After discovering the Grove of Dodona — a grove of ancient prophetic trees — in the woods at Camp Halfblood, former Greek god Apollo receives a prophecy instructing him to go to Indiana in search of another oracle. Before he departs, an acquaintance of Apollo’s, Leo Valdez, returns to Camp Halfblood. Leo had been missing for months after flying his bronze mechanical dragon to rescue his girlfriend, Calypso, from Ogygia — the enchanted island where she had been imprisoned for thousands of years. When Apollo fills Leo and Calypso in on what has happened at the camp over the past few months, they volunteer to fly to Indiana with him and help complete the quest.
When the three friends land in Indianapolis, a woman named Nanette greets them. The friends are wary and attempt to leave. She states that they can’t go until they have been welcomed properly and makes a phone call, stating that Apollo has arrived and that she needs back up. Calypso attempts to punch the woman, but breaks her hand on Nanette’s metal face. Apollo then realizes that the woman is actually a creature called a blemmyae disguising herself as a human. Blemmyae are headless human-like monsters, whose faces stretch across their chests and abdomens.
Nanette states that she is arresting the three friends in the name of the Triumvirate — a group of three immortal evil Roman emperors that are planning to overthrow the gods by cutting them off from all their sources of prophecy. An army of blemmyae begins approaching Leo, Calypso and Apollo, but the three friends are able to outsmart the creatures and escape on a nearby bulldozer. They don’t make it very far, however, before they are cornered in a plaza by the blemmyae. They are beginning to lose hope when an older gray-haired woman with a crossbow begins fighting off the blemmyae. She instructs Leo, Calypso and Apollo to follow close behind her, saying that she will take them to a secret fortress called the Waystation.
The woman introduces herself as Emmie, and once everyone is safely inside the Waystation, she explains that she and her partner, Josephine, were former Hunters of Artemis — a group of maiden warriors serving Apollo’s sister, the goddess of hunting and the moon. When Jo and Emmie fell in love, they decided to leave the Hunters to build a life together. Artemis made them the caretakers of the Waystation, which is a magical haven for demigods and other magical creatures that need protection and a home.
Lately, however, their family of demigods has been slowly disappearing, including their adopted daughter, Georgina. Emmie and Jo decide that they should share their respective stories over dinner and come up with a plan. Leo, Apollo and Calypso explain that they have come on a quest to find an oracle. Emmie and Jo state that the oracle they are looking for is called Trophonius. It is located in a system of caverns outside of Indianapolis.
The armies of Commodus, a member of the Triumvirate, guard the caves. Jo and Emmie warn the three friends that the cave is dangerous. The supplicant must drink from two magical springs to prepare his/her mind before speaking with the oracle. The prophecy then must be extracted from the supplicant using a magical item called the Throne of Memory or the prophecy will remain fragmented in the supplicant’s head and drive the person mad.
Jo and Emmie’s 7-year-old daughter, Georgina, thought that she could find the missing demigods, who had been kidnapped by Commodus, if she got a prophecy from the cave, but since Jo and Emmie didn’t have access to the Throne, Georgie was driven mad. Then Commodus kidnapped the girl.
While Jo and Emmie are explaining this, they are interrupted by a crash from the rooftop. They go to investigate and return with Britomartis, the goddess of nets and the creator of the Waystation. She instructs Apollo and Calypso to free her griffins, which have been captured by Commodus. The female is with egg, and the goddess is worried about them. Apollo is reluctant, but Britomartis promises that if they bring back the griffins, she will help them infiltrate Commodus’ palace to free the other prisoners and steal back the Throne of Memory.
Apollo and Calypso agree, and Britomartis informs them that the griffins are being kept at the Indianapolis Zoo. Apollo and Calypso leave early the next morning so they can sneak into the zoo before it opens. Once inside, however, they realize that one of Commodus’ commanding officers, Lityerses, is guarding the train to the hidden griffin enclosure. They manage to get onto the train without being captured, but Lityerses knows where they are and is following them. The train takes Apollo and Calypso to a hidden enclosure filled with mythical creatures and torture devices. They realize that Commodus is planning to stage a huge gladiator-style tournament and is using this space to prepare the animals to fight. They realize that the captured demigods are probably intended as sacrifices as well.
Calypso and Apollo are able to open the griffin cage, but it takes them a bit too long to earn the griffins’ trust, and Lityerses catches up with them. Suddenly, however, Apollo’s old friend Meg appears and fights Lityerses. Apollo has not seen Meg since she revealed that she was Nero’s stepdaughter and had been helping him locate the Grove of Dodona. Meg tells Apollo and Calypso to escape with the griffins, but Apollo refuses to leave without her. Meg wounds Lityerses, and they all mount the griffins and fly back to the Waystation.
Apollo thanks Meg for rescuing him. He tells her that he doesn’t blame her for anything that happened at the Grove of Dodona. She was manipulated by Nero and didn’t understand what he was trying to do. Meg explains that she was coming to Indianapolis to try to stop the Triumvirate because she overheard Commodus’ plan to rechristen Indianapolis in his own honor and to celebrate the occasion by sacrificing countless animals and demigods. Apollo and Meg agree to work together.
