A great plague has eradicated all vampires from the planet except for four lone female survivors of the species. They must integrate into society or risk extinction.

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.