Trama

During French Revolution, English nobles Sir Rodney and Lord Darcy aid French aristocracy against Robespierre. Disguised as "Black Fingernail", Sir Rodney battles Camembert and Bidet, French secret police leaders.

The time of the French Revolution, and Citizen Robespierre is beheading the French aristocracy. When word gets to England, two noblemen, Sir Rodney Effing and Lord Darcy take it upon themselves to aid their French counterparts. Sir Rodney is a master of disguise, and becomes "The Black Fingernail", scourge of Camembert and Bidet, leaders of the French secret police.—Simon N. McIntosh-Smith <Simon.N.Smith@cs.cf.ac.uk>

In the new French Republic during the Revolution, members of the French aristocracy are being rounded up and publicly executed by beheading via guillotine, these executions overseen by the head of the secret police, Citizen Camembert. Learning what is happening across the channel, Sir Rodney Ffing and Lord Darcy Pue, English dandies and layabouts, decide, using various disguises, to rescue their aristocratic French counterparts from execution, they leaving a calling card of a diagram of two upturned fingers in a sign of victory, one finger with a black fingernail. As such, Ffing, the unknown rescuer, is given the moniker the Black Fingernail. Ffing and Pue return to England with the Duke de Pomme Frits after rescuing him for his own protection. Believing the diagram to be a sign of English aristocracy, Camembert and his sidekick, the dim Citizen Bidet, go on a mission to apprehend the Black Fingernail. Camembert finds what he believes to be the Black Fingernail's weakness after it is discovered that he has fallen in love with a French maiden, Jacqueline, who is used as a pawn to lure the Black Fingernail out into the open.—Huggo

It is the time of the French Revolution, and the French nobles are being routinely hunted by the masses. Many nobles are dragged from their hiding place at the Bastille to the several guillotines all over the city for their execution by beheading. At its peak the guillotines are executing one noble every five minutes.

Two bored English noblemen, Sir Rodney Ffing (pronounced "Effing") (Sid James) and his best friend Lord Darcy Pue (Jim Dale), decide to have some fun. Ffing and Darcy were spending all their time in England with women, tea parties and balls and had nothing better to do. They are informed that the French Ladies and Nobles are being executed just for being nobles, and nobody is lifting a finger to do anything for them. Ffing and Darcy decide to save their French counterparts from beheading by the guillotine. Thus, they started a rescue operation which led to the rescue of many nobles from France. Ffing left his calling card "two digits rampant" (which is considered a sign of contempt back in England) after each rescue, in which one fingernail is painted black.

Enraged revolutionary leader and the chief of the secret police, Citizen Camembert (Kenneth Williams) and his toadying lackey, Citizen Bidet (Peter Butterworth), scour France and England for the elusive savior of the French nobles, who has become known as The Black Fingernail. Camembert is tasked with the finding and execution of French nobles and as such is the most dreaded man in all of France. Duke De Pommefrite (Charles Hawtrey) is the next target who is due for beheading shortly.

Ffing and Darcy get Camembert's guillotine to malfunction (by wedging an obstruction in the path of the blade of the guillotine). Ffing gets on the platform in the guise of inspecting the guillotine and declares that the design is flawed, to which Camembert objects. As a bet (for 100 Francs) Ffing gets Camembert to behead his own executioner Malabonce (Leon Greene), by way of testing the guillotine.

As a result, the crowd riots and allows Ffing to escape with Pommefrite. Pommefrite is an influential leader of the French nobility and hence an important figure for the revolutionists to get rid of. The revolutionists believe that the Black Fingernail had a role in helping Pommefrite escape.

Ffing and Darcy use various disguises to fool Camembert and use him to get out of Paris, which is heavily blockaded. Ffing disguises as a woman (headed to Calais) and seduces Camembert for a kiss and leaves his Black Fingernail calling card. Camembert panics and decides to pursue Ffing to Calais, but Ffing & Darcy were now disguised as his coachmen and travelling with Camembert's entourage to Calais.

They get into Calais, from where they get into a boat to escape France and into England with Pommefrite. During his escape from France, Ffing meets his true love, Jacqueline (Dany Robin), leaving her with a silver locket containing a set of his mother's false teeth. When Ffing met Jacqueline, it was late at night and very dark, so Ffing did not get a very good look at her face.

Camembert is chastised by his superior Maximillien Robespierre (Peter Gilmore) and threatened with the guillotine, unless he captures the Fingernail. Camembert is ordered to go to England and get Pommefrite back.

Ffing, who is the real Black Fingernail, had become a hero to many Frenchmen for his exploits & Jacqueline had fallen in love with him. Camembert and Bidet knew of Jacqueline's love for the Black Fingernail. Bidet and Camembert apprehend Jacqueline (Dany Robin) and imprison her.

Camembert decides to use Jacqueline as leverage to lure The Black Fingernail back. Using the locket (which they knew was given to Jacqueline by the Black Fingernail) as a trap, they travel to England to uncover the real identity of The Black Fingernail. They are accompanied by Camembert's lover, Desiree (Joan Sims), who is on the lookout to marry a man with a title, disguised as the Comte and Comtesse De La Plume De Ma Tante. Desirée pretends to be Camembert's flamboyant sister, whilst wearing the locket.

Camembert, Bidet and Desiree attend an aristocrats balls thrown by Ffing to inquire about the Black fingernail. At the ball Camembert uses Desiree to wear the locket given by the Black Fingernail to Jacqueline. Ffing notices the locket, believes Desiree to be Jacqueline & proceeds to meet her secretly & kiss her. However, Desiree reveals that she is not Jacqueline & that she is under Camembert's control at Bastille. Bidet overhears the entire conversation & tells Camembert that Ffing is the Black Fingernail.

Now Camembert is unaware that Desiree (Joan Sims), Camembert's flamboyant mistress, is herself in love with the hero and will do all she can to save him from the guillotine. Using Desiree's honor as bait, Camembert challenges Ffing to a duel, which he accepts. Camembert rigs the contest by instructing Bidet to load only 1 pistol. But Ffing plays a trick which makes Camembert end up in a human excreta pit & Ffing escapes with Darcy & Pommefrit to France, where he intends to rescue Jacqueline from Bastille.

Camembert arrives with reinforcements & transfers Jacqueline to his private castle, which he presented to himself for his services to the revolution. Ffing gets inside the castle with the help of Desiree who only agrees to assist in return for a marriage title. Ffing gets to Jacqueline to rescue her & then fights his way out of the castle, with the help of Darcy & Pommefrit, who help fight Camembert's soldiers and, in the process, destroy all the priceless artifacts of the castle.

For their incompetence, Robespierre (Peter Gilmore) orders the execution of Camembert and Bidet on a double guillotine. They are relieved to know that the Fingernail is not there to see it, until the executioner reveals that he is The Black Fingernail himself.

Afterwards, in England, Ffing marries Jacqueline, who becomes Lady Ffing, whilst he keeps his promise to Desiree, who has married the Duke De Pommfrite (as he has a title), much to her own chagrin.