One of seven espionage movies that Charles Gray made around the mid to late 60s and early 70s, the others being Doubles masques et agents doubles (1965), On ne vit que deux fois (1967) , The Man Outside (1967), Les diamants sont éternels (1971), Le gang de l'oiseau d'or (1969), and Commando pour un homme seul (1971), (Gray does not appear in the last-named of these films, but does speak on the soundtrack, dubbing all the lines for Jack Hawkins).
Reportedly, around the time that this movie was made and released, George Peppard (John Shay) and Judy Geeson (Polly Bendel) were involved in a personal relationship.