Amateur sleuth Flavia de Luce, along with her pestilent younger cousin, investigates the murder of a former public hangman and uncovers secrets that bring the greatest shock of her life.

Flavia de Luce has taken on the mentorship of her odious, moon-faced cousin Undine, who has come to live at Buckshaw following the death of her mother. Undine's main talent seems to be getting underfoot, although in her best moments she shows potential for trespassing and trickery.

When Major Greyleigh, a local recluse and former hangman, is found dead from ingesting poisonous mushrooms, suspicion fall on the de Luce family's longtime cook, Mrs. Mullet. After all, it was she who picked the mushrooms, cooked the omelette, and served it to Greyleigh in the moments before his death. "I have to admit," says Flavia, a budding expert in the chemical nature of poisons, "that I'd been praying to God for a mushroom poisoning. Not that I wanted anyone to die, but why give a gift such as mine without giving her the opportunity to use it?"

But Flavia knows the beloved Mrs. Mullet is innocent. Together with Dogger, estate gardener and partner-in-crime, and Undine, whose help she does not welcome but begrudgingly accepts, she sets out to find the real killer and clear Mrs. Mullet's good name. Little does she know that following the case's twists and turns will lead her to a most surprising discovery—one with the power to upend her entire life.