Thomas Lynley, Barbara Havers, and others are back after some fairly harrowing events in previous novels.
Havers has a transfer to Berwick-on-Tweed hanging over her, with Chief Superintendent Ardery ready to sign it at the least excuse. She's gotten completely restrained in dress and behavior--and Lynley feels he's effectively lost the use of his partner, previously an excellent detective. Havers needs some leeway if she's to do anything useful. Dorothea Harriman, department secretary, tries befriending Havers in the hope of changing her focus just a bit, with mixed results.
Then a famous feminist author Havers has met briefly, Clare Abbott, dies suddenly, and her editor, Rory Statham, doesn't believe it's "just" a heart attack. At Havers' urging, Lynley pushes for and gets a second autopsy--which finds evidence of poison, specifically sodium azide.
Then Statham is found barely alive, apparently of the same poison.