A second Regency featuring the incorrigible Kat, and the question of her sisters' marriages and her own magic. Spoilers for the first ahead.
It opens with Elyssa's wedding. Which is interrupted by an irate Mrs. Carlyle, arguing that her son is too young to marry without parental permission. Not even being told that her son is the best man, not the bridegroom, keeps her from spewing all sorts of nastiness, down to Angeline's being a witch. And assuring them that Lady Fotherington assured her on the best authority that it was so.
It ruins the wedding, of course -- even Elyssa and Mr. Collingwood do not stay for the breakfast -- and Kat uses her mother's magical mirror to get to the Guardians' hall to get Lady Fotherington. Unfortunately, this attempt creates a negative impression on Lord Ravenscroft, who is the head of the order and gets to say whether Kat can be trained as a Guardian. He forbids it.
Back in her own home, her stepmother packs up her and Angeline and sweeps them off to Bath, giving Angeline the direct order to find herself an eligible match before the scandal reaches her. Angeline sets out to attract a notorious rake and so technically fulfill the order. And their brother Charles meets up with some old friends from before he was sent down from Oxford.
The rest of the story involves the Roman roots of the baths, the use of the water to activate a magic mirror, a desire to win at gambling, a sacrifice, being caught kissing, Kat seeing Lord Ravenscroft somewhere other than he had told someone he would be, and her stepmother's prosperous connections, who take them after some judicious insinuations from Kat and Angeline.