Riordan had the idea to make a new series in the universe of Percy Jackson after he wrote Percy Jackson's Greek Gods, when he discovered two myths about Zeus punishing Apollo, turning him into a mortal. He liked the concept and decided to write a series centered on Apollo's trials, while many of the characters from Percy Jackson and the Olympians and The Heroes of Olympus return in the new series. Because Apollo is the god of poetry, the name of each chapter would be a bad haiku.
On December 10, 2015, the third chapter was made available for free download by USA Today. The first chapter was released along with the collection of short stories Demigods & Magicians on April 5, 2016. Three trailers were also released on YouTube for promotion. The Harvard Book Store organized a launch event, which was attended by Riordan.
The Fall of the Sun
Apollo, the Greek god of archery, poetry, and healing (among others), wakes up falling through the air and crash lands in a dumpster in Manhattan, having little to no memory of what happened to him. All he remembers is that his father, Zeus, is enraged at him for succumbing to vanity and flattery by his legacy, Octavian, thus fueling the Second Giant War; he has been punished by being turned into a mortal. After checking his wallet, he sees a New York State junior driver’s license and learns that he is now a curly-haired, flabby sixteen year-old human whose name is Lester Papadopoulos. Apollo is disgusted, but reassures himself, thinking that he must have retained some of his godly abilities. Once he is out of the dumpster, he is confronted by two thugs—Cade and Mikey—who say they were sent by their boss. They demand he gives them his wallet and all his money. Apollo attempts to fight back using his godly powers, which doesn't work, and gets knocked back to the ground. Cade and Mikey 'stomp' Apollo, leaving him badly injured.
Apollo discovers that he no longer has golden ichor, but instead bleeds mortal blood. He is saved by Meg McCaffrey, daughter of Demeter , who pelts Cade and Mikey with several rotted fruits. She claims to be the owner of the alley. She seems to be unsurprised by the fact that she is a demigod and that Apollo is a god, appearing unfazed by the situation. Apollo considers walking to the Empire State Building, but reconsiders, feeling that Zeus wouldn't have made it that easy for him to reclaim his godhood. Apollo tells Meg he was hoping that Percy Jackson or another demigod would claim his service, meaning he has to do tasks for his master, and asks her to guide him to Percy's house. Meg suddenly claims Apollo's service, binding him to her until his debt is paid. Despite feeling humiliated, Apollo agrees to lead Meg to Percy's house, telling her that Percy will guide them to Camp Half-Blood.
As Apollo and Meg walk to Percy's apartment, Meg claims to see 'shiny blobs' in the distance and the pair fear that the blobs are malicious spirits following them causing them both to develop an eerie feeling. They finally reach a red bricked, five-story flat. Apollo sees a Toyota with Pegasus hoof prints and is certain that this is the home of Percy. They ring the buzzer and, after Lester announces that he is the god Apollo, are let into the flat. Percy greets Apollo unenthusiastically, while Meg attempts to hide behind Apollo's back. Apollo reveals that Meg is a demigod who needs to get to Camp Half-Blood, and that earlier Meg saved Apollo from being mugged on the street. Percy sighs and invites Meg and Apollo into his apartment.
After walking into the apartment, Apollo questions why Sally Jackson is so big, only to discover that she is seven-months pregnant with a baby girl. Apollo and Meg get cleaned up while Percy gets the first aid kit and Sally makes snacks for them. After getting cleaned up, Percy tells Apollo that no one has heard anything about him for six months, but Apollo states that he has memory gaps. Percy agrees to drive Meg and Apollo to Camp Half-Blood, but he can't do much because his girlfriend, Annabeth Chase, who is currently in Boston for a family emergency, would kill him if he went on another quest.
