Before they left Camp Jupiter, Nico told Hazel he was going in search of the Doors of Death. He makes her promise to be careful even as they both worry that if she is successful at freeing Thanatos, it will mean her immediate return to the Underworld. On the boat, Hazel suffers terrible sea sickness, and her conversations with Frank and Percy cause her to fall into another blackout of memories.

Hazel and her mother now live in Alaska in a small, rented house. Her mother works as a maid, but at night she is possessed by the same woman who sent them to Alaska. The woman possessing Marie is Gaea and she is using Marie to force Hazel to help her raise her son, the giant Alcyoneus. Hazel escapes as often as she can and one day spends the whole day chasing a horse. When she arrives home late, she finds a note from Gaea ordering her to come to the island where the work is being done, in a cavern she calls the Heart of the Earth. On the way, a bird tells Hazel this will be the final night she will be required to come there.