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, aside from cultivating disgusting habits, seems to be raising Flavia's hackles, although in her best moments she shows potential for trespassing, trickery, and other assorted mayhem. When Major Greyleigh, a local recluse and former hangman, is found dead from a breakfast of poisonous mushrooms, suspicion falls on the de Luce family's longtime cook, Mrs. Mullet. After all, wasn't it she who picked the mushrooms, cooked the omelette, and served it to Greyleigh in the moments before his death? But Flavia knows the beloved Mrs. Mullet is innocent. Together with Dogger, estate gardener and partner-in-crime, and the obnoxious Undine, she sets out to find the real killer and clear Mrs. Mullet's good name.