It would appear that most, if not all, Hitchcock's devotees have little to admire in this work. Some complain of plodding and poor acting, and so on. I disagree. All this (and I omit most of it!) may explain why I have difficulty with the Hitchcock oeuvre overall. For me "Topaz" at least attempts to deal with real people within a synthetic world (the world of espionage).
Forget for a moment the intransigencies of the much-vaunted "Vertigo" and concentrate instead on the opening shots in the film under discussion. The display of "red" military might should be enough to chill the spine of any thinking, caring individual. Hitchcock is to be thanked for that.
The film juxtaposes the human conditions of power (violence) and tenderness that make for the aphorism "life is the medium in which love making and conflict effectively cancel one another out".
Yes, there are a number of implausible patches - would the episode within Cuba itself have been effected with such naivety, for example?
And the ending, like the beginning, is for me quite satisfactory tinged, as it is, with a sense of irony and humour as the high ranking French double agent boards a plane for "sanctuary" in the USSR.
I lived through the Cuban Missile crisis as a young man - perhaps this helps me identify with this film?