It opens with Alicia assuring her sister Kate that the Queen won't be angry with her for writing her a letter telling her that things are horrible at the castle where she keeps Princess Elizabeth. And to be sure, she's right.
The Queen's angry with Kate, instead, because Alicia is too innocent to think of writing such a letter, and obviously Kate put her up to it. So she packs her off to the Elvenwood, because she knows the lord there can be trust to keep Kate in bounds. (Alicia's distraught statement that she will write to the Queen to tell her that Kate didn't do it is put down with vigor.)
Sir Geoffrey is kindly enough, but he brings her to Elvenwood, where there is a Holy Well that pilgrims approach, and stories about the Fair Folk. (One flaw was that the Elizabethans figured out Victorian origin stories for the Fair Folk, which struck me as a bit anachronistic. Oh, well.) There's also the way that Sir Geoffrey's only child vanished while she was with his brother Christopher.
Thing after thing happens. It involves a bit of cold iron, a child caught in a flooding river, bowls of gold, a wish made on the well, a bonfire, talking about a manor and deciding where to put the new dairy, living deep down underground, a dancing day, a servant describing her as having run away, in love, and many other things before it becomes unraveled.