In this thrilling tale, Inspector Charlton has to not only discover a murderer but determine some family genealogy in order to ensure fair play to all.
It all begins when his Lulverton police headquarters receives a telephone call from a seemingly deranged Sir Victor Warringham who is at his nearby country estate Elmsdale. He says that he has been attacked and almost strangled by what appeared to have been a ghost. The report is taken with scepticism but even so, mainly because of Sir Victor's standing in the community, Charlton goes to investigate. But he is met by so many obstructions from a most unhelpful housekeeper and Sir Victor's bad-tempered son-in-law that he is unable to get into the house.
Charlton is told that Sir Victor is unwell and his doctor has been called to attend to him and that once he has been seen to it might then be possible for him to see him. Sir Victor's solicitor has also been called to transact some unknown business.
From then on the whole episode and its aftermath is full of intrigue involving family history and the question of inheritance in the, then unlikely, event of Sir Victor's death. Charlton does not know what to believe and he is not helped by the lies and deceit that he is told from everyone involved.
The tension mounts but little by little Charlton and his right hand men Sergeant Martin, and later Sergeant Bradfield, begin to uncover some unwholesome truths, helped by 10-year-old Jack Campbell, whose parentage is unclear. In the meantime Sir Victor meets with a mortal accident and the housekeeper is unexpectedly murdered.
As is to be expected, all this adds complications to Charlton's investigations, which get more complicated when a person from the past arrives at the house with a series of convoluted stories. But Charlton perseveres and, in an absolutely unexpected and exciting ending, 'Let X Be The Murderer' lives up to its title.