The cinema film was an expanded re-working of James Mitchell's original 1967 TV pilot episode, "A Magnum for Schneider". Taking much of its new material from Mitchell's own novelisation of that teleplay, it was based more on the novel than on the original television script. For legal reasons, no mention is made of the pilot teleplay or the TV series. The film's credits mention only the novel as its source, identifying it by the title – "Red File for Callan" – under which it had been published by Simon & Schuster in the United States in 1969, so as to avoid any reference to the title of the ABC television pilot.