In this case, it’s Sir Richard Wyndham, second in society only to Beau Brummell, who has just decided that he must marry Melissa Brandon, a rather sour young lady who his parents and hers engaged each other two as infants. He’s on his way to doing the deed when Penelope Creed climbs out her bedroom window and encounters him.
Pen is trying to escape her Aunt – she just has to get back to her country home, where she’s convinced her childhood friend will marry her, if she can just see him. Richard, not really wanting to get engaged himself, agrees to accompany her, as long as she’s disguised as a boy, and he can go as her tutor.
What’s the twist here is that Richard actually falls for Pen – usually the younger lady has some other suitable suitor, and there’s a slightly older lady somehow involved that the older gentleman falls for. The story is still fairly preposterous, but it’s definitely Heyer fun.