This episode was originally scheduled for broadcast on 11 September 2001, but was dropped to accommodate America under Siege, a BBC News Special on the events that took place in New York and Washington that day.
The killer uses a multiple-exposure set-up in the hope of photographing the souls of the murder victims at the moment they leave the dying bodies.
Arthur Conan Doyle believed that the spirit world could be photographed, and published a book titled The Coming of the Fairies in which he defended the validity -- and expressed the importance -- of "The Cottingley Fairies", a series of photos purporting to show two young girls interacting with fairies around the time of World War I.
Despite his being "a man of science", the memory of dead fiancée Elspeth Scott causes Arthur Conan Doyle to become increasingly obsessed with spiritualism through attending Helena Petchey's séances and his perceived visions of Miss Scott.
In real life, author Arthur Conan Doyle became an ardent spiritualist following the deaths of his wife Louisa, brother Innes and other family members. Doyle often wrote and gave interviews about the subject.