Former god Apollo was reunited with his friend Meg while he was searching for the oracle in Indianapolis. She agrees to help him complete the rest of his quest, which involves freeing all of the ancient sources of prophecy from the control of the Triumvirate — a group of three ancient Roman emperors who have been plotting for centuries to overthrow the gods.
The prophecy that Apollo and Meg received in Indianapolis instructs them to travel west through a magical maze called the Labyrinth. A satyr will be their guide, and he will help them find the next oracle. So when a satyr called Grover Underwood arrives at the Waystation, Apollo and Meg immediately enlist his help.
Despite the instructions to follow a satyr guide, Grover doesn’t know exactly where to go, and the three friends end up wandering through the Labyrinth for several days. Grover eventually begins to sense where the Labyrinth is leading them. Unfortunately the path leads them to an army of murderous birds called Strixes. Meg manages to block the path by growing a thick patch of strawberries from floor to ceiling in the tunnel via plant magic. This only leaves the friends access to one exit from the Labyrinth, however, so they head up to the surface.
To Grover’s surprise, the Labyrinth has brought him back to the base that he has been working out of for the past year in Southern California. He believes that the fires and droughts in California have a more sinister and possibly magical source than mortals understand. Grover and his team of dryads, nymphs and satyrs have been attempting to find and counteract this source. They chose this particular site because it held traces of strong nature magic despite the only remaining structures being a cistern and a few small green houses.
Grover explains that the greenhouses are full of plant dryads that go dormant during the day to avoid the heat and drought but will emerge in the evening. Meg is distraught that the maze has brought them to this ruined building. The home is called Aeithales, and Meg grew up there with her father before they moved to New York and he was murdered by Nero.
Meg eventually confides in Apollo, explaining that she was too young to fully understand, but that her father was working on an important project involving seven glowing seeds when he received a packet of yellow papers that greatly upset him. He and Meg fled the house, and she remembers seeing it go up in flames behind them.
Meg, Apollo and Grover have a meeting with the dryads once they wake up. The dryads explain that part of the Labyrinth has been burning and that they believe it to be the source of California’s drought and fires. They have been sending scouting parties into the maze, but none have ever returned. They suspect that the third member of the Triumvirate, Caligula, who is based in Southern California, may have somehow caused the maze to burn.
Since Apollo had dreamed that the oracle was chained in a burning prison, they conclude that all of their problems must be connected. Apollo decides that they should attempt to free the oracle first, hoping that she can offer guidance about how to defeat Caligula and restore the maze. The dryads also mention that two demigods named Piper and Jason attempted to find the oracle and managed to make it out of the maze alive, so Meg, Grover and Apollo decide to visit the demigods and see if they have any useful information.
When they arrive at Piper’s house, the three friends are shocked to find Piper and her dad, famous movie star Tristan McLean, packing their belongings to move to Oklahoma. Piper explains that the Triumvirate’s business front had paid her dad’s accountant to give him bad financial advice. The McLeans have lost everything and are moving to Oklahoma to be with family. Meg sees yellow Triumvirate Holdings envelopes on the McLeans’ desk and realizes that Caligula must have driven her and her father out of their home as well.
Piper tells them that she didn’t see much of the maze before being driven out by the spreading fire. She got separated from Jason near the entrance of the maze, however, so she doesn’t know exactly what he found inside of it. She suspects that he wasn’t completely honest with her and believes he may have found the oracle, though he refused to tell her. She offers to take Grover, Apollo and Meg to the entrance of the maze, and they borrow a car to drive to downtown Los Angeles.
Piper takes them to the entrance, and they sneak past a guard stationed there by Caligula. They don’t make it far, however, before they encounter a witch named Medea. She thanks Piper for bringing Apollo to her, commenting that it will make her job so much easier. She plans to extract the remaining divine essence from Apollo and combine it with the essence of the former sun god, Helios. Helios had faded from memory centuries before, and Apollo had succeeded him as the sun god.
Caligula wanted Medea to combine the essences of the two gods and infuse them into him, so he could become the new god of the sun. Helios, however, is no longer a god, but simply a semiconscious, fiery force. His presence in the maze is causing the surface of California to burn. Medea is able to control him to an extent, but it requires a great deal of concentration.
