"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.
Colin convinces himself to confess his crime, but Hazel pleads not guilty.
With lingering police suspicions Colin and Hazel are prevented from being together.
Lesley forces Colin and Hazel to put their plan into action straight away.
Colin Howell, a leading member of the Baptist community, embarks on an affair.