The trio start driving towards Camp only to only find out they are being followed by the three shiny blobs Meg saw earlier. They take a short cut to Camp, but are eventually caught up by the three blobs, who are nosoi. The trio attempt to fight them, but Percy and Apollo run straight into the nosoi who are mist-like beings and get infected with a disease. Meg attacks them with peaches from a nearby peach tree, although she has no idea how she did it. The nosoi try to attack her when her guardian, a karpoi shows up to protect her. The karpoi repetitively says the phrase "Peaches", so Meg names him "Peaches". Percy attempts to kill Peaches, but Meg pleads for him not to. Before Percy can finish escorting the duo to camp, a police cruiser comes to check out the damage to Percy's car. Percy leaves them to walk to camp through the forest while he deals with the officer.
While Meg and Apollo walk through the woods, Apollo starts to hallucinate and hears voices chanting "the fall of the sun, the final verse" and other commands. Meg has to drag Apollo the rest of the way through the forest to camp. They walk in on a campfire and Apollo passes out. While Apollo is unconscious, he dreams of a woman telling him to find the gates. He also dreams of a man also telling him to find the gates so that he may burn the Oracle.
Apollo awakens in Cabin 7 to Will Solace healing him. Apollo's other children, Kayla Knowles and Austin Lake, visit to confirm his mortal status and to contact Chiron about Apollo awakening. Apollo leaves for the Big House and meets Nico di Angelo, where he learns that Nico is dating his son Will. After going inside the Big House, they discuss the Oracle of Delphi and how the Oracle can no longer issue prophecies. Travel and communication also do not work for any of the demigods: Iris Messages and cell phones don't work, and Camp Jupiter has not communicated with Camp Half-Blood in weeks. Chiron also mentions that campers have been randomly disappearing after looking into the woods.
They leave the Big House for dinner. Nico is sitting at the Apollo cabin table since he has a "doctor's note" from Will. Meg sits over at the Hermes cabin, as she is unclaimed. While at the table, Apollo questions the whereabouts of the demigods who "went on the boat". After being told where the Seven are, Apollo's child, Kayla, asks Apollo when he will be getting his power back. Kayla and the rest of his children show support for their father and promise to help him get his powers back. Chiron announces a three-legged death race invented by a child of Hephaestus, Harley, will be held the next day.
Before the game tomorrow, Chiron assures everyone that the game is safe to play in the Labyrinth since its essence does not feel as evil after it had been brought back by Pasiphaë. Sherman Yang and Connor Stoll attempt to dunk Meg in the canoe lake for revenge since Meg humiliated them when she first got to camp. Instead, Meg accidentally calls Peaches again, who attacks Sherman. The campers try to attack Peaches, but Meg suddenly brings out her weapon, two siccae made from Imperial Gold. After saving Peaches, Meg is claimed as a child of Demeter. Everyone calls it a day at camp.
The next day everyone goes to breakfast, where they are allowed to sit anywhere they would like. Apollo sits next to Meg and inquires about her blades, which are Roman. Meg claims that her step-dad raised her and gave her the blades, which shrink down into rings. After breakfast, Apollo goes to demigod training that is run by Sherman. Apollo heads to a music class taught by the satyr Woodrow, where Apollo plays such a beautiful song on the guitar it brings people to tears. Apollo is quickly drained by the use of his talent on the guitar. Later, he goes to archery practice, where he can only make one bullseye, upsetting the former god. In frustration, Apollo swears on the River Styx that he will never use a bow or musical instrument until he will become a God again.
The Three-Legged Death Race
Suddenly Sherman goes off into the woods muttering about finding his girlfriend, Miranda Gardiner, who has gone missing. After snapping out of it, Sherman states he doesn't remember what happened and that if anyone tells what happened he'll beat them up. Apollo realizes how the woods are managing to capture people, but no one will listen to him as the three-legged death race is going to happen. For the race, Apollo is strapped to Meg. All the campers are instructed to find three golden apples in the Labyrinth. While in the Labyrinth, they end up below Delphi, which is currently guarded by Python. The two hear someone who Python calls the Beast talking above. They discuss how they need to control all of the Oracles, but first, they need to destroy the fifth Oracle, the Grove of Dodona. Apollo and Meg accidentally slip, fall, and land into a trash dump. After they find their way out of the garbage dump, they find the three golden apples, and quickly leave the Labyrinth, only to find out that Kayla and Austin went missing.
