The book opens two days after the close of the preceding novel, The Dark Prophecy. During this period, Apollo, Meg McCaffrey, and Grover Underwood have been navigating the Labyrinth. They escape from attacking strixes sent by the third emperor and break out of the Labyrinth. Meanwhile, Apollo has a dream vision of the captured Sibyl Herophile, beckoning him to rescue her. After awakening, they travel to Aeithales, a greenhouse built by Meg's father that currently serves as Grover's base. They meet with the spirits of a Joshua tree, a prickly pear, and aloe vera among others. They also meet Mellie, who guides them to find her husband, Gleeson Hedge, at an army-surplus store, where they battle Naevius Sutorius Macro's automation and spy upon a talking horse instructing a servant to destroy Aeithales. Apollo comes to the conclusion that the horse is Incitatus, and its owner is the third emperor, Caligula.
They return to Aeithales and convey the news to the spirits. Grover tells Apollo about a part of the Labyrinth that is burning, causing droughts and water shortage in the upper world. Hedge and Mellie reveal that Piper McLean and Jason Grace had explored the Labyrinth. Apollo decided to meet with them. Meg plants the seeds with which her father had planned to reincarnate the Meliai. Apollo, Meg and Grover meet Piper and bring Jason, who had broken up with Piper, from his school the following day. Jason confidentially tells Apollo that Herophile told him secretly that if he and Piper went into the Labyrinth together, one of them would die. He makes Apollo promise to carry on his work of temples for all gods at both camps in case he dies. They are captured by pandai while attempting to steal Caligula's caligae to enter the burning maze. Apollo threatens to kill himself until Caligula releases his friends as the latter requires Apollo's essence to make himself the sun god. Apollo stabs himself with an arrow, while Jason unleashes a tornado to rescue them, battles Caligula and gets stabbed twice by him, causing his death.
Piper and Apollo carry Jason's body to the former's house. Grover, Apollo, and Meg find the entrance to the burning maze with the help of Crest, a young, friendly, music-loving pandos. Wearing the caligae, Apollo guides Meg and Grover into the maze, solving a series of puzzles to advance, which together give a prophecy. Apollo makes a mistake, and he falls into a pit of fire. Apollo convinces the essence of Helios, which causes the fire, to retreat in return for killing Medea. They meet a chained Herophile and continue solving the prophecy. They meet Medea, who plans to combine Apollo and Helios' essence to create ichor for Caligula and to turn him into a new sun god. A chained Apollo resists Medea's chants with the help of Crest, whom Medea kills, and completes the prophecy. Piper arrives with the newly-reincarnated Meliai and kills Medea. Helios disintegrates to Tartarus, thereby ending the fires. Leo Valdez arrives; heartbroken after hearing Jason's fate, he conveys that Camp Jupiter won the battle with heavy casualties. Apollo feels personally responsible for Jason's death and vows to remember his promise.
The book opens two days after the close of the preceding novel, The Dark Prophecy. During this period, Apollo, Meg McCaffrey, and Grover Underwood have been navigating the…
