The "black robe" in the title of this suspense film belongs to a female lawyer, Florence Nat (Annie Girardot) who has just lost a case in which she defended Simon Risler (Claude Brasseur), a man wrongly accused of murder. Risler escapes before he can be put in prison, and seeks help from attorney Nat in finding the real killer, partly by going after the police inspector who framed him in the first place. A retired surgeon, in the process of setting up a drug rehab clinic gets involved in solving Risler's case, and soon the solution seems to be pointing to high-ranking figures with every desire and ample means to keep the truth well-hidden.