The story opens in Paris in September 1792, when the guillotine is busy and crowds gather at the city gates to watch aristocrats caught trying to flee. Rumor says a band of Englishmen keeps spiriting the condemned to safety, led by a man no one has seen, who leaves behind only a small red flower drawn on a scrap of paper: the Scarlet Pimpernel.
The scene shifts to England, to a Dover inn and the high society around it. We meet Sir Percy Blakeney, reputedly the richest and best-dressed idler in the land, and his French-born wife Marguerite, once the most admired woman in Paris. Their marriage has cooled into politeness. He plays the amiable fool, and she cannot understand how the brilliant man she married became so dull.
The French agent Chauvelin arrives determined to unmask the Pimpernel. He discovers that Marguerite's beloved brother Armand is tied to the League, and uses that to force her hand: help identify the leader, or her brother dies. Caught between two loyalties, she gathers a clue at Lord Grenville's ball and passes it on, not yet knowing whose life she is selling.
Slowly the pieces fit together. A seal-ring, a habit of disappearing, and a chance remark lead Marguerite to a discovery that overturns everything: the daring leader is her own husband. The lazy pose was a mask all along, and her unwitting betrayal has set Chauvelin on the trail of the man she finds she still loves.
She crosses to France to warn him, walking straight into the closing trap. The final chapters are a chase along the cliffs near Calais, where Percy, disguised and outnumbered, outwits Chauvelin one more time with a planted false note and a hidden boat. Husband and wife are reconciled, the fugitives reach the yacht, and the rescuer slips away unbeaten.