After the death game, many of the campers are injured and need serious medical attention. Apollo tries to cheer up Harley by telling him the frequency at which Celestial bronze automatons prefer to hear sound for a beacon that Harley is building for Leo Valdez to come home. Later, Apollo tries to go after Kayla and Austin but is stopped by Meg and Chiron. He settles for waiting by the woods with two books he grabbed from the Apollo cabin on oracles and himself.
When he gets back out to the outskirts of the woods, he notices that Meg has shown up. After reading about himself, Meg tells him that the Beast killed her father and that he's located in New York City. As the morning comes, a helicopter lands outside the camps magical barriers outside holding Rachel Elizabeth Dare. The duo go to meet Rachel along and decide to talk in Rachel's oracle cave. Once in the cave, Rachel says she trashed the cave because she was frustrated about not receiving prophecies anymore. Meg brings Chiron to the cave and Rachel states that the two satyrs who were sent to retrieve her have died. Rachel reveals that she's been doing some digging since the problem with the Oracle and has uncovered that Triumvirate Holdings paid for the weapons that Octavian used to attempt to destroy Camp Half-Blood and the mortal backing that Luke Castellan had to build his army against the gods. Apollo believes that Triumvirate Holdings are attempting to control all five of the Oracles, starting with the most ancient Oracle, Dodona, which is located at camp. Apollo thinks that the campers are being drawn into the Grove of Dodona.
Into the Woods
After a night's rest, Apollo and Meg leave to go find the grove. He is seen off by campers Nico, Will, Billie, Malcolm, and Paolo. Will and Nico give him a combat ukulele created by Harley and ambrosia for Meg. Billie gives Meg a flashlight and a packet of flower seeds, and Paolo gives Apollo a magic Brazilian scarf. After receiving their gifts, they go into the woods. Eventually they come across a dead myrmeke, which freaks Meg out. Apollo tells her not to worry and they continue farther into the woods until they come across a geyser.
At the geyser, they meet palikoi (geyser gods) who the duo try to get information out of about the grove and the missing campers. The palikos, Pete, refuses because his partner, Paulie, went missing while trying to find the grove. Suddenly, they are attacked by myrmekes. Apollo manages to defeat the three myrmekes by using his ukulele, breaking his oath on the River Styx, only to realize after the fight that they have kidnapped Meg.
After searching around the forest for Meg, he goes back to the palikos and asks him how to get to the myrmekes' lair. Pete finally tells Apollo after also telling him to get treatment at Camp and that Meg has 24 hours before she's killed at the lair. Apollo also learns that three men speaking Latin are headed towards the Grove of Dodona. Before Apollo leaves for camp, he makes another oath with Pete that he will save Meg or be boiled alive if he doesn't.
Apollo heads to camp, but collapses while in the middle of the forest. He starts hallucinating about Daphne, one of his loves. After he passes out, Apollo is woken up by Rhea, who is now a hippy living in upstate New York. Rhea explains that Triumvirate Holdings has been around for centuries waiting for the perfect moment to strike. Before leaving, Rhea hands Apollo wind chimes that will help focus the voices of the Oracle after being hung on the largest tree in the Grove of Dodona. She tells Apollo to save her Grove before teleporting Apollo back to camp. Apollo ends up passing out in front of Chiara.
Calming the Grove
After waking up, Apollo tells Nico and Will that the enemy is Nero, a Roman emperor. Before leaving the camp, he grabs a bow although he swore on the River Styx to never use one again. Apollo goes into the forest and finds the ants' lair. He manages to throw the ants off by singing, but eventually is worn out and uses his arrows. Once in the center of it, he finds Meg and they attempt to escape, but are stopped by the queen ant. Apollo uses the last of his strength to perform a rap appreciating mothers, which sways the queen ant into letting him go.
