The monster grumbled, still not happy with his deal, but he left Grover to his weaving and rolled the boulder back into place.

Grover closed his eyes and took a shaky breath, trying to calm his nerves.

"Hurry, Percy," he muttered. "Please, please, please!"

I woke to a ship's whistle and a voice on the intercom— some guy with an Australian accent who sounded way too happy.

"Good morning, passengers! We'll be at sea all day today. Excellent weather for the poolside mambo party! Don't forget million-dollar bingo in the Kraken Lounge at one o'clock, and for our special guests, disemboweling practice on the Promenade!"

I sat up in bed. "What did he say?"

Tyson groaned, still half asleep. He was lying facedown on the couch, his feet so far over the edge they were in the bathroom. "The happy man said… bowling practice?"

I hoped he was right, but then there was an urgent knock on the suite's interior door. Annabeth stuck her head in—her blond hair in a rat's nest. "Disemboweling practice?"

Once we were all dressed, we ventured out into the ship and were surprised to see other people. A dozen senior citizens were heading to breakfast. A dad was taking his kids to the pool for a morning swim. Crew members in crisp white uniforms strolled the deck, tipping their hats to the passengers.

Nobody asked who we were. Nobody paid us much attention. But there was something wrong.

As the family of swimmers passed us, the dad told his kids: "We are on a cruise. We are having fun."

"Yes," his three kids said in unison, their expressions blank. "We are having a blast. We will swim in the pool."

They wandered off.

"Good morning," a crew member told us, his eyes glazed. "We are all enjoying ourselves aboard the Princess Andromeda. Have a nice day." He drifted away.

"Percy, this is weird," Annabeth whispered. "They're all in some kind of trance."

Then we passed a cafeteria and saw our first monster. It was a hellhound—a black mastiff with its front paws up on the buffet line and its muzzle buried in the scrambled eggs. It must've been young, because it was small compared to most—no bigger than a grizzly bear. Still, my blood turned cold. I'd almost gotten killed by one of those before.

The weird thing was: a middle-aged couple was standing in the buffet line right behind the devil dog, patiently waiting their turn for the eggs. They didn't seem to notice anything out of the ordinary.

"Not hungry anymore," Tyson murmured.

Before Annabeth or I could reply, a reptilian voice came from down the corridor, "Ssssix more joined yesssterday."

Annabeth gestured frantically toward the nearest hiding place—the women's room—and all three of us ducked inside. I was so freaked out it didn't even occur to me to be embarrassed.

Something—or more like two somethings—slithered past the bathroom door, making sounds like sandpaper against the carpet.

"Yesss," a second reptilian voice said. "He drawssss them. Ssssoon we will be sssstrong."

The things slithered into the cafeteria with a cold hissing that might have been snake laughter.

Annabeth looked at me. "We have to get out of here."

"You think I want to be in the girls' restroom?"

"I mean the ship, Percy! We have to get off the ship."

"Smells bad," Tyson agreed. "And dogs eat all the eggs. Annabeth is right. We must leave the restroom and ship."

I shuddered. If Annabeth and Tyson were actually agreeing about something, I figured I'd better listen.

Then I heard another voice outside—one that chilled me worse than any monster's.

"— only a matter of time. Don't push me, Agrius!"

It was Luke, beyond a doubt. I could never forget his voice.

"I'm not pushing you!" another guy growled. His voice was deeper and even angrier than Luke's. "I'm just saying, if this gamble doesn't pay off—"

"It'll pay off," Luke snapped. "They'll take the bait. Now, come, we've got to get to the admiralty suite and check on the casket."

Their voices receded down the corridor.

Tyson whimpered. "Leave now?"

Annabeth and I exchanged looks and came to a silent agreement.

"We can't," I told Tyson.

"We have to find out what Luke is up to," Annabeth agreed. "And if possible, we're going to beat him up, bind him in chains, and drag him to Mount Olympus."

Annabeth volunteered to go alone since she had the cap of invisibility, but I convinced her it was too dangerous. Either we all went together, or nobody went.

"Nobody!" Tyson voted. "Please?"

But in the end he came along, nervously chewing on his huge fingernails. We stopped at our cabin long enough to gather our stuff. We figured whatever happened, we would not be staying another night aboard the zombie cruise ship, even if they did have million-dollar bingo. I made sure Riptide was in my pocket and the vitamins and thermos from Hermes were at the top of my bag. I didn't want Tyson to carry everything, but he insisted, and Annabeth told me not to worry about it. Tyson could carry three full duffel bags over his shoulder as easily as I could carry a backpack.

We sneaked through the corridors, following the ship's YOU ARE HERE signs toward the admiralty suite. Annabeth scouted ahead invisibly. We hid whenever someone passed by, but most of the people we saw were just glassy-eyed zombie passengers.

As we came up the stairs to deck thirteen, where the admiralty suite was supposed to be, Annabeth hissed, "Hide!" and shoved us into a supply closet.

I heard a couple of guys coming down the hall.

"You see that Aethiopian drakon in the cargo hold?" one of them said.

The other laughed. "Yeah, it's awesome."

Annabeth was still invisible, but she squeezed my arm hard. I got a feeling I should know that second guy's voice.

"I hear they got two more coming," the familiar voice said. "They keep arriving at this rate, oh, man—no contest!"

The voices faded down the corridor.

"That was Chris Rodriguez!" Annabeth took off her cap and turned visible. "You remember—from Cabin Eleven."

I sort of recalled Chris from the summer before. He was one of those undetermined campers who got stuck in the Hermes cabin because his Olympian dad or mom never claimed him. Now that I thought about it, I realized I hadn't seen Chris at camp this summer. "What's another half-blood doing here?"

Annabeth shook her head, clearly troubled.

We kept going down the corridor. I didn't need maps anymore to know I was getting close to Luke. I sensed something cold and unpleasant—the presence of evil.

"Percy." Annabeth stopped suddenly. "Look."

She stood in front of a glass wall looking down into the multistory canyon that ran through the middle of the ship. At the bottom was the Promenade—a mall full of shops— but that's not what had caught Annabeth's attention.

A group of monsters had assembled in front of the candy store: a dozen Laistrygonian giants like the ones who'd attacked me with dodge balls, two hellhounds, and a few even stranger creatures—humanoid females with twin serpent tails instead of legs.

"Scythian Dracaenae," Annabeth whispered. "Dragon women."

The monsters made a semicircle around a young guy in Greek armor who was hacking on a straw dummy. A lump formed in my throat when I realized the dummy was wearing an orange Camp Half-Blood T-shirt. As we watched, the guy in armor stabbed the dummy through its belly and ripped upward. Straw flew everywhere. The monsters cheered and howled.

Annabeth stepped away from the window. Her face was ashen.

"Come on," I told her, trying to sound braver than I felt. "The sooner we find Luke the better."