The Secret is the story of a real-life double murder. James Nesbitt plays Colin Howell, a respectable dentist and pillar of the community, who became a killer in partnership with a Sunday school teacher, Hazel Buchanan, played by Genevieve O'Reilly.
When Colin Howell and Hazel Buchanan meet at their local Baptist Church in rural Northern Ireland, they start an affair which will have cataclysmic consequences.
Colin and Hazel carry out his plan to kill Lesley and Trevor, and stage it to look like a suicide pact. But will the community and the police believe their story?
As the inquest approaches, Colin and Hazel have to hold their nerve. But even if they do get away with it, can their relationship survive?