Ever come home and found your room messed up? Like some helpful person (hi, Mom) has tried to «clean» it, and suddenly you can't find anything? And even if nothing is missing, you get that creepy feeling like somebody's been looking through your private stuff and dusting everything with lemon furniture polish?

That's kind of the way I felt seeing Camp Half-Blood again.

On the surface, things didn't look all that different. The Big House was still there with its blue gabled roof and its wraparound porch. The strawberry fields still baked in the sun. The same white-columned Greek buildings were scattered around the valley—the amphitheater, the combat arena, the dining pavilion overlooking Long Island Sound. And nestled between the woods and the creek were the same cabins—a crazy assortment of twelve buildings, each representing a different Olympian god.

But there was an air of danger now. You could tell something was wrong. Instead of playing volleyball in the sandpit, counselors and satyrs were stockpiling weapons in the tool shed. Dryads armed with bows and arrows talked nervously at the edge of the woods. The forest looked sickly, the grass in the meadow was pale yellow, and the fire marks on Half-Blood Hill stood out like ugly scars.

Somebody had messed with my favorite place in the world, and I was not… well, a happy camper.

As we made our way to the Big House, I recognized a lot of kids from last summer. Nobody stopped to talk. Nobody said, "Welcome back." Some did double takes when they saw Tyson, but most just walked grimly past and carried on with their duties—running messages, toting swords to sharpen on the grinding wheels. The camp felt like a military school. And believe me, I know. I've been kicked out of a couple.

None of that mattered to Tyson. He was absolutely fascinated by everything he saw. "Whasthat!" he gasped.

"The stables for pegasi," I said. "The winged horses."

"Whasthat!"

"Um… those are the toilets."

"Whasthat!"

"The cabins for the campers. If they don't know who your Olympian parent is, they put you in the Hermes cabin—that brown one over there—until you're determined. Then, once they know, they put you in your dad or mom's group."

He looked at me in awe. "You… have a cabin?"

"Number three." I pointed to a low gray building made of sea stone.

"You live with friends in the cabin?"

"No. No, just me." I didn't feel like explaining. The embarrassing truth: I was the only one who stayed in that cabin because I wasn't supposed to be alive. The "Big Three" gods—Zeus, Poseidon, and Hades—had made a pact after World War II not to have any more children with mortals. We were more powerful than regular half-bloods. We were too unpredictable. When we got mad we tended to cause problems… like World War II, for instance. The "Big Three" pact had only been broken twice—once when Zeus sired Thalia, once when Poseidon sired me. Neither of us should've been born.

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!"