I lunged but Bull Number Two blew flames at me. I rolled aside as the air turned to pure heat. All the oxygen was sucked out of my lungs. My foot caught on something—a tree root, maybe—and pain shot up my ankle. Still, I managed to slash with my sword and lop off part of the monster's snout. It galloped away, wild and disoriented. But before I could feel too good about that, I tried to stand, and my left leg buckled underneath me. My ankle was sprained, maybe broken.

Bull Number One charged straight toward me. No way could I crawl out of its path.

Annabeth shouted: "Tyson, help him!"

Somewhere near, toward the crest of the hill, Tyson wailed, "Can't—get—through!"

"I, Annabeth Chase, give you permission to enter camp!"

Thunder shook the hillside. Suddenly Tyson was there, barreling toward me, yelling: "Percy needs help!"

Before I could tell him no, he dove between me and the bull just as it unleashed a nuclear firestorm.

"Tyson!" I yelled.

The blast swirled around him like a red tornado. I could only see the black silhouette of his body. I knew with horrible certainty that my friend had just been turned into a column of ashes.

But when the fire died, Tyson was still standing there, completely unharmed. Not even his grungy clothes were scorched. The bull must've been as surprised as I was, because before it could unleash a second blast, Tyson balled his fists and slammed them into the bull's face. "BAD COW!"

His fists made a crater where the bronze bull's snout used to be. Two small columns of flame shot out of its ears. Tyson hit it again, and the bronze crumpled under his hands like aluminum foil. The bull's face now looked like a sock puppet pulled inside out.

"Down!" Tyson yelled.

The bull staggered and fell on its back. Its legs moved feebly in the air, steam coming out of its ruined head in odd places.

Annabeth ran over to check on me.

My ankle felt like it was filled with acid, but she gave me some Olympian nectar to drink from her canteen, and I immediately started to feel better. There was a burning smell that I later learned was me. The hair on my arms had been completely singed off.

"The other bull?" I asked.

Annabeth pointed down the hill. Clarisse had taken care of Bad Cow Number Two. She'd impaled it through the back leg with a celestial bronze spear. Now, with its snout half gone and a huge gash in its side, it was trying to run in slow motion, going in circles like some kind of merry-go-round animal.

Clarisse pulled off her helmet and marched toward us. A strand of her stringy brown hair was smoldering, but she didn't seem to notice. "You—ruin—everything!" she yelled at me. "I had it under control!"

I was too stunned to answer. Annabeth grumbled, "Good to see you too, Clarisse."

"Argh!" Clarisse screamed. "Don't ever, EVER try saving me again!"

"Clarisse," Annabeth said, "you've got wounded campers."

That sobered her up. Even Clarisse cared about the soldiers under her command.

"I'll be back," she growled, then trudged off to assess the damage.

I stared at Tyson. "You didn't die."

Tyson looked down like he was embarrassed. "I am sorry. Came to help. Disobeyed you."

"My fault," Annabeth said. "I had no choice. I had to let Tyson cross the boundary line to save you. Otherwise, you would've died."

"Let him cross the boundary line? " I asked. "But—"

"Percy," she said, "have you ever looked at Tyson closely? I mean… in the face. Ignore the Mist, and really look at him."

The Mist makes humans see only what their brains can process… I knew it could fool demigods too, but…

I looked Tyson in the face. It wasn't easy. I'd always had trouble looking directly at him, though I'd never quite understood why. I'd thought it was just because he always had peanut butter in his crooked teeth. I forced myself to focus at his big lumpy nose, then a little higher at his eyes.

No, not eyes.

One eye. One large, calf-brown eye, right in the middle of his forehead, with thick lashes and big tears trickling down his cheeks on either side.

"Tyson," I stammered. "You're a…"

"Cyclops," Annabeth offered. "A baby, by the looks of him. Probably why he couldn't get past the boundary line as easily as the bulls. Tyson's one of the homeless orphans."

"One of the what?"

"They're in almost all the big cities," Annabeth said distastefully. "They're… mistakes, Percy. Children of nature spirits and gods… Well, one god in particular, usually… and they don't always come out right. No one wants them. They get tossed aside. They grow up wild on the streets. I don't know how this one found you, but he obviously likes you. We should take him to Chiron, let him decide what to do."

"But the fire. How—"

"He's a Cyclops." Annabeth paused, as if she were remembering something unpleasant. "They work the forges of the gods. They have to be immune to fire. That's what I was trying to tell you."

I was completely shocked. How had I never realized what Tyson was?

But I didn't have much time to think about it just then. The whole side of the hill was burning. Wounded heroes needed attention. And there were still two banged-up bronze bulls to dispose of, which I didn't figure would fit in our normal recycling bins.

Clarisse came back over and wiped the soot off her forehead. "Jackson, if you can stand, get up. We need to carry the wounded back to the Big House, let Tantalus know what's happened."

"Tantalus?" I asked.

"The activities director," Clarisse said impatiently.

"Chiron is the activities director. And where's Argus? He's head of security. He should be here."

Clarisse made a sour face. "Argus got fired. You two have been gone too long. Things are changing."

"But Chiron… He's trained kids to fight monsters for over three thousand years. He can't just be gone. What happened?"

"That happened," Clarisse snapped.

She pointed to Thalia's tree.

Every camper knew the story behind the tree. Six years ago, Grover, Annabeth, and two other demigods named Thalia and Luke had come to Camp Half-Blood chased by an army of monsters. When they got cornered on top of this hill, Thalia, a daughter of Zeus, had made her last stand here to give her friends time to reach safety. As she was dying, her father, Zeus, took pity on her and changed her into a pine tree. Her spirit had reinforced the magic borders of the camp, protecting it from monsters. The pine had been here ever since, strong and healthy.

But now, its needles were yellow. A huge pile of dead ones littered the base of the tree. In the center of the trunk, three feet from the ground, was a puncture mark the size of a bullet hole, oozing green sap.

A sliver of ice ran through my chest. Now I understood why the camp was in danger. The magical borders were failing because Thalia's tree was dying.

Someone had poisoned it.