What motivates Marguerite to follow Chauvelin and rush to Calais?

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.