When Miles (Earl of Walmesley) shamelessly puts pressure on his friend Caro (Lady Caroline Eddington) to pose as his betrothed so that he can avoid a marriage planned for him by his great aunt (the Grand-duchess of Schaffenzeits) Caro at first refuses. This is because she had been in love with him for years, but realises the feeling is not mutual - and, very sensibly, is wary of what a sham engagement/eventual jilting might mean to her reputation.
Miles persists and, in a moment of weakness (and irritation with her carping/horrible aunt) Caro agrees. not just to help him, but also to send a message to a pompous and persistent suitor who refuses to believe she will not accept his (real) marriage proposals.
What happens next is predictable. Most reviewers agree that Caro is the star of this story, that she is too clever and interesting for Miles, and could’ve done much better looking elsewhere for her HEA.