After they make it out of the lair, they go to the place where the entrance of the grove is located. To the side of the entrance of the grove were the five missing demigods and the geyser god Paulie tied up to posts. Apollo attempts to free them to no avail. Suddenly, two Germani, Roman imperial guards, and Nero appear from the ants nest. Nero greets Apollo and lets him know that he sent his guards to attack him in New York disguised as thugs. Nero makes a bargain with Apollo by saying if Apollo and Meg open the grove's gates, he will release the six who are tied up. Apollo states he will never make this deal and attempts to get reassurance from Meg who is deathly quiet and crying. Nero reveals that he sent Meg to get Apollo in New York. Meg admits that Nero "adopted" her when she was a child and considers him her step-father, even though the "Beast" side of him killed her real father.
Apollo attempts to get Meg on his side, but fails. She commands him to help her open the gates, which he must follow since he took an oath to follow her as his sponsor. Once the gates are open, the voices in the grove tell Meg that Nero means to burn the grove and the hostages to the ground. As Nero attempts to burn the grove, Peaches attacks him, but Nero fights him off. Nero orders his guards to kill Peaches and Apollo, which makes Meg rebel against her step-father. Apollo gives her the wind chimes that Rhea gave him earlier to put on the trees, which make incoherent sentences. Nero and Apollo briefly fight each other, before Nero uses Greek Fire in a last attempt to destroy the grove.
Once the fire is started, Apollo goes straight into the hostages who are still unconscious and manages to get them all before being burned alive. Apollo begs for anyone to help him and receives help from dryads who are willing to consume the Greek fire to save the grove. Austin wakes up and Apollo tells him to take care of the rest of the campers so he can go inside the grove. Inside the grove, the trees are whispering a limerick over and over again. Apollo helps Meg put the wind chimes that Rhea gave them on the main tree that Meg is standing in front of. After the wind chime is on the tree branch, it gives another limerick to Apollo directly. Meg then tells Apollo that she releases him and runs away. As he starts to leave the grove, he finds an arrow on the ground and puts it in his quiver.
Fighting the Colossus Neronis
Apollo heads back to the campers and attempts to catch up all the demigods on what has been happening while they were unconscious. Apollo tells them that the Colossus Neronis was most likely attacking camp right now and that the Athena Parthenos wouldn't be able to defend camp with most of the campers gone. In an attempt to get out of the forest, Apollo raps into the myrmeke lair and gets the queen ant to take them all back to camp.
When they get to camp, the campers are attempting to fight off the giant statue of Apollo. The ants drop them off by the Beach, so that they can regroup with Will and Nico there. Apollo takes Will, Nico, Austin, and Kayla with him to help defeat the statue. Will and Nico shadow travel to the chariot, while Apollo tells Austin and Kayla that he's going to shoot the statue in the ear with a plague arrow. In the chariot with Austin and Kayla, Apollo hears the arrow he found in the grove talk to him. After multiple attempts at getting close to the statue, Percy Jackson enters the battle with a Hellhound, Mrs. O'Leary. Percy attempts to distract the statue by leading it towards the beach, while Apollo gets the plague enchantment to work. The enchantment starts to work too well and gets both of his children sick, so they crash-land in the sand.
Kayla and Austin are taken to the Apollo cabin as Chiron and Percy attempt to draw the statue back into the ocean since Percy has an advantage at sea. Apollo finally shoots his arrow and defeats the Colossus after it sneezes its head off. After the battle is over, everyone comes down with hay fever and needs to be treated. In the morning, Percy and Rachel go to talk to Apollo about securing the rest of the Oracles around the country. Apollo recites the prophecy and Percy realizes that the prophecy includes someone special. Shortly after, Apollo looks into the sky and sees Leo and Calypso on Festus entering camp.
