In the absence of adult supervision, Flavia has taken on the mentorship of her “pestilent” younger cousin Undine, who came to live at Bucksaw when her mother died (volume 10). Undine too seems fascinated by mayhem. Flavia has nigglings of doubt about her motivations. Has Undine inherited her dead mother’s darkness?  Or worse?

A mysterious villager, Major Greyleigh, a virtual hermit and former public hangman with stomach-curdling deeds in his past, has been found dead, killed by ingesting poisonous mushrooms.

“I have to admit,” says Flavia, “that I’d been praying to God, the Virgin Mary, and all the Saints for a mushroom poisoning. Not that I wanted anyone to die, but why give a girl a gift such has mine without giving her the opportunity to use it?”

The “odious, moon-faced” Undine is becoming increasingly crude and gregarious, and constantly tests her limits by pushing Flavia to undertake rash deeds, while Undine in turn is urged on in her daring by one of Flavia’s sister Ophelia’s former suitors, the rascally Carl Pendracka from Cincinnati, who seems, for some reason of his own, to be tutoring the child in unsavoury habits.

In her search for the murderer, Flavia becomes entangled with some rather strange American servicemen, only to be led to the most unlikely of suspects.

Many loose threads from books 1-10 are pulled together in this volume. In the end, the unimaginable comes to light:  Flavia — with the help and hindrance of the unpredictable Undine — discovers what really happened to Flavia’s father, whom Flavia was not allowed to visit on his death bed (at the end of volume 8).

The discovery reveals details about his involvement with the shadowy Nide (introduced in volumes 6-8) — the infamous ultra-secret espionage organization that operated during World War II and that seems to have been involved also in Flavia’s mother Harriet’s mountaintop death (introduced in volume 1 and developed in later volumes as Flavia follows clues to the circumstances of her mother’s death). Flavia’s unexpected prying into secrets old and new has produce the greatest shock of her life, inspiring a rage that will change the course of everyone’s lives, paving the way for future investigations.