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.
Although the origin of the story is unclear, it is rooted in Eleanor Brewer Philby's 1967 biography Kim Philby: The Spy I Loved, which delves into the life of a spy. The screenplay was penned by Jim Piddock.