Once everyone is safely settled at the Waystation, Britomartis returns. She tells Apollo that there is a secret entrance into Commodus’ palace through the sewer system, and she recommends that he take Meg and Leo with him to rescue the prisoners and find the throne.
They rest that night and leave the next morning. The three friends find the entrance fairly easily, only having to sneak by one monster on the way in. They find the prisoners’ cells and are outraged at Commodus’ cruelty. The prisoners have been starved and mistreated, but all of the former occupants of the Waystation, including Georgina, are alive.
Apollo, Meg and Leo free the prisoners and decide that Leo should guide the escapees back to the Waystation, while Meg and Apollo search for the Throne of Memory. They find the throne fairly quickly, but on their way out of the palace, Meg abruptly turns around and says that there’s something important in the other direction. Apollo reluctantly follows her back into the palace, and they find themselves in the middle of the arena where Commodus is holding a dress rehearsal for the next day’s sacrifices. In the center of the arena, tied up and hanging from the ceiling is Meg’s companion Peaches, the grain spirit.
Commodus greets Apollo and Meg and challenges them to a gladiatorial contest in order to save Peaches. Later in the dress rehearsal, he plans to sacrifice Lityerses for allowing Apollo and Calypso to escape with the griffins. Meg and Apollo accept his challenge, having no other choice, but it quickly becomes apparent that they are outmatched.
Suddenly Leo breaks through the roof, riding his bronze dragon. Leo, along with Jo, Emmie and a few Hunters of Artemis, are able to rescue Apollo, Meg and Peaches. Apollo frees Lityerses as well and convinces Jo and Emmie to bring him along.
Back at the Waystation, Jo and Emmie immediately attempt to use the Throne of Memory to extract the prophecy from Georgina. Rather than an ordinary prophecy, however, the Trophonius addresses Apollo directly, commanding him and Meg to come to the oracle’s cave at first light. The oracle explains that unless Apollo comes and receives a prophecy of his own, Commodus will succeed in rechristening Indianapolis and in destroying the Waystation.
Jo and Emmie are confident that Commodus will not be able to find the Waystation, but Lityerses explains that Commodus had trackers placed on the griffins before they were rescued and is planning an assault on the Waystation. Jo, Emmie and the demigods plan to stay and fight while Apollo goes to consult the Oracle of Trophonius.
Apollo and Meg leave the next morning to travel to the Bluespring Caverns, where the oracle is located. They search for the two magical springs, since Apollo must drink from both to prepare his mind. They find them at the entrance of one of the caves and conclude that the oracle must be inside. Apollo drinks from the springs and immediately forgets who he is and why he is there. Meg leads him into the cave, but a lake infested with water moccasins blocks their path.
In his delirious state, Apollo wanders into the lake. In an attempt to save him from the snakes, Meg sings a song about her deepest fears and failures. (She had seen Apollo successfully calm animals this way in the past.) In doing so, however, she opens herself up to the oracle to receive a prophecy, even though her mind was not prepared through drinking the springs.
Meg immediately collapses and begins to shake. Apollo knows that the oracle will kill Meg if he does not intervene, so he attempts to resuscitate her mouth to mouth and ends up sharing the weight of the prophecy with her.
Once the prophecy ends, Meg is left unconscious. The oracle tells Apollo that there are blemmyae at the entrance of the cave preparing to kill Apollo and Meg and destroy the cave with explosives. The oracle requests that Apollo ensure the cave’s destruction if he and Meg defeat the blemmyae, so that no one can use the prophecies for their own purposes like Commodus has. Apollo carries the unconscious Meg out of the cave with him. He encounters three blemmyae and knows that he will not be able overpower them.
He tricks one of the blemmyae into setting the explosives deeper in the cave, knowing that she will not have time to get back out before the cave collapses. Apollo then defeats the other two blemmyae and heads back to the Waystation to see how his friends are faring.
The battle is almost over by the time Apollo arrives, but Commodus is still fighting and has managed to recapture Georgina. He uses her as leverage against Jo, Emmie and the rest of the residents of the Waystation. Apollo arrives to find a standoff.
Apollo threatens to reveal his true godly form and burn Commodus to a crisp if the emperor doesn’t release Georgina. Apollo is simply bluffing, as he shouldn’t have the power to do this in his mortal state, but as soon as he makes the threat, Apollo feels a surge of power. He warns his friends to close their eyes as he reveals his godly form. Commodus is not burned up, but he is blinded by the blast. Emmie grabs Georgina, and Commodus charges in a blind rage. Commodus miscalculates and falls out of a window. When Apollo goes to see if he is still alive, the emperor has vanished.
The Waystation is safe and secure once more. Apollo places Meg on the Throne of Memory and receives a new prophecy instructing him to travel south with Meg to continue his search for the oracles. Meg recovers from the effects of the oracle within a few days, and they prepare for their new quest.