Michel Deville, previously a director of comedies, turns in Dossier 51 to a realistic account of secret service intelligence activities.

The story tells of the efforts of the spies of a foreign power, who compile a dossier on a French diplomat, in an effort to recruit him while he is still relatively obscure. Their target is watched, recorded, filmed ... and above all, analysed, to uncover a trait, one personal secret, that will render him vulnerable to manipulation.

The diplomat, Dominique Auphal, is destined to become a key figure in Third World trade relations.

As the dossier is being compiled we see how his activities are recorded by a multiplicity of surveillance devices. Those who spy on him, the technicians, are detached and dispassionate, though involved in petty administrative squabbles within their own hierarchy.

Dominique is observed in both his public and private life. The agents construct a psychological portrait of him, building it up painstakingly detail by detail, constantly reinterpreting the result in the light of newly emerging evidence. Finally, the technicians winkle out Dominique's secret: behind the carefully developed smokescreen. he is discovered to be a homosexual.

The flaw exposed, the agents set the trap in which they hope to catch their compromised quarry.