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.

Inspector John Marlott investigates a series of crimes in 19th Century London, which may have been committed by a scientist intent on re-animating the dead.