"You're saying I'm being used."

"I'm saying it's no accident Poseidon has claimed you now. It's a very risky gamble, but he's in a desperate situation. He needs you."

My dad needs me.

Emotions rolled around inside me like bits of glass in a kaleidoscope. I didn't know whether to feel resentful or grateful or happy or angry. Poseidon had ignored me for twelve years. Now suddenly he needed me.

I looked at Chiron. "You've known I was Poseidon's son all along, haven't you?"

"I had my suspicions. As I said ... I've spoken to the Oracle, too."

I got the feeling there was a lot he wasn't telling me about his prophecy, but I decided I couldn't worry about that right now. After all, I was holding back information too.

"So let me get this straight," I said. "I'm supposed go to the Underworld and confront the Lord of the Dead."

"Check," Chiron said.

"Find the most powerful weapon in the universe."

"Check."

"And get it back to Olympus before the summer solstice, in ten days."

"That's about right."

I looked at Grover, who gulped down the ace of hearts.

"Did I mention that Maine is very nice this time of year?" he asked weakly.

"You don't have to go," I told him. "I can't ask that of you.   

"Oh ..." He shifted his hooves. "No ... it's just that satyrs and underground places ... well..."   

He took a deep breath, then stood, brushing the shredded cards and aluminum bits off his T-shirt. "You saved my life, Percy. If ... if you're serious about wanting me along, I won't let you down."

I felt so relieved I wanted to cry, though I didn't think that would be very heroic. Grover was the only friend I'd ever had for longer than a few months. I wasn't sure what good a satyr could do against the forces of the dead, but I felt better knowing he'd be with me.

"All the way, G-man." I turned to Chiron. "So where do we go? The Oracle just said to go west."

"The entrance to the Underworld is always in the west. It moves from age to age, just like Olympus. Right now, of course, it's in America."

"Where?"

Chiron looked surprised. "I thought that would be obvious enough. The entrance to the Underworld is in Los Angeles."

"Oh," I said. "Naturally. So we just get on a plane—"

"No!" Grover shrieked. "Percy, what are you thinking? Have you ever been on a plane in your life?"

I shook my head, feeling embarrassed. My mom had never taken me anywhere by plane. She'd always said we didn't have the money. Besides, her parents had died in a plane crash.

"Percy, think," Chiron said. "You are the son of the Sea God. Your father's bitterest rival is Zeus, Lord of the Sky. Your mother knew better than to trust you in an airplane. You would be in Zeus's domain. You would never come down again alive."

Overhead, lightning crackled. Thunder boomed.

"Okay," I said, determined not to look at the storm. "So, I'll travel overland."

"That's right," Chiron said. "Two companions may accompany you. Grover is one. The other has already volunteered, if you will accept her help."

"Gee," I said, feigning surprise. "Who else would be stupid enough to volunteer for a quest like this?"

The air shimmered behind Chiron.

Annabeth became visible, stuffing her Yankees cap into her back pocket.

"I've been waiting a long time for a quest, seaweed brain," she said. "Athena is no fan of Poseidon, but if you're going to save the world, I'm the best person to keep you from messing up."

"If you do say so yourself," I said. "I suppose you have a plan, wise girl?"

Her cheeks colored. "Do you want my help or not?"

The truth was, I did. I needed all the help I could get.

"A trio," I said. "That'll work."

"Excellent," Chiron said. "This afternoon, we can take you as far as the bus terminal in Manhattan. After that, you are on your own."

Lightning flashed. Rain poured down on the meadows that were never supposed to have violent weather.

"No time to waste," Chiron said. "I think you should all get packing."

10. I RUIN A PERFECTLY GOOD BUS

It didn't take me long to pack. I decided to leave the Minotaur horn in my cabin, which left me only an extra change of clothes and a toothbrush to stuff in a backpack Grover had found for me.

The camp store loaned me one hundred dollars in mortal money and twenty golden drachmas. These coins were as big as Girl Scout cookies and had images of various Greek gods stamped on one side and the EmpireStateBuilding on the other. The ancient mortal drachmas had been silver, Chiron told us, but Olympians never used less than pure gold. Chiron said the coins might come in handy for non-mortal transactions—whatever that meant. He gave Annabeth and me each a canteen of nectar and a Ziploc bag full of ambrosia squares, to be used only in emergencies, if we were seriously hurt. It was god food, Chiron reminded us. It would cure us of almost any injury, but it was lethal to mortals. Too much of it would make a half-blood very, very feverish. An overdose would burn us up, literally.

Annabeth was bringing her magic Yankees cap, which she told me had been a twelfth-birthday present from her mom. She carried a book on famous classical architecture, written in Ancient Greek, to read when she got bored, and a long bronze knife, hidden in her shirt sleeve. I was sure the knife would get us busted the first time we went through a metal detector.

Grover wore his fake feet and his pants to pass as human. He wore a green rasta-style cap, because when it rained his curly hair flattened and you could just see the tips of his horns. His bright orange backpack was full of scrap metal and apples to snack on. In his pocket was a set of reed pipes his daddy goat had carved for him, even though he only knew two songs: Mozart's Piano Concerto no. 12 and Hilary Duff's "So Yesterday," both of which sounded pretty bad on reed pipes.

