Not so much "spy" as "assassin," though, for Edward Fox is cast as an assassin, doing a job that will earn him his retirement. He is to kill President Charles de Gaulle, "le Grand Charles" himself, in Liberation Day, as we eventually find out--but not until much later in the film.
The killer goes through many personalities, disguises, and changes, and manages to kill a few innocents on the way to his final conquest. He is unknown to the gendarme, who are on his trail early, with the help of an informer on the inside, but eventually his appearance becomes known to them--and even then, in spite of an alert commissioner of police in Paris who is every bit his match, Inspector Lebel (Michel Lonsdale)--he almost makes his kill. Had he done so, of course history would have had to be re-written because of a movie, an unlikely event--so we knew that he would fail, but still the plot was so well played that the suspense was never lost.
Written by Frederick Forsyth and directed by Fred Zinneman, perhaps this film was predestined for greatness, but in any case it is one of the great ones.
Joseph (Joe) Pierre
author of Handguns and Freedom...their care and maintenance
and other books