Who really poisoned Philip Boyes?
His cousin Norman Urquhart, the respectable Bedford Row solicitor at whose house Boyes ate his last dinner. Urquhart held Mrs. Wrayburn's fortune under a deed of trust and had secretly gambled it away in the Megatherium crash; her true will left the estate and fifty thousand pounds to Boyes. With Boyes dead before the old lady, Urquhart became heir to everything and his embezzlement would never be exposed.
How was arsenic given at a dinner where every dish was shared?
Through the sweet omelette Boyes cooked himself at table. Urquhart had spent about two years taking regular doses of solid white arsenic — the packet Miss Murchison found in his office panel — building an immunity that let him eat his half of a poisoned dish unharmed. He loaded the dose into the cracked egg he had brought home and directed into the bowl, then ostentatiously shared everything, having even dried out the bedroom water bottle to keep suspicion trained on the dinner and on Harriet's coffee afterward.
What was the powder Boyes swallowed at the Nine Rings that night?
Harmless bicarbonate of soda — a stomach remedy he took when the poison began to gripe him on the way home. Its discovery briefly promised a suicide defence, but the analysis destroyed that theory and forced Wimsey to look for a murderer instead.
How does Wimsey actually prove it?
By converging lines his agents dig up: Miss Climpson's fake seances locate the real will, Miss Murchison's lock-picking exposes the trust papers, the Megatherium losses and the hidden arsenic, and the office typewriter convicts Urquhart of forging the draft he showed Wimsey. Nail and hair samples obtained via Urquhart's manicurist come back saturated with arsenic, proving habitual dosing. Finally Wimsey bluffs that the Turkish delight Urquhart has just eaten carried a lethal dose — and the solicitor's total lack of symptoms, followed by his attempted flight into Parker's arms, gives the game away.
Is Harriet freed, and does she marry Wimsey?
The Crown withdraws the charge at the retrial, stating complete conviction of her innocence, and the jury returns a formal Not Guilty. She has refused Wimsey throughout — including a final offer to live with him unmarried, which he declines — and he drives off without waiting for thanks. The book ends with him telling his family he still intends to marry the prisoner if she will have him.
Themes and What the Ending Means
Justice and its machinery: The hung jury turns on one woman's refusal to convict on prejudice dressed as proof, and the book keeps asking how far a watertight case can differ from the truth.
The double standard: Harriet is tried as much for living unmarried with Boyes as for his murder; the judge's moralising and the public's relish show respectability itself operating as evidence against her.
Love without coercion: Wimsey holds the power of rescue over a condemned woman and repeatedly refuses to use it, accepting rejection rather than letting gratitude stand in for consent.
Money, wills and hidden rot: A trust deed, a suppressed will and a solicitor's speculation drive the murder; Sayers locates the real corruption in sleek professional respectability, not in Bohemia.
The uses of superfluous women: Spinsters, typists and ex-barmaids — the women society discounts — do the decisive detective work, from the jury room to the picked lock to the faked seance.
Performance and misdirection: The suspicion-proof dinner, the forged draft and Miss Climpson's spirit-rapping all trade on how easily people believe a well-staged show, for good and for ill.