The Hidden Oracle (2016) is the first installment in Rick Riordan’s The Trials of Apollo, a five-book fantasy series for young readers. Inspired by Greek and Roman mythology and history, the books follow the fallen god Apollo on a quest to rescue the five Oracles from a group of foes. The Trials of Apollo series takes place in the same universe as Riordan’s popular Percy Jackson oeuvre. Praised for its humor, The Hidden Oracle earned places on the New York Times and Publishers Weekly bestseller lists shortly after its release. The book also won the 2016 Goodreads Choice Award for Middle Grade and Children's Fiction. Before becoming a full-time writer, Riordan taught Greek mythology at the middle-school level for several years. Riordan’s writing often places elements from various mythologies in a contemporary context. This guide follows the 2017 Hyperion Paperbacks edition.

Plot Summary

The story opens with the book’s first-person narrator and protagonist Apollo, the Olympian God of the sun, prophecies, archery, music, and healing, falling into a garbage dump in an alley in Manhattan, New York. Apollo has been cast out of heaven by his father, Zeus, who blames Apollo for helping facilitate a war between Greek and Roman demigods, the powerful offspring of Gods and mortals. As punishment, Zeus has turned Apollo into a mortal teenager named Lester Papadopoulos, complete with acne and flab. Apollo has lost all his godly powers. Apollo is mugged and beaten up in the alley by two mysterious youth who appear to have been sent by someone to specifically target him. Twelve-year-old Meg McCaffrey rescues Apollo by raising a volley of rotten fruit against the muggers. Recognizing Meg as a demigod, Apollo reveals his true identity to her and tells her he can turn back into a god once he has been in the service of a mortal. Meg immediately claims Apollo's service, which means that he is bound to her until she releases him or considers his service complete.

Apollo suggests that they seek out Percy Jackson, a demigod whom he knows well and has helped train, so Percy can take them to Camp Half-Blood, a training school for demigods, which features prominently in Rick Riordan’s Percy Jackson books. However, Percy agrees to help the two only get as far as the camp, after which Percy will leave them. On the way to Long Island, the site of the camp, the three are attacked by nosoi, or ancient plague spirits. Percy, the son of the ocean god Poseidon, summons water to drench and dilute the power of the nosoi. Meg inadvertently summons a vicious grain spirit or karpos, who defends her fiercely. Since the karpos keeps repeating the word “Peaches,” Meg decides to call him by that name. Percy’s car having crashed in the fight against the nosoi, Meg and Apollo walk to Camp Half-Blood. Apollo hallucinates about a man and a woman telling him to find a mysterious gateway and faints.

Waking at Camp Half-Blood, Apollo meets his demigod children Will Solace, Kayla Knowles, and Austin Lake. Apollo learns from them and the camp administrator, Chiron, a centaur, that the camp is in the middle of its own set of crises. The Oracle of Delphi can longer foretell the future, and the demigods are unable to communicate in and out of the camp. More worryingly, campers have been mysteriously disappearing into the woods surrounding the camp. Apollo wants Chiron to prioritize his problems but learns that his fall and the events at the camp may be linked. Meanwhile, as Meg defends herself against a prank, she displays her formidable powers. Demeter, the goddess of harvest and agriculture, claims Meg as her child by displaying her symbol of a sickle and sheaves of wheat above Meg’s head.

At the camp, though Apollo finds himself still excellent at music, he is encumbered by his mortal shortcomings, such as a hand that cramps after playing an instrument. After he fails at archery, his other beloved skill, Apollo swears on the River Styx to never again engage in either activity. Meanwhile, in the afternoon, the campers form teams of two for a three-legged death race in the ancient maze called the Labyrinth. The object of the race is to claim three golden apples from the Labyrinth. Meg and Apollo are paired. While lost in the Labyrinth, they overhear a conversation between the monster Python, who has seized the original cave of Delphi, and a shadowy human called “the Beast.” The Beast tells Python that he will be able to control all the Oracles once he has destroyed the fifth and oldest Oracle, the Grove of Dodona.

Apollo and Meg retrieve the apples and escape the Labyrinth. Back at the camp, Apollo is shocked to learn that his children Kayla and Austin have disappeared into the woods. Apollo tries to go after them, but Meg orders him to wait. Meg reveals to Apollo that she knows the Beast from his voice. He is the man who killed Meg’s father. After the murder, Meg was adopted and raised by her stepfather. The next day, Rachel Elizabeth Dare, Apollo’s priestess and the medium of the Oracle of Delphi, flies into the camp using a helicopter. She shares her suspicions that Triumvirate Holdings, a company headed by the three worst Roman emperors in history, is behind the attempt to control the Oracles. She thinks the missing campers are being led to the Grove of Dodona.

Apollo and Meg enter the woods in search of the grove. A geyser spirit or palikos directs them to the gateway to the grove, which is next to a nest of giant ants. Apollo and Meg are attacked by the ants or myrmeke. Apollo distracts the ants by playing the ukulele, breaking his oath to the River Styx. In the process, another set of ants carry off Meg to their lair. Apollo rescues Meg and wins over the queen ant with his music. Apollo and Meg go to the gateway, where they find the missing campers, bound, unconscious, and tied to stakes. As Apollo and Meg attempt to free the captives, they are seized by gigantic elite Roman guards. Nero, the infamous Roman emperor and the leader of the triumvirate, finally makes an appearance.

Nero reveals that Apollo’s attackers in New York were his henchmen while Meg is none other than his adopted daughter. Meg purposely claimed Apollo’s service so she could bring him to Camp Half-Blood and Nero. To Apollo’s shock, Meg admits Nero’s story is true. Even though Nero’s cruel side, the Beast, murdered her biological father, Meg is loyal to her stepfather. Meg commands Apollo to help her open the gates to the grove. Bound to her, Apollo has no choice but to obey.

Inside the grove, the trees whisper to Meg that Nero intends to burn them. Sensing Meg’s horror at the idea, Peaches fights Nero in defense of the grove, and Nero orders his guards to kill Peaches. Meg jumps to the aid of the karpos, finally standing up to Nero. Nero sets the grove on fire and flees the scene. Dryads or tree spirits come to Apollo’s aid, drawing the fire into their bodies and sacrificing themselves to save the grove and Apollo and his friends. The grove tells Meg and Apollo a prophecy, which holds clues for Apollo’s next quest. Freeing Apollo of her service, Meg rushes off to find her stepfather Nero, whom she still believes will keep her safe. Apollo frees the captives.

Apollo and the former hostage demigods return to Camp Half-Blood, where another crisis unfolds: Colossus Neronis, Nero's giant automaton, is attacking the camp. With the help Percy Jackson, who arrives just in time, and the other campers, Apollo enchants an arrow with a plague and shoots the Colossus in the ear. The infected Colossus sneezes his head off and dies. Apollo relays his prophecy to Rachel and Percy. Apollo believes the next Oracle he has to seek and protect is the Oracle of Trophonius. Percy decodes that Apollo’s companion on the quest will be the demigod Leo Valdez, the son of Hephaestus, Roman god of machines. As if on cue, Leo flies into the camp on his metal dragon Festus, with the nymph Calypso in mortal form in tow. Apollo, Leo, and Calypso form an alliance and decide to search for the next Oracle in Indiana, as per the grove’s direction, setting up the stage for The Dark Prophecy, book two of the series.