An elaborate surveillance operation is mounted by a French intelligence agency on a French diplomat, codenamed 51. While his professional life is not suspect, clandestine investigations into his private life reveal an increasing number of vulnerabilities.
Only surviving child of his adoring mother, she reveals that he was hated by her husband, who had betrayed his biological father to the Gestapo. At his Catholic school, the priests recall his propensity for fantasy about other boys. Military service kept him in an all-male environment and former comrades remember his construction of alternative realities about them. After a student affair with an anarchist of boyish appearance, he hastily married a girl of good family, who gave him two children but sleeps with other men.
When an agent wins his confidence and starts revealing knowledge of his past, he kills himself by driving into a tree. The operation is wound up.
