Already the most powerful man in France, Maximilien Robespierre (Richard Basehart) wants to become the nation's dictator. He summons François Barras (Richard Hart), the only man who can nominate him before the National Convention. Barras refuses to do so and goes into hiding.
Meanwhile, patriot Charles D'Aubigny (Robert Cummings) secretly kills and impersonates Duval (Charles Gordon), the bloodstained prosecutor of Strasbourg, who had been summoned to Paris by Robespierre for some unknown purpose (which Robespierre's enemies want very much to ascertain). Neither Robespierre nor Fouché (Arnold Moss), the chief of his secret police, have met Duval before, so the substitution goes undetected. Robespierre informs D'Aubigny that his black book, containing the names of those he intends to denounce and have executed, has been stolen. Robespierre's numerous foes are kept in check by the uncertainty of whether their names are on the list. If they were to learn for certain that they are, they would band together against him. He gives D'Aubigny authority over everyone in France, save himself, and 24 hours to retrieve the book.
D'Aubigny meets Barras (Richard Hart) through his sole contact, Madelon (Arlene Dahl), whom D'Aubigny once loved. However, he was followed, and Barras is arrested by the police, led by Saint-Just. D'Aubigny finds himself in an uncomfortable position, but manages to allay the suspicions of both sides that he has betrayed them.
He goes to visit Barras in prison, and informs him that three of his best men have been murdered. Strangely, their rooms had not been ransacked to search for the book, leading D'Aubigny to surmise that it was never stolen in the first place, and that Robespierre is using the alleged theft to distract his foes. Saint-Just, still suspicious, sends for Duval's wife to identify her husband. Through quick thinking, Madelon pretends to be Madame Duval and extricates her former lover just before the real Madame Duval arrives.
Before news of his impersonation gets out, D'Aubigny returns to Robespierre's private office to look for the book. There he encounters the opportunistic Fouché, who is seemingly willing to sell out his master. When D'Aubigny finds the book, however, Fouché tries to stab him. D'Aubigny strangles him into unconsciousness and escapes.
He and Madelon hide out at the farmhouse of a fellow conspirator, but their location is extracted through torture. A nighttime chase ensues. D'Aubigny gets away, but Madelon is caught, taken back to Paris, and tortured. She refuses to talk.
As the Convention is about to convene the next day, Fouché shows up and offers to trade Madelon for the book. D'Aubigny turns him down. The book is passed from hand to hand among the delegates. Thus, when Robespierre arrives to denounce Barras, the crowd turns on him instead. He nearly brings the mob to heel with his golden words, but Fouché has his henchman shoot Robespierre through the jaw, silencing him. Robespierre is taken to meet Madame Guillotine.
D'Aubigny searches Robespierre's office, finds a secret room, and rescues Madelon. Fouché falls into conversation with an army officer as the crowd celebrates the death of Robespierre. Fouché, about to take leave of the officer, asks his name. The man replies, "Bonaparte. Napoleon Bonaparte." Fouché, unimpressed, still promises to remember the name.Robert Cummings as Charles D'Aubigny
Richard Basehart as Maximilien Robespierre
Richard Hart as François Barras
Arlene Dahl as Madelon
Arnold Moss as Fouché
Norman Lloyd as Tallien, Barras' associate
Charles McGraw as Sergeant
Beulah Bondi as Grandma Blanchard, the woman who shelters D'Aubigny and Madelon at the farmhouse
Jess Barker as Saint-Just
Robert Warwick as the Leader of the Convention
Producer Walter Wagner, director Anthony Mann, cinematographer John Alton and production designer William Cameron Menzies used their combined talents to make a low budget "epic" using Broadway stars and shooting on sets only costing $40,000.