One of seven espionage movies that Charles Gray made around the mid to late 60s and early 70s. The others being Masquerade (1965), You Only Live Twice (1967) , The Man Outside (1967), Diamonds Are Forever (1971), The File of the Golden Goose (1969), and When Eight Bells Toll (1971),
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.