The first Carry On to be produced by the Rank Organisation, Don't Lose Your Head, was not conceived as a part of the series and was first released without the Carry On prefix. However, the ongoing popularity of the series persuaded Rank to add the prefix to the titles of this and the following film, Follow That Camel, when they were re-released.
French actress Dany Robin makes here her only Carry On appearance.
The film was followed by Carry On Follow That Camel (1967).
It is the time of the French Revolution. Whilst the French aristocracy is losing their heads (literally), two bored English noblemen, Sir Rodney Ffing (pronounced "Effing") and his best friend Lord Darcy Pue, bored with the endless rounds of country pursuits, decide to have some fun and save their French counterparts from beheading by the guillotine.
The enraged and incompetent revolutionary leader, Citizen Camembert, and his toadying lackey, Citizen Bidet, scour France for the elusive saviour of the nobles, who is nicknamed “The Black Fingernail” after his calling card of “two digits rampant”. After a series of audacious rescues, the Fingernail succeeds in rescuing the Duc de Pommfrit whilst disguised as an insurance salesman, and in the process, tricks Camembert into guillotining his own executioner. Camembert is chastised by his superior Maximillien Robespierre and threatened with the guillotine, unless he captures the Fingernail.
During his escape from France, Sir Rodney meets his true love, Jacqueline, leaving her with a silver locket containing a set of his mother's false teeth. On discovering Jacqueline, Camembert and Bidet imprison her. Using the locket as a trap, they travel to England to uncover the real identity of The Black Fingernail. They are accompanied by Camembert's lover, Desirée, 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.
After a series of intrigues at a ball at Ffing House, everyone's identity is unknowingly revealed. Foppish Sir Rodney challenges Camembert to a rigged duel in order to get a head start on his journey to Paris to rescue Jacqueline. Desirée is now herself in love with the hero and will do all she can to save him from the guillotine in return for his promise that she will marry her titled man.
On arrival in Paris, the Fingernail discovers that Jacqueline has been moved from the Bastille to the Château Neuf, the former home of an avid art collector and member of the aristocracy, recently presented to Citizen Camembert - by himself. Ffing, Lord Darcy, and the Duc de Pommfrit travel there to rescue her. During the ensuing fight between the rescuers and the French soldiers, most of Camembert's new art collection is destroyed. With the help of Desirée, Jacqueline is rescued. All five flee the collapsing château to safety, whilst Camembert and Bidet attempt to stop it from falling down.
For their incompetence, Robespierre 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 Desirée, who has married the Duc de Pommfrit (as he has a title), much to her own chagrin.