We waved good-bye to the other campers, took one last look at the strawberry fields, the ocean, and the Big House, then hiked up Half-Blood Hill to the tall pine tree that used to be Thalia, daughter of Zeus.

Chiron was waiting for us in his wheelchair. Next to him stood the surfer dude I'd seen when I was recovering in the sick room. According to Grover, the guy was the camp's head of security. He supposedly had eyes all over his body so he could never be surprised. Today, though, he was wearing a chauffeur's uniform, so I could only see extra peepers on his hands, face and neck.

"This is Argus," Chiron told me. "He will drive you into the city, and, er, well, keep an eye on things."

I heard footsteps behind us.

Luke came running up the hill, carrying a pair of basketball shoes.

"Hey!" he panted. "Glad I caught you."

Annabeth blushed, the way she always did when Luke was around.

"Just wanted to say good luck," Luke told me. "And I thought ... um, maybe you could use these."

He handed me the sneakers, which looked pretty normal. They even smelled kind of normal.

Luke said, "Maia!"

White bird's wings sprouted out of the heels, startling me so much, I dropped them. The shoes flapped around on the ground until the wings folded up and disappeared.

"Awesome!" Grover said.

Luke smiled. "Those served me well when I was on my quest. Gift from Dad. Of course, I don't use them much these days...." His expression turned sad.

I didn't know what to say. It was cool enough that Luke had come to say good-bye. I'd been afraid he might resent me for getting so much attention the last few days. But here he was giving me a magic gift.... It made me blush almost as much as Annabeth.

"Hey, man," I said. "Thanks."

"Listen, Percy ..." Luke looked uncomfortable. "A lot of hopes are riding on you. So just ... kill some monsters for me, okay?"

We shook hands. Luke patted Grover's head between his horns, then gave a good-bye hug to Annabeth, who looked like she might pass out.

After Luke was gone, I told her, "You're hyperventilating."

"Am not."

"You let him capture the flag instead of you, didn't you?"

"Oh ... why do I want to go anywhere with you, Percy?"

She stomped down the other side of the hill, where a white SUV waited on the shoulder of the road. Argus followed, jingling his car keys.

I picked up the flying shoes and had a sudden bad feeling. I looked at Chiron. "I won't be able to use these, will I?"

He shook his head. "Luke meant well, Percy. But taking to the air ... that would not be wise for you."

I nodded, disappointed, but then I got an idea. "Hey, Grover. You want a magic item?"

His eyes lit up. "Me?"

Pretty soon we'd laced the sneakers over his fake feet, and the world's first flying goat boy was ready for launch.

"Maia!" he shouted.

He got off the ground okay, but then fell over sideways so his backpack dragged through the grass. The winged shoes kept bucking up and down like tiny broncos.

"Practice," Chiron called after him. "You just need practice!"

"Aaaaa!" Grover went flying sideways down the hill like a possessed lawn mower, heading toward the van.

Before I could follow, Chiron caught my arm. "I should have trained you better, Percy," he said. "If only I had more time. Hercules, Jason—they all got more training."

"That's okay. I just wish—"

I stopped myself because I was about to sound like a brat. I was wishing my dad had given me a cool magic item to help on the quest, something as good as Luke's flying shoes, or Annabeth's invisible cap.

"What am I thinking?" Chiron cried. "I can't let you get away without this."

He pulled a pen from his coat pocket and handed it to me. It was an ordinary disposable ballpoint, black ink, removable cap. Probably cost thirty cents.

"Gee," I said. "Thanks."

"Percy, that's a gift from your father. I've kept it for years, not knowing you were who I was waiting for. But the prophecy is clear to me now. You are the one."

I remembered the field trip to the Metropolitan Museum of Art, when I'd vaporized Mrs. Dodds. Chiron had thrown me a pen that turned into a sword. Could this be ... ?

I took off the cap, and the pen grew longer and heavier in my hand. In half a second, I held a shimmering bronze sword with a double-edged blade, a leather-wrapped grip, and a flat hilt riveted with gold studs. It was the first weapon that actually felt balanced in my hand.

"The sword has a long and tragic history that we need not go into," Chiron told me. "Its name is Anaklusmos."

"'Riptide,'" I translated, surprised the Ancient Greek came so easily.

"Use it only for emergencies," Chiron said, "and only against monsters. No hero should harm mortals unless absolutely necessary, of course, but this sword wouldn't harm them in any case."

I looked at the wickedly sharp blade. "What do you mean it wouldn't harm mortals? How could it not?"

"The sword is celestial bronze. Forged by the Cyclopes, tempered in the heart of Mount Etna, cooled in the River Lethe. It's deadly to monsters, to any creature from the Underworld, provided they don't kill you first. But the blade will pass through mortals like an illusion. They simply are not important enough for the blade to kill. And I should warn you: as a demigod, you can be killed by either celestial or normal weapons. You are twice as vulnerable."