Paris, 1879 and the Great Revolution has begun. Every day scores of the noblest heads in France are lost in the fearful embrace of Madame la Guillotine. Every few minutes a freshly sliced 'loaf'. Watching these proceedings with malevolent glee are Citizen Camembert, Chief of the Secret Police and his assistant Bidet. Meanwhile across the Channel, the English carried on with their normal pursuit of pleasure. The same old round; the same old people; the same old house parties; the same old concerts; the same old balls.
Learning of the plight of their aristocratic counterparts. Two chivalrous Englishmen decide to lift a finger (or even two) and rush to the rescue. These are Sir Rodney Ffing and Lord Darcy. Coincident with their arrival in France, the guillotine is cheated of a number of noble heads. Using a variety of disguises, Sir Rodney and Darcy affect a number of spectacular rescues, always leaving in their wake a slip of paper bearing the contemptuous sign of two extended fingers - one digit having a blackened fingernail - the Black Fingernail has struck again!
Robespierre, head of the Revolution, conveys his displeasure to Camembert and warns him that on no account must a prominent leader of the Royalists, the Duke de Pommfrit be allowed to escape. In spite of increased precautions the Black Fingernail succeeds, and the head executioner, Malabonce, loses his head in the subsequent confusion. Camembert and Bidet are left to contemplate the accursed mocking sign attached to the guillotine itself.
The trail now leads to a coaching inn at Calais and it would appear that Sir Rodney and his gallant companions are trapped. It's thanks to the timely intervention of a beautiful French girl, Jacqueline, that Sir Rodney is able to escape, leaving his heart and a locket behind as a token of his undying love. Sadly, Jacqueline is captured and taken to Paris for interrogation by Robespierre. As a result, Camembert and Bidet are despatched to England to continue the search, taking with them Desiree Dubarry.
Assuming the mantle of escaped French aristocrats, they stumble across Sir Rodney's country mansion where a ball is in progress. There they discover the presence of the Duke de Pommfrit. Desiree uses her ample charms in an endeavour to extract some information regarding the Black Fingernail. But it is Sir Rodney himself, caught in an unguarded moment, who gives the game away to Desiree.
Camembert forces a duel with Sir Rodney, who having learned of Jacqueline's imprisonment in the Bastille is anxious to return to Paris and rescue her. Needless to say, the duel is reduced to a farce and the opposing factions head back to France with Sir Rodney intent on rescuing his fair lady. Too late, he discovers that Jacqueline has been removed from the Bastille to Camembert's country residence - the Chateau Neuf.
Camembert has set his final trap. Knowing that Sir Rodney will attempt to rescue Jacqueline, he has the Chateau well guarded, with the lady as bait. Unfortunately, our villain does not reckon with the fatal fascination which our hero exercises over Desiree and it is with her help that he rescues Jacqueline - not before a stirring passage of arms inside the Chateau that reduces the whole place and its priceless objects d'art to a shambles.
Pausing only long enough for a double wedding in an English country church, Sir Rodney returns once more to Paris to witness the execution of two of its leading citizens. This time, however, no rescue is planned. Camembert and Bidet meet their fates at the hands of the Black Fingernail.
- Sid James as Sir Rodney Ffing/The Black Fingernail
- Kenneth Williams as Citizen Camembert
- Jim Dale as Lord Darcy Pue
- Charles Hawtrey as Duke de Pommefrite
- Joan Sims as Desiree Dubarry
- Peter Butterworth as Citizen Bidet
- Dany Robin as Jacqueline
- Peter Gilmore as Maximilien Robespierre
- Marianne Stone as Landlady
- Michael Ward as Henri
- Leon Greene as Malabonce