Once news spreads that Leo is back in camp, the campers take turns "greeting" Leo by punching him in an orderly fashion. Leo tells them that since Iris messages were down they couldn't get a hold of anyone and had been stuck in the Sea of Monsters. Calypso explains that since leaving her island, Ogygia, she no longer has any powers and is simply a regular mortal. Percy leaves before dinner, but not without saying goodbye to Leo and Calypso. Later, Leo and Calypso inform the campers that they are going to help Apollo on his quest for the second Roman ruler. The trio walk to the campfire for a sing-along.
There are two prophecies given to Apollo while he is in the Grove of Dodona.
Caves of blue.
Strike the hue.
Serpents and roaches.
The first of these prophecies most likely gives a summary of each of the trials Apollo will have to face to take the respective oracles out of the control of the Triumvirate.
Unraveling the First Prophecy
- The Oracle of Trophonius was located in the Bluespring Caverns.
- Currently unknown
- the third line is talking about the burning maze in California. The maze put Southern California in a drought. Since California is in the West it would be westward from New York.
- Currently unknown (possibly references the Sibylline Books)
- The Bluespring Caverns are in Indiana.
- Currently unknown
- Festus means happy in Latin. Given that Festus returns to Camp Half-Blood at the end of the book, this line can be taken as having that meaning.
- Serpents could mean Python
There once was a god named Apollo
Who plunged in a cave blue and hollow
Upon a three-seater
The bronze fire-eater
Was forced death and madness to swallow
The second prophecy, which is in the form of a limerick (a type of poem that Apollo in no uncertain terms does not like), tells him where to go, how he is to get to the Oracle and which one he is to secure. Which in this case is the Oracle of Trophonius in the next book.
Unraveling The Second Prophecy
- Line one is self-evident that there once was a god named Apollo before he was turned into a human.
- Line two is making a reference to Bluespring Caverns (which is in Indiana), the current location of the Oracle Apollo is to secure.
- Lines three and four refer to Festus, the bronze dragon who seats three people.
- Line five tells that the Oracle of Trophonius was the one to secure because people had been known to go insane from what they hear. Furthermore, in order to save Meg McCaffrey from the plague of Trophonius ("was forced"), Apollo uses mouth-to-mouth resuscitation ("to swallow") techniques to draw the hallucinatory plague smoke ("death and madness") out of Meg's lungs.
- Percy Jackson
- Leo Valdez
- Paolo Montes
- Rachel Elizabeth Dare
- Chiara Benvenuti
- Hoodlums punch my face/ I would smite them if I could/ Mortality blows
- A girl from nowhere/ Completes my embarrassment/ Stupid bananas
- Used to be goddy/ Now uptown feeling shoddy/ Bah, haiku don't rhyme
- Casa de Jackson/ No gold-plated throne for guests/ Seriously dude?
- Seven-layer dip/ Chocolate chip cookies in blue/ I love this woman
- Aquaman driving/ Couldn't possibly be worse/ Oh, wait, now it is
- Tag with plague spirits/ You're it, and you're infectious/ Have fun with that, LOL
- Peaches in combat/ I am hanging it up right now/ My brain exploded
- A walk through the woods/ Voices driving me bonkers/ I hate spaghetti
- My bus is in flames/ My son is older than me/ Please, Zeus, make it stop
- Check your spam folder/ The prophecies might be there/ No? Well, I'm stumped. Bye
- Ode to a hot dog/ With bug juice and tater chips/ I got nothing, man
- Three-legged death race/ Five terrible syllables/ Oh, gods. Please not Meg
- You've got to be kid–/ Well, crud, what just happened there?/ I ran out of syl–
- Practice makes perfect/ Ha, ha, ha, I don't think so/ Ignore my sobbing
- Tied to McCaffrey/ We might end up in Lima/ Harley is evil
- Bowling balls of death/ Rolling toward my enemies/ I'll trade you problems
- The Beast is calling/ Tell him I'm not here. Let's hide/ Where? In garbage. Natch
- They have gone missing?/ No, no, no, no, no, no, no/ No, et cetera
- Don't paint over gods/ If you're redecorating/ That's, like, common sense
- Up in my business/ Always burning Oracles/ Romans gonna hate
- Armed to the eyeballs/ A combat ukulele/ Magic Brazil scarf
- Scale of one to ten/ How would you rate your demise?/ Thanks for your input
- Breaking my promise/ Spectacularly failing/ I blame Neil Diamond
- I'm on a roll now/ Boiling, burning, throwing up/ Lions? Hey, why not?