Apollo, Meg, Piper and Grover escape back the way they’ve entered by dividing her attention. Medea taunts them, however, saying they will only be able to find the oracle if she allows them to or if they have access to Caligula’s enchanted shoes. She mentions that Jason only found the oracle because she allowed him to and that the prophecy he received is slowly driving him mad.
Apollo and his friends return to Aeithales to rest. They decide to go visit Jason at his boarding school the next day. They hope that his prophecy might help them find Caligula’s base. They also want to ask for his help locating and stealing Caligula’s shoes.
When they find Jason, he agrees to help, but confides to Apollo that the prophecy stated that if Jason and Piper faced Caligula, one of them would die. He was not planning to tell Piper. Instead, he was waiting until she moved to Oklahoma before seeking Caligula by himself. Then he’d be the one who died rather than her. Apollo convinces Jason that he needs to tell Piper the truth and that she has the right to make her own decision. After talking, Jason, Piper, Apollo and Meg decide to find Caligula together, despite the consequences.
They discover the location of Caligula’s base from the prophecy, but when they arrive, they are surprised to discover that his base is actually a fleet of yachts. They have no way of knowing where Caligula or his shoes are in the fleet. They commandeer a Coast Guard ship and begin searching, starting in the middle of the fleet. Soon they discover which boat Caligula is on, as well as which boat his shoes are on, by eavesdropping on some of his guards. Unfortunately, the two boats are in opposite directions, so they decide to split up with Apollo and Piper searching for the shoes, and Jason and Meg planning to confront Caligula and defeat him once and for all.
Apollo and Piper find the shoes on a boat dedicated entirely to the emperor’s footwear, and Apollo stashes them in his quiver. Before they can escape, however, one of Caligula’s highest-ranking officials, who happens to be a talking horse, finds them in the boat and captures them. He mentions that Jason and Meg have already been captured and that they will all be escorted to Caligula’s throne room.
Caligula greets the four adventurers. Medea, who plans to begin extracting Apollo’s divine essence immediately, is also with him. Once reunited, the four friends are able to fight their way free, but Jason is mortally wounded in the process. Before he dies, he is able to summon a cloud spirit to carry his friends to safety. Once they land, Piper sends the spirit back to retrieve Jason’s body.
Piper, distraught over Jason’s death, refuses to help Apollo and Meg any further. Apollo and Meg return to Aeithales to rest before attempting to enter the maze again. While there, one of the dryads approaches Meg and gives her the glowing seeds that she remembers her father working to cultivate.
The dryad explains that they have been protecting the seeds and waiting for her or her father to return. The seeds were her father’s attempts to reincarnate the oldest and most powerful dryads — a grove of ash trees called the silver wives. Meg plants the seeds before she and Apollo leave for the maze.
They reenter the maze, but with Caligula’s sandals they are guided safely through the maze via a series of word puzzles. They finally arrive at the oracle’s prison. She is surrounded by a river of lava and chained to a rock wall. She is only able to speak in riddles and word puzzles but is able to guide them across the river.
Suddenly Medea enters the chamber behind Apollo and Meg. Meg quickly attempts to free the oracle by slicing her chains off with a sword. The chains immediately attach themselves to Apollo.
Once Apollo is chained, Medea attempts to extract Apollo’s godly essence. Meg attempts to use nature magic to summon help, but she worries that they are too far underground for any dryads to hear her. The oracle tells Apollo to resist Medea’s attempts to dissolve him, and he realizes that he must leverage the fact that he took the oracle’s place and rely on his prophetic powers.
Apollo speaks a prophecy, which is able to momentarily disrupt Medea’s spell and delays her long enough for help to arrive. Meg’s nature song summoned the silver wives, accompanied by Piper. The silver wives had already sprouted into small trees since Meg had planted them.
Piper and the silver wives are able to kill Medea and free Apollo. Apollo is able to speak to the essence of Helios and convince him to leave the maze and allow his spirit to rest. The silver wives escort Meg, Apollo and the oracle back to the surface.
The oracle decides to make her home in Aeithales, where the silver wives and other dryads will protect her. The silvers wives are charged with guarding all of Southern California from the Triumvirate and other threats to nature. Meg and Apollo work to interpret the new prophecy and conclude that it is sending them to the Roman demigod camp, New Rome. Meg hopes to return home to Aeithales someday, but Apollo’s quest is not over yet.