Percy is absolutely miserable in cabin three, all by himself.
Just when he started to feel like part of the community, "I'd been separated out as if I had some rare disease" (9.4).
Luke begins to train Percy one-on-one, pushing him harder and harder. He tells Percy that he's going to need all the training he can get if he's going to protect himself from monsters like the hell-hound.
Percy discovers that he is a wanted criminal in the mortal world, accused of his mother's disappearance. Somebody who doesn't like Percy posts an article about this on his cabin door.
Percy's worried that the gods are fighting because of him, because his existence means that Poseidon broke a very serious oath. He figures that Mr. D is going to kill him or something.
The storm is getting bigger; it never rains at Camp Half-Blood – storm clouds usually roll on past the camp. But, today, it looks like it's going to rain.
Mr. D gives Percy two choices. Percy can either let Mr. D turn him into an Atlantic bottlenose dolphin, or Percy can go on a quest.
Mr. D is on his way to Mount Olympus for a meeting, and he says that if Percy is still at Camp Half-Blood when he returns, he'll turn him into a dolphin for sure.
Percy wants to know what this "quest" business is all about.
Chiron tells Percy that Zeus and Poseidon are having the worst fight in centuries. Zeus's symbol of power, a master bolt (the source of Zeus's lighting bolts), has been stolen. Zeus blames Poseidon and thinks he got a hero to steal it for him. Now that Poseidon has openly claimed Percy as his son, everything thinks that Percy stole the bolt.
Ever since Poseidon, Hera, and some other gods trapped Zeus in a golden net in order to make him be a better ruler, Zeus has never trusted Poseidon.
Poseidon rules the ocean, and the forges of the Cyclopses are under the ocean. The Cyclopses made Zeus's master bolt, so Zeus thinks that Poseidon is having tons of copies made so that he can wage war on his brother.
Now it's time for someone to find the master bolt before World War III erupts. The deadline for finding the master bolt is the summer solstice: June 21st.
This is where Percy comes in as the dude who goes on the quest to find the master bolt.
Chiron sends Percy upstairs to the attic to have a chat with the Oracle.
The attic is full of ancient stuff – "Greek hero junk" (9.123) and pickled monster parts. And it smells like reptiles. Mmmmmmm.
Suddenly the mummy of a woman opens its mouth, and green smoke pours out. The green smoke forms the image of Smelly Gabe and his buddies playing poker. This is what the Oracle says: "I am the spirit of Delphi, speaker of prophecies of Phoebus Apollo, slayer of the mighty Python. Approach, seeker, and ask" (9.125).
The smoke image of Smelly Gabe turns to Percy and says: "You shall go west, and face the god who has turned" (9.131).
Another smoke dude says: "You shall find what was stolen, and see it safely returned" (9.132).
A third smoke dude says: "You shall be betrayed by one who calls you a friend" (9.133).
And a fourth smoke dude says: "And you shall fail to save what matters most, in the end" (9.134).
Then the mummy inhales the green mist, closes her mouth, and Percy is left with tons of questions. What friend? What matters most?
Percy reports back to Chiron and Grover about everything he heard – everything except the being-betrayed-by-a-friend part and the failing-to-save-what-matters-most part. Percy decides to keep this info from Chiron and Grover.
Chiron senses that Percy is keeping something from him, but doesn't push Percy to tell him.
Chiron helps Percy and Grover figure out that they must travel to Hades in order to recover the master bolt. Hades is the god who has turned against his brothers, Zeus and Poseidon. Hades must have had someone steal the master bolt for him so that Zeus and Poseidon would wage war on one another.
Hades is the god of the Underworld, which means that Percy and Grover have a ticket to Worm City.
Chiron supposes that Hades must have a spy at Camp Half-Blood – because only someone within camp boundaries could have summoned a hellhound. Hellhounds come from Hades.
Chiron guesses that Hades wants to kill Percy before he begins his quest to recover the master bolt – Hades doesn't want peace restored between Zeus and Poseidon.
Percy asks why they can't just tell the gods that they suspect Hades has had the master bolt stolen for him. Chiron responds that suspecting and knowing are two different things. Gods can't invade each other's territories, so it has to be a hero that retrieves the bolt. A hero like Percy.
Chiron tells Percy that Poseidon needs him. Percy chews on this: "my dad needs me" (9.176).
Grover is terrified of the idea of going to the Underworld, but he agrees to come along on the quest. Percy is totally relieved to hear this. His BFF will be with him every step of the way.
The gates to the Underworld are, naturally, in Los Angeles.
Percy, Grover, and Annabeth will travel together across the country to fetch the master bolt.
But, they can't take any planes, because the air is Zeus's territory. Zeus already hates Percy enough – he might strike their plane and cause a crash.