- Imperators here?/ Gag me with a peace symbol/ Not groovy, Mama
- I apologize/ For pretty much everything/ Wow, I'm a good guy
- Parenting advice:/ Mamas, don't let your larvae/ Grow up to be ants
- Nightmares of torches/ And a man in purple clothes/ But that's not the worst
- I school McCaffrey/ Yo, girl, your stepdad is wack/ Why won't she listen?
- Listen to the trees/ The trees know what is up, yo/ They know all the things
- It takes a Village/ People to protect your mind/ "Y.M.C.A." Yeah
- Parting is sorrow/ Nothing about it is sweet/ Don't step on my face
- Uber's got nothing/ Lyft is weak. And taxis? Nah/ My ride is da mom
- Buck-naked statue/ A Neurotic Colossus/ Where art thy undies?
- I love me some plague/ When it's on the right arrow/ Ka-bam! You dead, bro?
- Hey, look! It's Percy/ Least he could do was help out/ Taught him everything
- After the sneezing/ Healing peeps, passing limericks/ Worst God Award? Me
- Want to hit Leo?/ That is understandable/ Hunk Muffin earned it
- As of this book, Apollo is the first god to ever have a point of view throughout all three Camp Half-Blood series, an experience that Riordan himself describes as "interesting".
- All of the chapter titles are in the form of a haiku.
- The existence of Aztec gods are implied when Apollo mentions Tonatiuh as one of the many gods maintaining control over the sun in his absence. 
- This is the first Rick Riordan book specializing in Classical Mythology to reference a character from the other series, with Apollo mentioning Ra by name.
- Although not mentioned by name, the "smoking hot god" and his talking sword is heavily implied to be the Norse god Frey and his former weapon Sumarbrander. 
- The Hidden Oracle is similar to the first Thor movie by Marvel. Apollo gets cast down to Earth without powers to learn humility because of a mistake he made. In the Thor movie, Thor gets cast down to Earth without powers to learn the same thing.
- The events of this novel run alongside the events of The Sword of Summer, as Percy mentions that Annabeth is in Boston on a family emergency.
- The events of this novel take place approximately three years after the events of The Titan's Curse, as Apollo mentions that the last time he had visited Camp Half-Blood in winter had been three years ago, when Thalia Grace crash-landed the sun chariot in the canoe lake.
- However, Percy was 13 during The Titan's Curse and as of Tower of Nero he's around 18, so this is an error as in actuality five years have passed.
- There is another anachronistical error as Percy refers to Peaches' repetition of his name as "Grooting" and references the Guardians of the Galaxy film. Since The Hidden Oracle takes place in 2010-11 (since Percy was born in 1993 and is 17 in the book) this isn't possible as Guardians of the Galaxy came out in 2014.
- However, Percy was 13 during The Titan's Curse and as of Tower of Nero he's around 18, so this is an error as in actuality five years have passed.
- This is the second novel to feature Percy Jackson but not Annabeth Chase. The first was The Son of Neptune.
- The short story Percy Jackson and the Singer of Apollo is included in the paperback version of this book.
- In this book, Apollo comments that the sun will continue to rise and fall, because of the other religions and science. However, when Thanatos was captured, death stopped happening even though other religions had death gods and psychopomps too.
- This could be in specific reference to Greek and Roman monsters and humans, while other pantheons were not affected.
- The book is dedicated to the Muse Calliope, with the quote “This is long overdue. Please don’t hurt me”.