Strong Poison opens in a courtroom where crimson roses sit on the bench and an aged judge sums up for the jury. Harriet Vane, a 29 year old writer of detective stories, stands accused of murdering her former lover Philip Boyes, a 36 year old author of five novels preaching atheism, anarchy, and free love. They met in 1927, lived together from March 1928, and quarrelled in February 1929 when Boyes, after long opposing marriage on principle, finally offered it. Harriet had also bought arsenic on April 10 as ‘Mary Slater’ at a Southampton Row chemist, and arsenical weedkiller on May 5 as ‘Edith Waters’, claiming it was research for a novel.

The judge walks through the fatal evening of June 20. Boyes dined with his cousin, the solicitor Norman Urquhart, in Woburn Square on sherry, bouillon, turbot, chicken en casserole, and an omelette he cooked himself, then took a taxi to Harriet’s flat at 100 Doughty Street for coffee at 9:25. By 10:10 he was hailing a cab in Guilford Street, and he arrived back at Woburn Square doubled over in pain. Watching from the gallery, Lord Peter Wimsey sits in gloom beside Freddy Arbuthnot, while his own Miss Climpson sits on the jury.

• The three gastric attacks of March 31, April 15, and May 12 that line up with Harriet’s meetings and poison purchases
• Prussic acid, strychnine, and a failed attempt at aconitine, each bought at a different shop under a different name
• The ambiguous letter from Harlech about taking ‘a trip out west’, read as suicide by the defence and a Barbados voyage by the Crown
• Why the omelette, the shared dishes, and the analysed bottle of Corton seem to clear Urquhart’s dinner table
• The courtroom fight over whether Harriet had ‘a cup of coffee ready’ or poured it from a saucepan in front of Boyes