Though not credited, the story is based on Eleanor Brewer Philby's 1967 book Kim Philby: The Spy I Loved, published in 1967. The screenplay was written by Jim Piddock.
The narrative is based on Eleanor Brewer Philby's 1967 book Kim Philby: The Spy I Loved, which explores the life of a spy. Although the authorship is not credited, the screenplay was written by Jim Piddock.