Fresh from his victory over the evil gebbeth, Ged the wizard goes to the Tombs of Atuan in search of the half of a ring. There he is captured by a teen-aged girl.
The girl is a priestess in a remote temple structure that houses Great Powers somewhere beneath the ground. The story is told from her perspective, and we learn that she is forced into this life as a pre-teen and made a special warden of the entity's treasures - including the ring that the wizard wants to steal. She also grows steadily more sure that Something Ain't Right Here, but she's not sure what or why. It's only when she begins to talk to her captive - he spends his days shuffling about in the Tombs without sustenance - that she begins to understand what it is that seems so off about the Powers of the Earth. It's not just that serving those Powers is hell, there's really something corrupt in that place.
The story that unfolds is quite brilliant, and my son and I plowed through this very quickly, reading a whole chapter each night.