutledge's head as a cynical voice always causing Rutledge to lose hope.
During the investigation, Inspector Rutledge questions many people but is only given many varying incomplete stories. The testimony of shell-shocked soldier Daniel Hickam and a doll found at the scene of the crime turn out to be the key pieces of evidence needed to make the case make sense. Still convinced the hero Captain Wilton is not the culprit despite all pointing evidence, further investigation leads Rutledge to discover the lover of the deceased Colonel Harris, the fact that Harris's steward Royston is guilty of having killed a child twenty years ago in an automobile accident, and that Colonel Harris was actually not the true target. The culprit had shot Harris believing him to be Royston, and the motive was purely for revenge.
Having solved the case, Rutledge's career as a detective is now secure. He even manages to save Royston because the culprit was about to take another shot at revenge.