May 1991. The location: a quiet, picturesque 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.

The 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 secretly before both later remarrying.

A series of disasters in Howell's life – the death of his eldest son, massive losses in an investment scam and the revelation that he has been sexually assaulting female patients – lead to him declaring that he is a fraud and a godless man.

He tells the elders of his Church that he and Hazel Stewart conspired together to murder their spouses nearly two decades earlier.

What follows the dramatic confession are two of the most sensational murder investigations ever seen in Ireland, leading to both Howell's conviction for murder in December 2010, and Stewart's in March 2011 – despite her protestations of innocence.