The Scarlet Pimpernel is set in 1792, during the early stages of the French Revolution. Marguerite St. Just, a beautiful French actress, is the wife of wealthy English fop Sir Percy Blakeney, a baronet. Before their marriage, Marguerite took revenge upon the Marquis de St. Cyr, who had ordered her brother to be beaten for his romantic interest in the Marquis' daughter, with the unintended consequence of the Marquis and his sons being sent to the guillotine. When Percy found out, he became estranged from his wife. Marguerite, for her part, became disillusioned with Percy's shallow, dandyish lifestyle.
Meanwhile, the "League of the Scarlet Pimpernel", a secret society of twenty English aristocrats, "one to command, and nineteen to obey", is engaged in rescuing their French counterparts from the daily executions (see Reign of Terror). Their leader, the mysterious Scarlet Pimpernel, takes his nickname from the drawing of a small red flower with which he signs his messages. Despite being the talk of London society, only his followers and possibly the Prince of Wales know the Pimpernel's true identity. Like many others, Marguerite is entranced by the Pimpernel's daring exploits.
We seek him here, we seek him there, Those Frenchies seek him everywhere. Is he in heaven?—Is he in hell? That demmed, elusive Pimpernel.Sir Percy Blakeney, Baronet (ch.12)
At a ball attended by the Blakeneys, a verse by Percy about the "elusive Pimpernel" makes the rounds and amuses the other guests. Meanwhile, Marguerite is blackmailed by the wily new French envoy to England, Citizen Chauvelin. Chauvelin's agents have stolen a letter incriminating her beloved brother Armand, proving that he is in league with the Pimpernel. Chauvelin offers to trade Armand's life for her help against the Pimpernel. Contemptuous of her seemingly witless and unloving husband, Marguerite does not go to him for help or advice. Instead, she passes along information which enables Chauvelin to learn the Pimpernel's true identity.
Later that night, Marguerite finally tells her husband of the terrible danger threatening her brother and pleads for his assistance. Percy promises to save him. After Percy unexpectedly leaves for France, Marguerite discovers to her horror (and simultaneous delight) that he is the Pimpernel. He had hidden behind the persona of a dull, slow-witted fop to deceive the world. He had not told Marguerite because of his worry that she might betray him, as she had the Marquis de St. Cyr. Desperate to save her husband, she decides to pursue Percy to France to warn him that Chauvelin knows his identity and his purpose. She persuades Sir Andrew Ffoulkes to accompany her, but because of the tide and the weather, neither they nor Chauvelin can leave immediately.
At Calais, Percy openly approaches Chauvelin in a decrepit inn (the Chat gris), whose owner is in Percy's pay. Despite Chauvelin's best efforts, the Englishman manages to escape by offering Chauvelin a pinch of snuff, which turned out to be pure pepper. When Chauvelin took this pinch, he effectively incapacitated himself. Through a bold plan executed right under Chauvelin's nose, Percy rescues Marguerite's brother Armand and the Comte de Tournay, the father of a schoolfriend of Marguerite's. Marguerite pursues Percy right to the very end, resolute that she must either warn him or share his fate. Percy, heavily disguised, is captured by Chauvelin, who does not recognise him, so he is able to escape.
With Marguerite's love and courage amply proven, Percy's ardour is rekindled. Safely back on board their schooner, the Day Dream, the happily reconciled couple returns to England. Sir Andrew marries the Count's daughter, Suzanne.