Based on the novel by Baroness Orczy, an English fop secretly saves French aristocrats from the guillotine without telling his blackmailed French wife.
In 1792 fifty are being guillotined per day in France. A soldier catches an aristocrat disguised as a farmer; but disguised as a poor woman the English Scarlet Pimpernel (Leslie Howard) rescues the family of Count de Tourney by telling guards plague is in the wagon. They are transferred to a coach and taken to England escorted by troops loyal to the Pimpernel. Robespierre recalls from London his ambassador Chauvelin (Raymond Massey) and orders him and the Count de Tourney to catch the Pimpernel.
In England Marguerite Blakeney (Merle Oberon) sits for a portrait. Her husband Percy Blakeney (Leslie Howard) recites his poem about the Scarlet Pimpernel and plays the fop, saying he is fatigued from taking the baths. Marguerite is estranged from him, saying he has changed. Chauvelin has men following her brother Armand St. Just (Walter Rilla), whom Marguerite asks not to risk his life for Percy's money. Percy loves Marguerite even though she caused some to be guillotined. Percy discusses clothes with the Prince of Wales (Nigel Bruce) and attends a boxing match. Chauvelin tells Marguerite that Armand is arrested and asks her to name the Scarlet Pimpernel. She asks Percy if it is Andrew.
At a ball Chauvelin tells Marguerite to get a note from the sleeve of Andrew (Anthony Bushell). She tells Andrew she has a headache and takes the burning note to breathe the smoke. She tells Chauvelin of a meeting in the library at midnight. At 12 Chauvelin finds Percy sleeping on a couch and then falls asleep in a chair. He awakes to find a note with a pimpernel.
At home Percy uses the pimpernel seal on a note to Andrew. Marguerite tells him Armand is arrested. Percy asks her why she denounced the Marquis, and she admits she told Chauvelin, thinking he was a friend. Now she has betrayed the Pimpernel to him to save her brother. Percy tells her she loves Pimpernel, but he would not leave his wife. She asks him to help Armand. When she sees the pimpernel on Percy's portrait, she goes to Andrew to warn him Chauvelin knows Percy is Pimpernel. Percy is disguised as a French soldier as they search for Pimpernel. Chauvelin learns of a meeting at an inn and finds Marguerite there, arresting her. Percy greets Chauvelin, and Wilmot arrives with a pistol. When Chauvelin tells Percy Marguerite is arrested, Percy offers his life for hers; but the firing squad turns out to be his men. Chauvelin is put in a dungeon, and Percy and Marguerite take a ship to England.
This aristocratic fantasy implies that a useless fop may be a courageous hero and that even his wife may not realize it. The English are portrayed as civilized compared to the French terror. Saving people from unjust death penalties is truly heroic though, and the irony of the story is delightful.