Plot

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 <[email protected]>

It is the time of the French Revolution, and 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 and save their French counterparts from beheading by the guillotine.

Enraged revolutionary leader 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, after his calling card of "two digits rampant". Duke De Pommefrite (Charles Hawtrey) is the next target who is due for beheading shortly.

Ffing and Darcy get Camembert's guillotine to malfunction & then as a bet get him to behead his own executioner & 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, & 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.

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.