The novel opens with mystery author Harriet Vane on trial for the murder of her former lover, Phillip Boyes: a writer with strong views on atheism, anarchy, and free love. Publicly professing to disapprove of marriage, he had persuaded a reluctant Harriet to live with him, only to renounce his principles a year later and to propose. Harriet, outraged at being deceived, had broken off the relationship.
Following the separation, the former couple had met occasionally, and the evidence at trial pointed to Boyes suffering from repeated bouts of gastric illness at around the time that Harriet was buying poisons under assumed names, to demonstrate – so she said – a plot point of her novel then in progress.
Returning from a holiday in North Wales in better health, Boyes had dined with his cousin, the solicitor Norman Urquhart, before going to Harriet's flat to discuss reconciliation, where he had accepted a cup of coffee. That night he was taken fatally ill, apparently with gastritis. Foul play was eventually suspected, and a post-mortem revealed that Boyes had died from acute arsenic poisoning. Apart from Harriet's coffee and the evening meal with his cousin (in which every item had been shared by two or more people), the victim appeared to have taken nothing else that evening.
The trial results in a hung jury. As a unanimous verdict is required, the judge orders a re-trial. Lord Peter Wimsey visits Harriet in prison, declares his conviction of her innocence and promises to catch the real murderer. Wimsey also announces that he wishes to marry her, a suggestion that Harriet politely but firmly declines.
Working against time before the new trial, Wimsey first explores the possibility that Boyes killed himself. Wimsey's friend, Detective Inspector Charles Parker, disproves that theory. The rich great-aunt of the cousins Urquhart and Boyes, Rosanna Wrayburn, is old and senile, and according to Urquhart (who is acting as her family solicitor) when she dies most of her fortune will pass to him, with very little going to Boyes. Wimsey suspects that to be a lie, and sends his enquiry agent to get hold of Rosanna's original will, which she does in a comic scene exposing the practices of fraudulent . The will in fact names Boyes as principal beneficiary.