This documentary examines the case of one of the UK's most notorious double killings. Colin Howell was a married dentist with four children and a pillar of the Baptist community in Coleraine, but was having an affair with married Sunday school teacher Hazel Buchanan. They agreed the only way they could be together was if they murdered their spouses and Howell devised a plot to poison them with carbon monoxide fumes and then stage their deaths as a double suicide