I ran after the tour group, through a tunnel carved out of solid rock. It seemed to run forever. The walls were moist, and the air hummed with electricity and the roar of water. I came out on a U-shaped balcony that overlooked this huge warehouse area. Fifty feet below, enormous turbines were running. It was a big room, but I didn’t see any other exit, unless I wanted to jump into the turbines and get churned up to make electricity. I didn’t.

Another tour guide was talking over the microphone, telling the tourists about water supplies in Nevada. I prayed that Thalia, Zoë, and Grover were okay. They might already be captured, or eating at the snack bar, completely unaware that we were being surrounded. And stupid me: I had trapped myself in a hole hundreds of feet below the surface.

I worked my way around the crowd, trying not to be too obvious about it. There was a hallway at the other side of the balcony—maybe some place I could hide. I kept my hand on Riptide, ready to strike.

By the time I got to the opposite side of the balcony, my nerves were shot. I backed into the little hallway and watched the tunnel I’d come from.

Then right behind me I heard a sharp Chhh! like the voice of a skeleton.

Without thinking, I uncapped Riptide and spun, slashing with my sword.

The girl I’d just tried to slice in half yelped and dropped her Kleenex.

“Oh my god!” she shouted. “Do you always kill people when they blow their nose?”

The first thing that went through my head was that the sword hadn’t hurt her. It had passed clean through her body, harmlessly. “You’re mortal!”

She looked at me in disbelief. “What’s that supposed to mean? Of course I’m mortal! How did you get that sword past security?”

“I didn’t— Wait, you can see it’s a sword?”

The girl rolled her eyes, which were green like mine. She had frizzy reddish-brown hair. Her nose was also red, like she had a cold. She wore a big maroon Harvard sweatshirt and jeans that were covered with marker stains and little holes, like she spent her free time poking them with a fork.

“Well, it’s either a sword or the biggest toothpick in the world,” she said. “And why didn’t it hurt me? I mean, not that I’m complaining. Who are you? And whoa, what is that you’re wearing? Is that made of lion fur?”

She asked so many questions so fast, it was like she was throwing rocks at me. I couldn’t think of what to say. I looked at my sleeves to see if the Nemean Lion pelt had somehow changed back to fur, but it still looked like a brown winter coat to me.

I knew the skeleton warriors were still chasing me. I had no time to waste. But I just stared at the redheaded girl. Then I remembered what Thalia had done at Westover Hall to fool the teachers. Maybe I could manipulate the Mist.

I concentrated hard and snapped my fingers. “You don’t see a sword,” I told the girl. “It’s just a ballpoint pen.”

She blinked. “Um . . . no. It’s a sword, weirdo.”

“Who are you?” I demanded.

She huffed indignantly. “Rachel Elizabeth Dare. Now, are you going to answer my questions or should I scream for security?”

“No!” I said. “I mean, I’m kind of in a hurry. I’m in trouble.”

“In a hurry or in trouble?”

“Um, sort of both.”

She looked over my shoulder and her eyes widened. “Bathroom!”

“What?”

“Bathroom! Behind me! Now!”

I don’t know why, but I listened to her. I slipped inside the boys’ bathroom and left Rachel Elizabeth Dare standing outside. Later, that seemed cowardly to me. I’m also pretty sure it saved my life.

I heard the clattering, hissing sounds of skeletons as they came closer.

My grip tightened on Riptide. What was I thinking? I’d left a mortal girl out there to die. I was preparing to burst out and fight when Rachel Elizabeth Dare started talking in that rapid-fire machine gun way of hers.

“Oh my god! Did you see that kid? It’s about time you got here. He tried to kill me! He had a sword, for god’s sake. You security guys let a sword-swinging lunatic inside a national landmark? I mean, jeez! He ran that way toward those turbine thingies. I think he went over the side or something. Maybe he fell.”

The skeletons clattered excitedly. I heard them moving off.

Rachel opened the door. “All clear. But you’d better hurry.”

She looked shaken. Her face was gray and sweaty.

I peeked around the corner. Three skeleton warriors were running toward the other end of the balcony. The way to the elevator was clear for a few seconds.

“I owe you one, Rachel Elizabeth Dare.”

“What are those things?” she asked. “They looked like—”

“Skeletons?”

She nodded uneasily.

“Do yourself a favor,” I said. “Forget it. Forget you ever saw me.”

“Forget you tried to kill me?”

“Yeah. That, too.”

“But who are you?”

“Percy—” I started to say. Then the skeletons turned around. “Gotta go!”

“What kind of name is Percy Gotta-go?”

I bolted for the exit.

The café was packed with kids enjoying the best part of the tour—the dam lunch. Thalia, Zoë, and Grover were just sitting down with their food.

“We need to leave,” I gasped. “Now!”

“But we just got our burritos!” Thalia said.

Zoë stood up, muttering an Ancient Greek curse. “He’s right! Look.”

The café windows wrapped all the way around the observation floor, which gave us a beautiful panoramic view of the skeletal army that had come to kill us.

I counted two on the east side of the dam road, blocking the way to Arizona. Three more on the west side, guarding Nevada. All of them were armed with batons and pistols.

But our immediate problem was a lot closer. The three skeletal warriors who’d been chasing me in the turbine room now appeared on the stairs. They saw me from across the cafeteria and clattered their teeth.

“Elevator!” Grover said. We bolted that direction, but the doors opened with a pleasant ding, and three more warriors stepped out. Every warrior was accounted for, minus the one Bianca had blasted to flames in New Mexico. We were completely surrounded.

Then Grover had a brilliant, totally Grover-like idea.

“Burrito fight!” he yelled, and flung his Guacamole Grande at the nearest skeleton.

Now, if you have never been hit by a flying burrito, count yourself lucky. In terms of deadly projectiles, it’s right up there with grenades and cannonballs. Grover’s lunch hit the skeleton and knocked his skull clean off his shoulders. I’m not sure what the other kids in the café saw, but they went crazy and started throwing their burritos and baskets of chips and sodas at each other, shrieking and screaming.

The skeletons tried to aim their guns, but it was hopeless. Bodies and food and drinks were flying everywhere.

In the chaos, Thalia and I tackled the other two skeletons on the stairs and sent them flying into the condiment table. Then we all raced downstairs, Guacamole Grandes whizzing past our heads.

“What now?” Grover asked as we burst outside.

I didn’t have an answer. The warriors on the road were closing in from either direction. We ran across the street to the pavilion with the winged bronze statues, but that just put our backs to the mountain.

The skeletons moved forward, forming a crescent around us. Their brethren from the café were running up to join them. One was still putting its skull back on its shoulders. Another was covered in ketchup and mustard. Two more had burritos lodged in their rib cages. They didn’t look happy about it. They drew batons and advanced.

“Four against eleven,” Zoë muttered. “And they cannot die.”

“It’s been nice adventuring with you guys,” Grover said, his voice trembling.

Something shiny caught the corner of my eye. I glanced behind me at the statue’s feet. “Whoa,” I said. “Their toes really are bright.”