He must've been surprised when I tightened my grip, the ocean filling me with extra strength. But Nereus had a few tricks left, too. He changed shape until I was holding a sleek black seal.

I've heard people make jokes about trying to hold a greased pig, but I'm telling you, holding on to a seal in the water is harder. Nereus plunged straight down, wriggling and thrashing and spiraling through the dark water. If I hadn't been Poseidon's son, there's no way I could've stayed with him.

Nereus spun and expanded, turning into a killer whale, but I grabbed his dorsal fin as he burst out of the water.

A whole bunch of tourists went, "Whoa!"

I managed to wave at the crowd. Yeah, we do this every day here in San Francisco.

Nereus plunged into the water and turned into a slimy eel. I started to tie him into a knot until he realized what was going on and changed back to human form. "Why won't you drown?" he wailed, pummelmg me with his fists.

"I'm Poseidon's son," I said.

"Curse that upstart! I was here first!"

Finally he collapsed on the edge of the boat dock. Above us was one of those tourist piers lined with shops, like a mall on water. Nereus was heaving and gasping. I was feeling great. I could've gone on all day, but I didn't tell him that. I wanted him to feel like he'd put up a good fight.

My friends ran down the steps from the pier.

"You got him!" Zoe said.

"You don't have to sound so amazed," I said.

Nereus moaned. "Oh, wonderful. An audience for my humiliation! The normal deal, I suppose? You'll let me go if I answer your question?"

"I've got more than one question," I said.

"Only one question per capture! That's the rule."

I looked at my friends.

This wasn't good. I needed to find Artemis, and I needed to figure out what the doomsday creature was. I also needed to know if Annabeth was still alive, and how to rescue her. How could I ask that all in one question?

A voice inside me was screaming Ask about Annabeth! That's what I cared about most.

But then I imagined what Annabeth might say. She would never forgive me if I saved her and didn't save Olympus. Zoe would want me to ask about Artemis, but Chiron had told us the monster was even more important.

I sighed. "All right, Nereus. Tell me where to find this terrible monster that could bring an end to the gods. The one Artemis was hunting."

The Old Man of the Sea smiled, showing off his mossy green teeth.

"Oh, that's too easy," he said evilly. "He's right there."

Nereus pointed to the water at my feet.

"Where?" I said.

"The deal is complete!" Nereus gloated. With a pop, he turned into a goldfish and did a backflip into the sea.

"You tricked me!" I yelled.

"Wait." Thalia's eyes widened. "What is that?"

"MOOOOOOOO!"

I looked down, and there was my friend the cow serpent, swimming next to the dock. She nudged my shoe and gave me the sad brown eyes.

"Ah, Bessie," I said. "Not now."

"Mooo!"

Grover gasped. "He says his name isn't Bessie."

"You can understand her… er, him?"

Grover nodded. "It's a very old form of animal speech. But he says his name is the Ophiotaurus."

"The Ophi-what?"

"It means serpent bull in Greek," Thalia said. "But what's it doing here?"

"Moooooooo!"

"He says Percy is his protector," Grover announced.

"And he's running from the bad people. He says they are close."

I was wondering how you got all that out of a single moooooo.

"Wait," Zoe said, looking at me. "You know this cow?"

I was feeling impatient, but I told them the story.

Thalia shook her head in disbelief. "And you just forgot to mention this before?"

"Well… yeah." It seemed silly, now that she said it, but things had been happening so fast. Bessie, the Ophiotaurus, seemed like a minor detail.

"I am a fool," Zoe said suddenly. "I know this story!"

"What story?"

"From the War of the Titans," she said. "My… my father told me this tale, thousands of years ago. This is the beast we are looking for."

"Bessie?" I looked down at the bull serpent. "But… he's too cute. He couldn't destroy the world."

"That is how we were wrong," Zoe said. "We've been anticipating a huge dangerous monster, but the Ophiotaurus does not bring down the gods that way. He must be sacrificed."

"MMMM," Bessie lowed.

"I don't think he likes the S-word," Grover said.

I patted Bessie on the head, trying to calm him down. He let me scratch his ear, but he was trembling.

"How could anyone hurt him?" I said. "He's harmless."

Zoe nodded. "But there is power in killing innocence.

Terrible power. The Fates ordained a prophecy eons ago, when this creature was born. They said that whoever killed the Ophiotaurus and sacrificed its entrails to fire would have the power to destroy the gods."

"MMMMMM!"

"Um," Grover said. "Maybe we could avoid talking about entrails, too."

Thalia stared at the cow serpent with wonder. "The power to destroy the gods… how? I mean, what would happen?"

"No one knows," Zoe said. "The first time, during the Titan war, the Ophiotaurus was in fact slain by a giant ally of the Titans, but thy father, Zeus, sent an eagle to snatch the entrails away before they could be tossed into the fire. It was a close call. Now, after three thousand years, the Ophiotaurus is reborn."

Thalia sat down on the dock. She stretched out her hand. Bessie went right to her. Thalia placed her hand on his head. Bessie shivered.

