Kate Sutton, neither charming, nor beautiful, is blamed for her sister’s too-frank letter to Queen Mary and sent away to an obscure holding in the north of England known as The Perilous Gard.
Here she meets her guardian’s younger brother, Christopher, who blames himself for his niece’s loss down the castle well. Kate can’t believe he’s responsible, and when she hears a local story about the “fairy folk” she suggests the child may have been kidnapped by the remnant of a druidic cult. Finally convinced, Christopher offers himself in exchange for the child, correctly guessing the folk are planning a human sacrifice like one described in the ballad Tam Lin.
Kate witnesses the exchange and is taken as a slave to the same underground world. She finds Christopher and works to counter the mind games designed to make him a willing sacrifice. When she escapes the caves to meet him by the bonfire on Halloween night she’s able to reach his mind—saving them both by breaking through the spell of words the folk use, but the next time she sees him he is with her sister. Kate rejects the Fairy Queen’s offer of “magic” to bring him back, only to learn it was one last trick, to make her question his love when he did choose her.
Nice job. It might have been nice to have a sense from the beginning that this was going to be fantasy/romance (it sounds very historical-fiction at first), but I'd definitely request a manuscript from this synopsis. "Neither charming nor beautiful" is a good beginning; it shows me a bit of the author's hand, and the plot sounds like an appealing twist on traditional stories of fairies.