As is her custom, Mrs Barbara Monkhouse is away for a week or two helping organise a women's emancipation movement but this time she returns home to find her husband Harold is dead.
The exact nature of chronic invalid Harold's illness has never been diagnosed although he's been ailing for years. During Barbara's absence Harold's brother Amos visited him and was so shocked by his appearance he insisted on an expert opinion. It is all to no avail, however, for a few days later Harold dies of arsenic poisoning. Was it administered in his food or drink or perhaps added to his medicine? Everyone in, or with access to, the house at the time is under suspicion -- domestic science teacher Madeline Norris (Harold's daughter by his first wife), Harold's highly strung secretary Anthony Wallingford, the servants, and even medical man Dr Dimsdale, not to mention regular visitor and narrator Rupert Mayfield, a barrister and inseparable childhood friend of Barbara Monkhouse and her now deceased stepsister Stella Keene. Mayfield asks Dr Thorndyke to investigate so that innocent parties can be cleared of suspicion, but the true depravity of the culprit is only revealed at the close of the novel.
My verdict: This entry in the Thorndyke saga revels in a particularly inventive plot, richly decorated with such details as mysterious bottles of unknown origin and an infernal machine in the post, not to mention what well may be the most inventive way of administering poison ever utilised in the annals of detective fiction.