A masked-club revel ends with a dancer found apparently poisoned, prompting the narrator—an assistant to a prominent toxicologist—and a Scotland Yard inspector to investigate. Their inquiries uncover secret societies, coded records, psychoanalytic clues, disputed testimonies, a locked safe, fingerprint evidence, and an elusive toxin, while suspects range among the club's members and an aristocratic circle. The narrative traces the piecing together of physical evidence, ciphers, and personal revelations to establish cause, motive, and method, culminating in forensic and psychological deductions that expose the mechanisms behind the death.