Three members of the interlocking Mortimer/Collard/Vandervent families are killed by arsenic poisoning. The family's money, widely thought to be considerable, comes from the family business as toy merchants; they also own a pair of spectacularly something houses near to each other at Blackheath. At the time of the poisonings, the matriarch of the family lives in one of the houses with one of her daughters as yet unmarried; her son and daughter with their respective spouses live in the other. Roger Vandervent, a widower, came to his marriage to the other Collard daughter with a son by a previous marriage. Those are the principals.
Whew. Have you got all that?
Roger Vandervent is the first to go, then next that matriarch, Harriet Collard. The daughter-in-law Ysabel Collard is arrested for the murders, is convicted, and sentenced to be hanged, when a third murder occurs, that of her husband George Collard. Since Ysabel is in prison, she's clearly innocent of the third murder, and now it's presumed she's innocent of the first two as well. She's released, and the murders go unsolved in the world of the novel, but a final postscript reveals that, in fact, a rough justice prevailed.
This warmed up as it went along. The initial exposition, which I boiled down to a paragraph above, was pretty leaden and a bit unclear; I had to keep referring back. Also Symons, who is a scholar of the mystery novel tradition among one of his many hats, wears his learning rather heavily in this, clanking along like a full suit of armor to prevent any accusations he didn't know his period. Consider:
She [Ysabel] sat alone in the coach marked "For Ladies Only" and carried with her George Newnes' publication The Strand Magazine. She turned the pages and glanced at the plentiful illustrations which, together with the stories by a writer named Doyle, she supposed to be the secret of the magazine's success. She read nothing, however,...I appreciate a sly Sherlock Holmes reference as well as the next person, but did we need George Newnes? Ah, well, your mileage may vary...
But the resolution was stronger. The solution was clever and there was even some emotional weight in the denouement. First the doctors and then the police are comically incompetent, and the solution to the case is revealed in that postscript by Paul Vandervent, the son by a previous marriage. Satisfactory, as Nero Wolfe might say.
Vintage Mystery Challenge. Silver Age. What. Means of Murder In The Title