Thalia had gotten herself turned into a pine tree when she was twelve. Me … well, I was doing my best not to follow her example. I had nightmares about what Poseidon might turn me into if I were ever on the verge of death— plankton, maybe. Or a floating patch of kelp.

When we got to the Big House, we found Chiron in his apartment, listening to his favorite 1960s lounge music while he packed his saddlebags. I guess I should mention—Chiron is a centaur. From the waist up he looks like a regular middle-aged guy with curly brown hair and a scraggly beard. From the waist down, he’s a white stallion. He can pass for human by compacting his lower half into a magic wheelchair. In fact, he’d passed himself off as my Latin teacher during my sixth-grade year. But most of the time, if the ceilings are high enough, he prefers hanging out in full centaur form.

As soon as we saw him, Tyson froze. “Pony!” he cried in total rapture.

Chiron turned, looking offended. “I beg your pardon?”

Annabeth ran up and hugged him. “Chiron, what’s happening? You’re not … leaving?” Her voice was shaky. Chiron was like a second father to her.

Chiron ruffled her hair and gave her a kindly smile. “Hello, child. And Percy, my goodness.

You’ve grown over the year!”

I swallowed. “Clarisse said you were … you were …”

“Fired.” Chiron’s eyes glinted with dark humor. “Ah, well, someone had to take the blame.

Lord Zeus was most upset. The tree he’d created from the spirit of his daughter, poisoned! Mr. D had to punish someone.”

“Besides himself, you mean,” I growled. Just the thought of the camp director, Mr. D, made me angry.

“But this is crazy!” Annabeth cried. “Chiron, you couldn’t have had anything to do with poisoning Thalia’s tree!”

“Nevertheless,” Chiron sighed, “some in Olympus do not trust me now, under the circumstances.”

“What circumstances?” I asked.

Chiron’s face darkened. He stuffed a Latin-English dictionary into his saddlebag while the Frank Sinatra music oozed from his boom box.

Tyson was still staring at Chiron in amazement. He whimpered like he wanted to pat Chiron’s flank but was afraid to come closer. “Pony?”

Chiron sniffed. “My dear young Cyclops! I am a centaur. ”

“Chiron,” I said. “What about the tree? What happened?”

He shook his head sadly. “The poison used on Thalia’s pine is something from the Underworld, Percy. Some venom even I have never seen. It must have come from a monster quite deep in the pits of Tartarus.”

“Then we know who’s responsible. Kro—”

“Do not invoke the titan lord’s name, Percy. Especially not here, not now.”

“But last summer he tried to cause a civil war in Olympus! This has to be his idea. He’d get Luke to do it, that traitor.”

“Perhaps,” Chiron said. “But I fear I am being held responsible because I did not prevent it and I cannot cure it. The tree has only a few weeks of life left unless …”

“Unless what?” Annabeth asked.

“No,” Chiron said. “A foolish thought. The whole valley is feeling the shock of the poison. The magical borders are deteriorating. The camp itself is dying. Only one source of magic would be strong enough to reverse the poison, and it was lost centuries ago.”

“What is it?” I asked. “We’ll go find it!”

Chiron closed his saddlebag. He pressed the stop button on his boom box. Then he turned and rested his hand on my shoulder, looking me straight in the eyes. “Percy, you must promise me that you will not act rashly. I told your mother I did not want you to come here at all this summer. It’s much too dangerous. But now that you are here, stay here. Train hard. Learn to fight. But do not leave.”

“Why?” I asked. “I want to do something! I can’t just let the borders fail. The whole camp will be—”

“Overrun by monsters,” Chiron said. “Yes, I fear so. But you must not let yourself be baited into hasty action! This could be a trap of the titan lord. Remember last summer! He almost took your life.”

It was true, but still, I wanted to help so badly. I also wanted to make Kronos pay. I mean, you’d think the titan lord would’ve learned his lesson eons ago when he was overthrown by the gods. You’d think getting chopped into a million pieces and cast into the darkest part of the Underworld would give him a subtle clue that nobody wanted him around. But no. Because he was immortal, he was still alive down there in Tartarus—suffering in eternal pain, hungering to return and take revenge on Olympus. He couldn’t act on his own, but he was great at twisting the minds of mortals and even gods to do his dirty work.

The poisoning had to be his doing. Who else would be so low as to attack Thalia’s tree, the only thing left of a hero who’d given her life to save her friends?

Annabeth was trying hard not to cry. Chiron brushed a tear from her cheek. “Stay with Percy, child,” he told her. “Keep him safe. The prophecy—remember it!”

“I—I will.”

“Um …” I said. “Would this be the super-dangerous prophecy that has me in it, but the gods have forbidden you to tell me about?”

Nobody answered.

“Right,” I muttered. “Just checking.”

“Chiron …” Annabeth said. “You told me the gods made you immortal only so long as you were needed to train heroes. If they dismiss you from camp—”

“Swear you will do your best to keep Percy from danger,” he insisted. “Swear upon the River Styx.”

“I—I swear it upon the River Styx,” Annabeth said.

Thunder rumbled outside.

