The Scarlet Pimpernel begins in the throes of the French Revolution, with the revolutionary masses at the West Barricade waiting for fleeing aristocrats to be captured and sent to the guillotine. But we learn that in recent times, more and more aristocrats have escaped because of the help of the famous Scarlet Pimpernel who comes in disguise to free the nobility from certain death.
Meanwhile in England, several French escapees gather with the League of The Scarlet Pimpernel in a small pub, where they wait the arrival of the latest refugees. The rescued Comtesse de Tournay soon arrives, with her daughter and son, but says that her husband remains in Paris. She wants to thank the Scarlet Pimpernel for rescuing her, but is told his identity must be kept a secret. She mentions that back in France, the women are terrible for their traitorous actions - and specifically mentions Marguerite St. Just, who condemned a family to die. At that precise moment, Marguerite St. Just arrives.
Here in England, Marguerite St. Just is known as Lady Blakeney, for she is married to Percy Blakeney, the richest and most fashionable man in England. But Percy is also seemingly a buffoon - imbecilic and dull, and when the Comtesse's son challenges him to a duel to avenge his mother's disdain of Marguerite, Percy looks even more the fool as his wife wittily defuses the situation.
Lady Blakeney's brother leaves for France, but before he leaves, he urges his sister to tell Percy why she denounced the St. Cyr family, but she says he already hates her for it, no matter the circumstances. As she goes back to the pub, she meets Chauvelin, a French officer, intent on discovering the identity of the Scarlet Pimpernel. He has been spying on the activities of the Englishmen at the pub, and says that Marguerite must help him find the Pimpernel. She refuses.
Later that night, two members of the League of the Pimpernel are ambushed by Chauvelin just as they discuss plans to rescue the Countess's husband. Chauvelin finds a letter from Marguerite's brother, Armand, and now he sees that he can blackmail her to help her find the Pimpernel.
Chauvelin corners Lady Blakeney at the opera and reveals the letter he has found. If she does not help him, he will ensure that her brother is executed.
At the ball that night, Lady Blakeney finds out that the Pimpernel will be waiting in one of the rooms at one o'clock that night. But when she tells Chauvelin of this, he goes to the room, only to find Percy stretched out on the couch taking a nap. Chauvelin tells Lady Blakeney that she better help him find the Scarlet Pimpernel or else her brother will be in danger.
Lady Blakeney and Percy go to their country home outside the city of London. Under the stress of all of her dealings with Chauvelin and the coolness of her husband, Lady Blakeney explains the circumstances that led her to denounce the Marquis de Cyr's family at the tribunal. But Percy says she's told him too late and he remains cool to her, even though deep down he still loves her. He promises to save her brother.
Later that night, Lady Blakeney is peeking around her husband's study when she finds a ring emblazoned with the image of the Scarlet Pimpernel -- she discovers his true identity. And indeed, now she realizes she has betrayed her own husband to Chauvelin - and must make the choice between saving him and her brother.
Lady Blakeney sails to Calais, where she hides at an inn, only to witness a tense encounter between Percy and Chauvelin, who have accidentally run into each other. Unbeknownst to Percy, however, Chauvelin has six soldiers on the way to arrest him.
But Percy manages to outwit Chauvelin and he escapes, and a chase ensues to find him. Marguerite follows behind as Chauvelin and his henchmen enlist the help of an Old Jew, who claims to know the way that Percy left.
The Jew takes them to a hut, where Marguerite realizes that the fugitives are hiding. She throws herself towards the hut in an effort to save the ones inside, but she's captured by Chauvelin. Chauvelin and his henchmen go inside the hunt and find that its empty. A moment later they see a boat drifting out of the harbor and realize the fugitives have escaped.
Chauvelin leads his men on a hunt for Percy, but not after making sure the old Jew is mercilessly beaten. Only after Chauvelin leaves does the old Jew get up groggily and reveals himself to the bound Marguerite as her husband Percy in diguise.
Percy and Marguerite set sail for England the next day, reconciled, having freed the fugitives and Percy promises to make sure Chauvelin never steps foot in English noble society again.