"Whoa," I said. "Slow down. Tell me what happened."

He took a deep breath. "I was . . . I was walking in the woods up by Harlem Meer. And I felt this tremble in the ground, like something powerful was near."

"You can sense stuff like that?" Nico asked.

Grover nodded. "Since Pan's death, I can feel when something is wrong in nature. It's like my ears and eyes are sharper when I'm in the Wild. Anyway, I started following the scent. This man in a long black coat was walking through the park, and I noticed he didn't cast a shadow. Middle of a sunny day, and he cast no shadow. He kind of shimmered as he moved."

"Like a mirage?" Nico asked.

"Yes," Grover said. "And whenever he passed humans—"

"The humans would pass out," Nico said. "Curl up and go to sleep."

"That's right! Then after he was gone, they'd get up and go about their business like nothing happened."

I stared at Nico. "You know this guy in black?"

"Afraid so," Nico said. "Grover, what happened?"

"I followed the guy. He kept looking up at the buildings around the park like he was making estimates or something. This lady jogger ran by, and she curled up on the sidewalk and started snoring. The guy in black put his hand on her forehead like he was checking her temperature. Then he kept walking. By this time, I knew he was a monster or something even worse. I followed him into this grove, to the base of a big elm tree. I was about to summon some dryads to help me capture him when he turned and . . ."

Grover swallowed. "Percy, his face. I couldn't make out his face because it kept shifting. Just looking at him made me sleepy. I said, 'What are you doing?' He said, 'Just having a look around. You should always scout a battlefield before the battle.' I said something really smart like, 'This forest is under my protection. You won't start any battles here!' And he laughed. He said, 'You're lucky I'm saving my energy for the main event, little satyr. I'll just grant you a short nap. Pleasant dreams.' And that's the last thing I remember."

Nico exhaled. "Grover, you met Morpheus, the God of Dreams. You're lucky you ever woke up."

"Two months," Grover moaned. "He put me to sleep for two months!"

I tried to wrap my mind around what this meant. Now it made sense why we hadn't been able to contact Grover all this time.

"Why didn't the nymphs try to wake you?" I asked.

Grover shrugged. "Most nymphs aren't good with time. Two months for a tree—that's nothing. They probably didn't think anything was wrong."

"We've got to figure out what Morpheus was doing in the park," I said. "I don't like this 'main event' thing he mentioned."

"He's working for Kronos," Nico said. "We know that already. A lot of the minor gods are. This just proves there's going to be an invasion. Percy, we have to get on with our plan."

"Wait," Grover said. "What plan?"

We told him, and Grover started tugging at his leg fur.

"You're not serious," he said. "Not the Underworld again."

"I'm not asking you to come, man," I promised. "I know you just woke up. But we need some music to open the door. Can you do it?"

Grover took out his reed pipes. "I guess I could try. I know a few Nirvana tunes that can split rocks. But, Percy, are you sure you want to do this?"

"Please, man," I said. "It would mean a lot. For old times' sake?"

He whimpered. "As I recall, in the old times we almost died a lot. But okay, here goes nothing."

He put his pipes to his lips and played a shrill, lively tune. The boulders trembled. A few more stanzas, and they cracked open, revealing a triangular crevice.

I peered inside. Steps led down into the darkness. The air smelled of mildew and death. It brought back bad memories of my trip through the Labyrinth last year, but this tunnel felt even more dangerous. It led straight to the land of Hades, and that was almost always a one-way trip.

I turned to Grover. "Thanks . . . I think."

"Perrrrcy, is Kronos really going to invade?"

"I wish I could tell you better, but yeah. He will."

I thought Grover might chew up his reed pipes in anxiety, but he straightened up and brushed off his T-shirt. I couldn't help thinking how different he looked from fat old Leneus. "I've got to rally the nature spirits, then. Maybe we can help. I'll see if we can find this Morpheus.'"

"Better tell Juniper you're okay, too."

His eyes widened. "Juniper! Oh, she's going to kill me!"

He started to run off, then scrambled back and gave me another hug. "Be careful down there! Come back alive!"

Once he was gone, Nico and I roused Mrs. O'Leary from her nap.

When she smelled the tunnel, she got excited and led the way down the steps. It was a pretty tight fit. I hoped she wouldn't get stuck. I couldn't imagine how much Drano we'd need to un-stick a hellhound wedged halfway down a tunnel to the Underworld.

"Ready?" Nico asked me. "It'll be fine. Don't worry."

He sounded like he was trying to convince himself.

I glanced up at the stars, wondering if I would ever see them again. Then we plunged into darkness.

The stairs went on forever—narrow, steep, and slippery. It was completely dark except for the light of my sword. I tried to go slow, but Mrs. O'Leary had other ideas. She bounded ahead, barking happily. The sound echoed through the tunnel like cannon shots, and I figured we would not be catching anybody by surprise once we reached the bottom.

Nico lagged behind, which I thought was strange.

"You okay?" I asked him.

"Fine." What was that expression on his face . . . doubt? "Just keep moving," he said.

I didn't have much choice. I followed Mrs. O'Leary into the depths. After another hour, I started to hear the roar of a river.

We emerged at the base of a cliff, on a plain of black volcanic sand. To our right, the River Styx gushed from the rocks and roared off in a cascade of rapids. To our left, far away in the gloom, fires burned on the ramparts of Erebos, the great black walls of Hades's kingdom.

I shuddered. I'd first been here when I was twelve, and only Annabeth and Grover's company had given me the courage to keep going. Nico wasn't going to be quite as helpful with the "courage" thing. He looked pale and worried himself.

Only Mrs. O'Leary acted happy. She ran along the beach, picked up a random human leg bone, and romped back toward me. She dropped the bone at my feet and waited for me to throw it.

"Um, maybe later, girl." I stared at the dark waters, trying to get up my nerve. "So, Nico . . . how do we do this?"

"We have to go inside the gates first," he said.

"But the river's right here."

"I have to get something," he said. "It's the only way."

He marched off without waiting.

I frowned. Nico hadn't mentioned anything about going inside the gates. But now that we were here, I didn't know what else to do. Reluctantly, I followed him down the beach toward the big black gates.

Lines of the dead stood outside waiting to get in. It must've been a heavy day for funerals, because even the EZ-DEATH line was backed up.

"Woof!" Mrs. O'Leary said. Before I could stop her she bounded toward the security checkpoint. Cerberus, the guard dog of Hades, appeared out of the gloom—a three-headed rottweiler so big he made Mrs. O'Leary look like a toy poodle. Cerberus was half transparent, so he's really hard to see until he's close enough to kill you, but he acted like he didn't care about us. He was too busy saying hello to Mrs. O'Leary.

"Mrs. O'Leary, no!" I shouted at her. "Don't sniff . . . Oh, man."

Nico smiled. Then he looked at me and his expression turned all serious again, like he'd remembered something unpleasant. "Come on. They won't give us any trouble in the line. You're with me."