Former detective Marcella is devastated when her husband of 15 years, Jason, suddenly announces he is leaving her and no longer loves her. She is visited by the detective in charge of an investigation linked to the Grove Park murders, one of Marcella’s former unsolved cases. The case involves a serial killer who the police believe has begun to kill again after 11 years. Marcella decides to return to work after several years spent raising their two children, Emma and Edward, who are away at boarding school. Reinstated with her old rank of detective sergeant, Marcella has a very different idea about who should be the main suspect and tells her new superior, to no avail. She discovers her husband, who works as a legal advisor for DTG Construction property developers, is having an affair with Grace Gibson, the CFO of the firm and daughter of the owner, Sylvie Gibson.
Grace Gibson is reported missing and Marcella vaguely remembers visiting her the previous evening to confront her about Jason. Marcella is warned by her superiors to stop harassing Peter Cullen, her prime suspect, who is in prison for killing his wife in a domestic incident but is allowed out several days a week to work at a bakery under supervision. Marcella believes Peter has a personality disorder and cannot handle rejection. Marcella herself is being secretly treated for a disorder that causes memory blackouts. Webcam girl Cara, who meets people via a cheating app and steals from them, is threatened in her internet chatroom by someone she has stolen from. Marcella is drawn to a wood on Hampstead Heath by memories and discovers Grace’s body bound with a plastic bag over her head — fitting the pattern of the Grove Park murders.
Marcella fails to report the body, which is later revealed to be that of Grace Gibson, now believed to be the fourth victim of the serial killer. Marcella’s early hopes of a reconciliation with her husband may be fruitless when she discovers that he had been having an affair for more than three years and Grace was pregnant. Jason loses his job when Sylvie discovers the affair. Marcella’s former partner, DCI Laura Porter, allows her to keep working on the case with some restrictions. The police realise Cara, who has been selling the items she steals at a pawn shop, can identify the killer but, as Marcella closes in, Cara is intentionally struck and killed in a hit-and-run. The man supposed to be supervising Peter Cullen during his work release is arrested for breaking and entering, confirming Marcella’s suspicion that Cullen had the opportunity to continue killing. Cullen is released on parole and begins stalking criminology student Maddy Stevenson, who has been meeting him for research for her dissertation. Studying CCTV, the police discover that Clive Bonn had been at Hampstead Heath around the time Gibson’s body was left there, and also learn that Marcella had been studying CCTV around Grace Gibson’s home — prior to her body being found.