The programme follows a team of London detectives led by DCI Cassie Stuart (Nicola Walker) (Series 1–4), DCI Jessie James (Sinéad Keenan) (Series 5-6) and DI Sunny Khan (Sanjeev Bhaskar) as they solve cold cases of disappearance and murder.

[4][5] A year later it was confirmed that Sinéad Keenan would replace Walker as Bhaskar's new partner, DCI Jessica "Jessie" James.

Each series deals with a new case, introducing seemingly unconnected characters who are gradually revealed to have some relationship with the victim.

[8][9] Series 1 to 4 include the story of DCI Cassie Stuart's personal life: of the typical but growing conflicts in her immediate family, of her spartan arms-length relationships with individuals who like her, of her drive to solve cases and of her coping both successfully and unsuccessfully with the relentless strain of her job.

During the historical cold cases, Cassie verbalizes her driving motives: find the truth of what happened, provide closure to the living and possibly bring a criminal to justice.

[12] Series 3 investigates the murder of schoolgirl Hayley Reid (Bronagh Waugh), who disappeared from a seaside resort town on New Year's Eve 1999.

[13] Series 4 follows the discovery in 2020 of Matthew Walsh's headless and handless corpse, found inside an old freezer removed during a house clearance in north London.

[14] Series 5 investigates the murder of Precious Falade, whose body is found in the chimney of a house undergoing renovation.

While DCI James tries to deal with the revelation that her husband has been having an affair with her sister, she and the team assemble the circumstances surrounding the death, including the troubled lives of the dead woman's mother, son and key social worker.

Series 6 follows the discovery of body parts belonging to a man called Gerry Cooper, in a London marsh.

Locations included Liverpool, the London suburbs, Kingston upon Thames, the Essex coast, Westminster and the Fens.

[19] Scenes were set in Lymington, Hampshire (which substituted for the fictional Middenham and its estuary), Uxbridge, Middlesex, Amersham in Buckinghamshire, Clifton in Bristol, Ealing in West London, and King's Lynn and Hunstanton in Norfolk.

An empty mansion at Bulstrode Park near Gerrards Cross, Buckinghamshire provided the setting for the police station and lab.

A fourth series was scheduled to be made in autumn 2019; however, ITV only confirmed in January 2020 that filming had commenced, with the planned broadcasting timeframe having been delayed.

In September 2020, it was announced that filming had recommenced and the series planned on being screened in 2021, with actors Walker, Bhaskar, Reeves and Egan all reprising their roles.

Dean emotionally expresses remorse over the murder and his failure to remove himself from his roots in the violent Quinn family.

In 2017, it was reported that a remake was in development for ABC in the United States, with Josh Berman, Sony Pictures Television and BBC Worldwide Productions producing.