'It was a dance between control and manipulation' - Colin Howell at the trial of Hazel Stewart

May 1991. The location: a quiet seaside town. The scene: two bodies in a car filled with carbon monoxide. Police officer Trevor Buchanan and nurse Lesley Howell have apparently taken their own lives, unable to live with the pain of their spouses' affair with each other.

Their adulterous pair - Sunday school teacher Hazel Buchanan and dentist Colin Howell - had met in the local Baptist Church. Following the apparent double-suicide, they continue their affair but later marry other people.

A series of disasters in Howell's life - bereavement, financial disaster, sexual scandal - prompt him share his darkest secrets to the elders of his church. He reveals that he and Hazel Stewart had conspired to murder their spouses nearly two decades earlier.

That confession leads to two of the most sensational murder investigations ever seen in the UK, Howell's conviction for murder in December 2010 and Stewart's in March 2011. She continues to protest her innocence.