Adventure in an alternate, magical Regency.
At Lady Truthful's eighteenth birthday, her father shows her and her three cousins the heirloom emerald. She's too young for it, but it will give her power to command weather. Except that, in an incident involving a lightning strike, it vanishes. Her father, horror-struck, is afflicted with fever. Her cousins make foolish plans to recover. Truthful herself goes to her aunt, where she was about to come out, and with her help, disguises herself as a (male) cousin of hers to search.
The plot involves King Canute, Napoleon's having been entombed in the Rock of Gibraltar, Truthful's colliding with someone in the street, being trapped in a barrel, a masquerade, a sea anchor, a maid who does not use iron pins, and much more.