"The Secret" is based on a true story that took place in the seaside town of Coleraine, Northern Ireland, in 1991. Respected dentist and leading member of the Baptist community, Colin Howell has an affair with fellow Baptist and Sunday school teacher Hazel Buchanan. Colin's wife, Lesley, is a former nurse and Hazel is married to Trevor, a police officer. Because they wish to remain in the Baptist church, divorce is not an option, prompting Colin and Hazel to plot something much darker in order to be together. The passing of Lesley's father leaves her suicidal, setting into motion Colin's plan to kill her and Trevor.