The story centers on a Kargish child who is taken from her family and dedicated as the high priestess in the service of the "Nameless Ones" on the island of Atuan. Her true name is Tenar, but she is renamed Arha, "the eaten one" when she is formally consecrated to the gods' service at age six, as all the high priestesses are considered reincarnations of a single priestess.
Tenar's youth is a haunting contrast between light-hearted childish escapades and dark, solemn rituals. Gradually she comes to accept her lonely, anonymous role, and to feel at home in the unlit underground labyrinth, the eponymous Tombs, where the malevolent, powerful Nameless Ones dwell. Indeed, as she becomes aware of the political machinations among the older priestesses, the Tombs become a refuge to her as she is the only one who may freely move through the labyrinth under them.
Ged, the protagonist of A Wizard of Earthsea, enters the story when Tenar is fifteen years old. He comes to the Tombs in order to find the long-lost half of the Ring of Erreth-Akbe, a magical talisman necessary for peace in Earthsea, which had been broken centuries before. (The other half is already in his possession, as described in A Wizard of Earthsea.) Arha finds him wandering, lost, in the Labyrinth, and traps him underground to die in order to punish what she sees as sacrilege. Yet in her loneliness, she is drawn to him and listens as he tells her of the outside world.
Tenar is eventually won over by Ged's kindness. She realizes that the Nameless Ones demand her service but give nothing and create nothing in return. Ged must expend his strength continually on hiding himself from the Nameless Ones, as they will kill him if they discover his presence. When he grows too weak to hide himself, they will kill him. She helps him escape from the Tombs with the ring, as he helps her escape from the priesthood.
Ged, while still a young man, is portrayed here as much wiser than in the first book. When Tenar asks him about the scar on his face, caused by the Shadow creature that he unleashed, he replies that it is the result of his foolishness in the past - his ambition has been tempered with experience. And it is his ambition and intelligence, combined with Tenar's budding wish for freedom and a wider world, that leads to their success.
Tenar reappears and plays a large role in the fourth book of the series, Tehanu.