originally titled 'don't lose your head' , this parody of the scarlet pimpernel story was the first carry on to be produced by rank film productions . 
two english fops , the 'powdered , be-wigged , be-ribboned' sir rodney ffing ( sidney james ) and his counterpart lord darcy pew ( jim dale ) decide to travel to revolutionary france in an attempt to rescue their fellow french royalists and aristocrats from losing their heads by the guillotine . 
due to a series of machinations and disguises , they are largely successful . 
ffing becomes known as 'the black fingernail' because he leaves a calling card behind which shows two fingers sticking up , one with a black fingernail . 
after the fingernail rescues a prominent royalist the duc de pommfrit ( charles hawtrey ) , citizen robespierre ( peter gilmore ) orders the head of the secret police citizen 'the big cheese' camembert ( kenneth williams ) and citizen bidet ( peter butterworth ) to follow the fingernail to england and do away with him . 
 ( in fact , darcy and ffing are their coachmen ! ) 
once at calais , the fingernail meets jacqueline ( dany robin ) and they fall in love instantly . 
he tells her his identity and gives her his locket . 
when camembert realises that the fingernail is nearby , he searches the inn at calais and captures jacqueline , thinking that she is wearing a diguise and is really the fingernail ! 
jacqueline is imprisoned in the bastille and camembert , his love desiree dubarry ( joan sims ) , and bidet all travel to london in pursuit of the fingernail . 
they pretend to be of noble stock , calling themselves the duc and duchesse de la plume de ma tante ( with bidet their assistant ) and are invited by darcy to a ball held by ffing . 
desiree finds out that ffing is the fingernail by wearing the locket around her neck , but she ends up falling in love with him . 
ffing attempts to stall camembert so that he can return to the bastille to rescue jacqueline , camembert has her moved to the 'chateau neuve' , and a climactic sword-fight decides who will lose their heads at the end of the film ! 
a more complex story than most carry ons , this film enjoys good production values ( sets , costumes ) and an on-form cast . 
sid james is excellent as the english fop and black fingernail , kenneth williams excels as the evil camembert , and peter butterworth expertly plays the substantial part of camembert's thick-witted crony . 
other acting honours go to joan sims who is perfect as desiree and charles hawtrey who is excellent as the french aristo pommfrit . 
although it suffers from a disasterously over-long sword fight at the end of the film , it is largely successful due to the performances of the main stars , its slick and professional production , and its better-than-usual script . 
definitely one of the best of the series and a joy to watch .