I took Annabeth’s magic cap out of my pocket and put it on.

I didn’t feel any different, but when I raised my arms I couldn’t see them. I was invisible.

I crept up to Nico and sneaked around him. I couldn’t see the girls very well in the dark, but I knew their voices: Zoë and Bianca. It sounded like they were arguing.

“It cannot be cured,” Zoë was saying. “Not quickly, at any rate.”

“But how did it happen?” Bianca asked.

“A foolish prank,” Zoë growled. “Those Stoll boys from the Hermes cabin. Centaur blood is like acid. Everyone knows that. They sprayed the inside of that Artemis Hunting Tour T-shirt with it.”

“That’s terrible!”

“She will live,” Zoë said. “But she’ll be bedridden for weeks with horrible hives. There is no way she can go. It’s up to me . . . and thee.”

“But the prophecy,” Bianca said. “If Phoebe can’t go, we only have four. We’ll have to pick another.”

“There is no time,” Zoë said. “We must leave at first light. That’s immediately. Besides, the prophecy said we would lose one.”

“In the land without rain,” Bianca said, “but that can’t be here.”

“It might be,” Zoë said, though she didn’t sound convinced. “The camp has magic borders. Nothing, not even weather, is allowed in without permission. It could be a land without rain.”

“But—”

“Bianca, hear me.” Zoë’s voice was strained. “I . . . I can’t explain, but I have a sense that we should not pick someone else. It would be too dangerous. They would meet an end worse than Phoebe’s. I don’t want Chiron choosing a camper as our fifth companion. And . . . I don’t want to risk another Hunter.”

Bianca was silent. “You should tell Thalia the rest of your dream.”

“No. It would not help.”

“But if your suspicions are correct, about the General—”

“I have thy word not to talk about that,” Zoë said. She sounded really anguished. “We will find out soon enough. Now come. Dawn is breaking.”

Nico scooted out of their way. He was faster than me.

As the girls sprinted down the steps, Zoë almost ran into me. She froze, her eyes narrowing. Her hand crept toward her bow, but then Bianca said, “The lights of the Big House are on. Hurry!”

And Zoë followed her out of the pavilion.

I could tell what Nico was thinking. He took a deep breath and was about to run after his sister when I took off the invisibility cap and said, “Wait.”

He almost slipped on the icy steps as he spun around to find me. “Where did you come from?”

“I’ve been here the whole time. Invisible.”

He mouthed the word invisible. “Wow. Cool.”

“How did you know Zoë and your sister were here?”

He blushed. “I heard them walk by the Hermes cabin. I don’t . . . I don’t sleep too well at camp. So I heard footsteps, and them whispering. And so I kind of followed.”

“And now you’re thinking about following them on the quest,” I guessed.

“How did you know that?”

“Because if it was my sister, I’d probably be thinking the same thing. But you can’t.”

He looked defiant. “Because I’m too young?”

“Because they won’t let you. They’ll catch you and send you back here. And . . . yeah, because you’re too young. You remember the manticore? There will be lots more like that. More dangerous. Some of the heroes will die.”

He shoulders sagged. He shifted from foot to foot. “Maybe you’re right. But, but you can go for me.”

“Say what?”

“You can turn invisible. You can go!”

“The Hunters don’t like boys,” I reminded him. “If they find out—”

“Don’t let them find out. Follow them invisibly. Keep an eye on my sister! You have to. Please?”

“Nico—”

“You’re planning to go anyway, aren’t you?”

I wanted to say no. But he looked me in the eyes, and I somehow couldn’t lie to him.

“Yeah,” I said. “I have to find Annabeth. I have to help, even if they don’t want me to.”

“I won’t tell on you,” he said. “But you have to promise to keep my sister safe.”

“I . . . that’s a big thing to promise, Nico, on a trip like this. Besides, she’s got Zoë, Grover, and Thalia—”

“Promise,” he insisted.

“I’ll do my best. I promise that.”

“Get going, then!” he said. “Good luck!”

It was crazy. I wasn’t packed. I had nothing but the cap and the sword and the clothes I was wearing. I was supposed to be going home to Manhattan this morning. “Tell Chiron—”

“I’ll make something up.” Nico smiled crookedly. “I’m good at that. Go on!”

I ran, putting on Annabeth’s cap. As the sun came up, I turned invisible. I hit the top of Half-Blood Hill in time to see the camp’s van disappearing down the farm road, probably Argus taking the quest group into the city. After that they would be on their own.

I felt a twinge of guilt, and stupidity, too. How was I supposed to keep up with them. Run?

Then I heard the beating of huge wings. Blackjack landed next to me. He began casually nuzzling a few tufts of grass that stuck through the ice.

If I was guessing, boss, I’d say you need a getaway horse. You interested?

A lump of gratitude stuck in my throat, but I managed to say, “Yeah. Let’s fly.”

NINE

I LEARN HOW TO GROW ZOMBIES

The thing about flying on a pegasus during the daytime is that if you’re not careful, you can cause a serious traffic accident on the Long Island Expressway. I had to keep Blackjack up in the clouds, which were, fortunately, pretty low in the winter. We darted around, trying to keep the white Camp Half-Blood van in sight. And if it was cold on the ground, it was seriously cold in the air, with icy rain stinging my skin.

I was wishing I’d brought some of that Camp Half-Blood orange thermal underwear they sold in the camp store, but after the story about Phoebe and the centaur-blood T-shirt, I wasn’t sure I trusted their products anymore.

We lost the van twice, but I had a pretty good sense that they would go into Manhattan first, so it wasn’t too difficult to pick up their trail again.

Traffic was bad with the holidays and all. It was mid morning before they got into the city. I landed Blackjack near the top of the Chrysler Building and watched the white camp van, thinking it would pull into the bus station, but it just kept driving.

“Where’s Argus taking them?” I muttered.

Oh, Argus ain’t driving, boss, Blackjack told me. That girl is.

“Which girl?”

The Hunter girl. With the silver crown thing in her hair.

“Zoë?”

That’s the one. Hey, look! There’s a donut shop. Can we get something to go?

I tried explaining to Blackjack that taking a flying horse to a donut shop would give every cop in there a heart attack, but he didn’t seem to get it. Meanwhile, the van kept snaking its way toward the Lincoln Tunnel. It had never even occurred to me that Zoë could drive. I mean, she didn’t look sixteen. Then again, she was immortal. I wondered if she had a New York license, and if so, what her birth date said.

“Well,” I said. “Let’s get after them.”

We were about to leap off the Chrysler Building when Blackjack whinnied in alarm and almost threw me. Something was curling around my leg like a snake. I reached for my sword, but when I looked down, there was no snake. Vines—grape vines—had sprouted from the cracks between the stones of the building. They were wrapping around Blackjack’s legs, lashing down my ankles so we couldn’t move.

“Going somewhere?” Mr. D asked.

He was leaning against the building with his feet levitating in the air, his leopard-skin warm-up suit and black hair whipping around in the wind.

God alert! Blackjack yelled. It’s the wine dude!

Mr. D sighed in exasperation. “The next person, or horse, who calls me the ‘wine dude’ will end up in a bottle of Merlot!”