The climax occurs when Percy tricks the Chauvelin into taking snuff that is really pepper. From that time he has the upper hand. This occurs in chapter 25.
TheChauvelin is the French envoy to England. He is blackmailing Marguerite, Percy’s wife, and has accused her brother Armand of being in league with the mysterious Scarlet Pimpernel, who is trying to rescue French aristocrats from the Guillotine.
When Percy tricks him into taking snuff that is actually pepper, the Chauvelin “never even remotely guessed the trick which was being played upon him” (ch 25). Percy is thus able to sneak away.
When she realised what had happened, a curious mixture of joy and wonder filled her heart. It all was so neat, so ingenious. Chauvelin was still absolutely helpless …whilst his cunning adversary had quietly slipped through his fingers. (ch 26)
A climax is a turning point, and the point where the story changes. The rising action is the series of interesting events leading up to the climax, and the falling action is the result of the events of the climax. In the rising action, Sir Percy is in danger from the Chauvelin, and in the falling action he escapes and rescues his wife’s brother.
The last 10 chapters are very important. If I were you, I would just read over all the summaries again