Thalia's expression bothered me. She almost looked… hungry.

"We have to protect him," I told her. "If Luke gets hold of him—"

"Luke wouldn't hesitate," Thalia muttered. "The power to overthrow Olympus. That's… that's huge."

"Yes, it is, my dear," said a man's voice in a heavy French accent. "And it is a power you shall unleash."

The Ophiotaurus made a whimpering sound and submerged.

I looked up. We'd been so busy talking, we'd allowed ourselves to be ambushed.

Standing behind us, his two-color eyes gleaming wickedly, was Dr. Thorn, the manticore himself.

"This is just pairrr-fect," the manticore gloated.

He was wearing a ratty black trench coat over his Westover Hall uniform, which was torn and stained. His military haircut had grown out spiky and greasy. He hadn't shaved recently, so his face was covered in silver stubble. Basically he didn't look much better than the guys down at the soup kitchen.

"Long ago, the gods banished me to Persia," the manticore said. "I was forced to scrounge for food on the edges of the world, hiding in forests, devouring insignificant human farmers for my meals. I never got to fight any great heroes. I was not feared and admired in the old stories! But now that will change. The Titans shall honor me, and I shall feast on the flesh of half-bloods!"

On either side of him stood two armed security guys, some of the mortal mercenaries I'd seen in D.C. Two more stood on the next boat dock over, just in case we tried to escape that way. There were tourists all around—walking down the waterfront, shopping at the pier above us—but I knew that wouldn't stop the manticore from acting.

"Where… where are the skeletons?" I asked the manticore.

He sneered. "I do not need those foolish undead! The General thinks I am worthless? He will change his mind when I defeat you myself!"

I needed time to think. I had to save Bessie. I could dive into the sea, but how could I make a quick getaway with a five-hundred-pound cow serpent? And what about my friends?

"We beat you once before," I said.

"Ha! You could barely fight me with a goddess on your side. And, alas… that goddess is preoccupied at the moment. There will be no help for you now."

Zoe notched an arrow and aimed it straight at the manticore's head. The guards on either side of us raised their guns.

"Wait!" I said. "Zoe, don't!"

The manticore smiled. "The boy is right, Zoe Nightshade. Put away your bow. It would be a shame to kill you before you witnessed Thalia's great victory."

"What are you talking about?" Thalia growled. She had her shield and spear ready.

"Surely it is clear," the manticore said. "This is your moment. This is why Lord Kronos brought you back to life. You will sacrifice the Ophiotaurus. You will bring its entrails to the sacred fire on the mountain. You will gain unlimited power. And for your sixteenth birthday, you will overthrow Olympus."

No one spoke. It made terrible sense. Thalia was only two days away from turning sixteen. She was a child of the Big Three. And here was a choice, a terrible choice that could mean the end of the gods. It was just like the prophecy said. I wasn't sure if I felt relieved, horrified, or disappointed. I wasn't the prophecy kid after all. Doomsday was happening right now.

I waited for Thalia to tell the manticore off, but she hesitated. She looked completely stunned.

"You know it is the right choice," the manticore told her. "Your friend Luke recognized it. You shall be reunited with him. You shall rule this world together under the auspices of the Titans. Your father abandoned you, Thalia. He cares nothing for you. And now you shall gain power over him. Crush the Olympians underfoot, as they deserve. Call the beast! It will come to you. Use your spear."

"Thalia," I said, "snap out of it!"

She looked at me the same way she had the morning she woke up on Half-Blood Hill, dazed and uncertain. It was almost like she didn't know me. "I… I don't—"

"Your father helped you," I said. "He sent the metal angels. He turned you into a tree to preserve you."

Her hand tightened on the shaft of her spear.

I looked at Grover desperately. Thank the gods, he understood what I needed. He raised his pipes to his mouth and played a quick riff.

The manticore yelled, "Stop him!"

The guards had been targeting Zoe, and before they could figure out that the kid with the pipes was the bigger problem, the wooden planks at their feet sprouted new branches and tangled their legs. Zoe let loose two quick arrows that exploded at their feet in clouds of sulfurous yellow smoke. Fart arrows!

The guards started coughing. The manticore shot spines in our direction, but they ricocheted off my lion's coat.

"Grover," I said, "tell Bessie to dive deep and stay down!"

"Moooooo!" Grover translated. I could only hope that Bessie got the message.

"The cow…" Thalia muttered, still in a daze.

"Come on!" I pulled her along as we ran up the stairs to the shopping center on the pier. We dashed around the corner of the nearest store. I heard the manticore shouting at his minions, "Get them!" Tourists screamed as the guards shot blindly into the air.

We scrambled to the end of the pier. We hid behind a little kiosk filled with souvenir crystals—wind chimes and dream catchers and stuff like that, glittering in the sunlight. There was a water fountain next to us. Down below, a bunch of sea lions were sunning themselves on the rocks. The whole of San Francisco Bay spread out before us: the Golden Gate Bridge, Alcatraz Island, and the green hills and fog beyond that to the north. A picture-perfect moment, except for the fact that we were about to die and the world was going to end.