“Very well,” Chiron said. He seemed to relax just a little. “Perhaps my name will be cleared and I shall return. Until then, I go to visit my wild kinsmen in the Everglades. It’s possible they know of some cure for the poisoned tree that I have forgotten. In any event, I will stay in exile until this matter is resolved … one way or another.”

Annabeth stifled a sob. Chiron patted her shoulder awkwardly. “There, now, child. I must entrust your safety to Mr. D and the new activities director. We must hope … well, perhaps they won’t destroy the camp quite as quickly as I fear.”

“Who is this Tantalus guy, anyway?” I demanded. “Where does he get off taking your job?”

A conch horn blew across the valley. I hadn’t realized how late it was. It was time for the campers to assemble for dinner.

“Go,” Chiron said. “You will meet him at the pavilion. I will contact your mother, Percy, and let her know you’re safe. No doubt she’ll be worried by now. Just remember my warning! You are in grave danger. Do not think for a moment that the titan lord has forgotten you!”

With that, he clopped out of the apartment and down the hall, Tyson calling after him, “Pony! Don’t go!”

I realized I’d forgotten to tell Chiron about my dream of Grover. Now it was too late. The best teacher I’d ever had was gone, maybe for good.

Tyson started bawling almost as bad as Annabeth. I tried to tell them that things would be okay, but I didn’t believe it.

The sun was setting behind the dining pavilion as the campers came up from their cabins.

We stood in the shadow of a marble column and watched them file in. Annabeth was still pretty shaken up, but she promised she’d talk to us later. Then she went off to join her siblings from the Athena cabin—a dozen boys and girls with blond hair and gray eyes like hers. Annabeth wasn’t the oldest, but she’d been at camp more summers than just about anybody. You could tell that by looking at her camp necklace—one bead for every summer, and Annabeth had six. No one questioned her right to lead the line.

Next came Clarisse, leading the Ares cabin. She had one arm in a sling and a nasty-looking gash on her cheek, but otherwise her encounter with the bronze bulls didn’t seem to have fazed her.

Someone had taped a piece of paper to her back that said, YOU MOO, GIRL! But nobody in her cabin was bothering to tell her about it.

After the Ares kids came the Hephaestus cabin—six guys led by Charles Beckendorf, a big fifteen-year-old African American kid. He had hands the size of catchers’ mitts and a face that was hard and squinty from looking into a blacksmiths forge all day. He was nice enough once you got to know him, but no one ever called him Charlie or Chuck or Charles. Most just called him Beckendorf.

Rumor was he could make anything. Give him a chunk of metal and he could create a razor-sharp sword or a robotic warrior or a singing birdbath for your grandmother’s garden. Whatever you wanted.

The other cabins filed in: Demeter, Apollo, Aphrodite, Dionysus. Naiads came up from the canoe lake. Dryads melted out of the trees. From the meadow came a dozen satyrs, who reminded me painfully of Grover.

I’d always had a soft spot for the satyrs. When they were at camp, they had to do all kinds of odd jobs for Mr. D, the director, but their most important work was out in the real world. They were the camp’s seekers. They went undercover into schools all over the world, looking for potential half-bloods and escorting them back to camp. That’s how I’d met Grover. He had been the first one to recognize I was a demigod.

After the satyrs filed in to dinner, the Hermes cabin brought up the rear. They were always the biggest cabin. Last summer, it had been led by Luke, the guy who’d fought with Thalia and Annabeth on top of Half-Blood Hill. For a while, before Poseidon had claimed me, I’d lodged in the Hermes cabin. Luke had befriended me … and then he’d tried to kill me.

Now the Hermes cabin was led by Travis and Connor Stoll. They weren’t twins, but they looked so much alike it didn’t matter. I could never remember which one was older. They were both tall and skinny, with mops of brown hair that hung in their eyes. They wore orange CAMP HALF-BLOOD T-shirts untucked over baggy shorts, and they had those elfish features all Hermes’s kids had: upturned eyebrows, sarcastic smiles, a gleam in their eyes whenever they looked at you—like they were about to drop a firecracker down your shirt. I’d always thought it was funny that the god of thieves would have kids with the last name “Stoll,” but the only time I mentioned it to Travis and Connor, they both stared at me blankly like they didn’t get the joke.

As soon as the last campers had filed in, I led Tyson into the middle of the pavilion.

Conversations faltered. Heads turned. “Who invited that? ” somebody at the Apollo table murmured.

I glared in their direction, but I couldn’t figure out who’d spoken.

From the head table a familiar voice drawled, “Well, well, if it isn’t Peter Johnson. My millennium is complete.”

I gritted my teeth. “Percy Jackson … sir.”

Mr. D sipped his Diet Coke. “Yes. Well, as you young people say these days: Whatever.”

He was wearing his usual leopard-pattern Hawaiian shirt, walking shorts, and tennis shoes with black socks. With his pudgy belly and his blotchy red face, he looked like a Las Vegas tourist who’d stayed up too late in the casinos. Behind him, a nervous-looking satyr was peeling the skins off grapes and handing them to Mr. D one at a time.