True crime drama A Confession continues on our screens with the fourth episode to air at 9pm tonight.
The acclaimed six-part ITV series is based on Detective Superintendent Steve Fulcher's investigation into the disappearance of Sian O'Callaghan in 2011.
But Fulcher, played in this series by Martin Freeman, broke police protocol in the hunt to find Sian, who is played by Florence Howard.
The cast also includes Siobhan Finneran as O'Callaghan's mother Elaine Pickford, Imelda Staunton as Karen Edwards (whose daughter also went missing) and This County's Charlie Cooper as Sian's boyfriend.
Here is all you need to know about the drama.
What is the true story A Confession is based on?
O'Callaghan was 22-years-old when she went missing after leaving Suju's nightclub in Swindon in the early hours of March 19, 2011 following a night out with friends.
She had planned to go to her boyfriend's house, which was only a 15 minute walk away, but she never turned up and he reported her missing.
With the police investigation underway officers visited her mother, who at first thought her daughter must have stayed overnight with a girlfriend as it was uncharacteristic of her to be out of touch with her loved ones.
Detective Superintendent Fulcher led a team to piece together her last movements, scouring CCTV footage from the club and re-tracing her steps home before analysis revealed her mobile phone signal to be bounding off a mast 14 miles away in Savernake Forest.
Fulcher ordered a full-scale enquiry into the missing person operation, and knowing the clock was ticking, started operating on the premise that O'Callaghan was alive and being held against her will.
Diligent and painstaking police work soon revealed CCTV footage of her in the High Street, which led to vital clues in the investigation.
Karen Edwards, whose own daughter Becky Godden-Edwards disappeared a number of years before, watched the investigation unfold and became convinced that her daughter was involved in a similar case as the search for O'Callaghan went on.
Fulcher extracts a confession from Halliwell, who reveals the location of O'Callaghan's body to police but confesses to the murder of Godden-Edwards as well.
However, he had broken a number of rules to get Halliwell to confess and a judge later ruled the confessions were inadmissible as evidence.
Halliwell was convicted of murdering O'Callaghan as the case was strong enough without the confession, but it wasn't until 2016 that he was charged with Godden-Edwards' murder. He was found guilty in September 2016 and sentenced to life in prison.
While Fulcher kept his job, he was found guilty of gross misconduct and received a final written warning in January 2014, before he resigned in May that year.
When will A Confession air on TV?
The series will air for six episodes from Monday, September 2 on ITV at 9pm.
Who is in the cast?
Martin Freeman (The Hobbit, The Office, Sherlock) plays the main character Detective Superintendent Fulcher, who is leading the hunt for O'Callaghan's killer.
Imelda Saunton plays Karen Edwards, the mother of Becky Godden-Edwards (another of Halliwell's victims and who plays a major part in the story) while Joe Absolom is Christopher Halliwell.
Alongside them is Charlie Cooper as Kevin Reape, Siobhan Finneran as Sian's mother Elaine Pickford and Stephanie Hyam as Becky.
Sian herself is played by Florence Howard, in her second on-screen role.
There is, and it it shows us how a weary Fulcher picks up the case after receiving a call telling him a young woman has disappeared.
Intent on catching the culprit and on finding O'Callaghan, Fulcher risks his job and reputation for the way he handled the case.