I heard rumbling deep in the ship. Monsters yelled at me from above. A spear sailed past my ear. An arrow pierced my thigh, but I barely had time to register the pain. I plunged into the sea and willed the currents to take me far, far away—a hundred yards, two hundred yards.

Even from that distance, the explosion shook the world. Heat seared the back of my head. The Princess Andromeda blew up from both sides, a massive fireball of green flame roiling into the dark sky, consuming everything.

Beckendorf, I thought.

Then I blacked out and sank like an anchor toward the bottom of the sea.

Demigod dreams suck.

The thing is, they're never just dreams. They've got to be visions, omens, and all that other mystical stuff that makes my brain hurt.

I dreamed I was in a dark palace at the top of a mountain. Unfortunately, I recognized it: the palace of the Titans on top of Mount Othrys, otherwise known as Mount Tamalpais, in California. The main pavilion was open to the night, ringed with black Greek columns and statues of the Titans. Torchlight glowed against the black marble floor. In the center of the room, an armored giant struggled under the weight of a swirling funnel cloud—Atlas, holding up the sky.

Two other giant men stood nearby over a bronze brazier, studying images in the flames.

"Quite an explosion," one said. He wore black armor studded with silver dots like a starry night. His face was covered in a war helm with a ram's horn curling on either side.

"It doesn't matter," the other said. This Titan was dressed in gold robes, with golden eyes like Kronos. His entire body glowed. He reminded me of Apollo, God of the Sun, except the Titan's light was harsher, and his expression crueler. "The gods have answered the challenge. Soon they will be destroyed."

The images in the fire were hard to make out: storms, buildings crumbling, mortals screaming in terror.

"I will go east to marshal our forces," the golden Titan said. "Krios, you shall remain and guard Mount Othrys."

The ram horn dude grunted. "I always get the stupid jobs. Lord of the South. Lord of Constellations. Now I get to babysit Atlas while you have all the fun."

Under the whirlwind of clouds, Atlas bellowed in agony, "Let me out, curse you! I am your greatest warrior. Take my burden so I may fight!"

"Quiet!" the golden Titan roared. "You had your chance, Atlas. You failed. Kronos likes you just where you are. As for you, Krios, do your duty."

"And if you need more warriors?" Krios asked. "Our treacherous nephew in the tuxedo will not do you much good in a fight."

The golden Titan laughed. "Don't worry about him. Besides, the gods can barely handle our first little challenge. They have no idea how many others we have in store. Mark my words, in a few days' time, Olympus will be in ruins, and we will meet here again to celebrate the dawn of the Sixth Age!"

The golden Titan erupted into flames and disappeared.

"Oh, sure," Krios grumbled. "He gets to erupt into flames. I get to wear these stupid ram's horns."

The scene shifted. Now I was outside the pavilion, hiding in the shadows of a Greek column. A boy stood next to me, eavesdropping on the Titans. He had dark silky hair, pale skin, and dark clothes—my friend Nico di Angelo, the son of Hades.

He looked straight at me, his expression grim. "You see, Percy?" he whispered. "You're running out of time. Do you really think you can beat them without my plan?"

His words washed over me as cold as the ocean floor, and my dreams went black.

"Percy?" a deep voice said.

My head felt like it had been microwaved in aluminum foil. I opened my eyes and saw a large shadowy figure looming over me.

"Beckendorf?" I asked hopefully.

"No, brother."

My eyes refocused. I was looking at a Cyclops—a misshapen face, ratty brown hair, one big brown eye full of concern. "Tyson?"

My brother broke into a toothy grin. "Yay! Your brain works!"

I wasn't so sure. My body felt weightless and cold. My voice sounded wrong. I could hear Tyson, but it was more like I was hearing vibrations inside my skull, not the regular sounds.

I sat up, and a gossamer sheet floated away. I was on a bed made of silky woven kelp, in a room paneled with abalone shell. Glowing pearls the size of basketballs floated around the ceiling, providing light. I was under water.

Now, being the son of Poseidon and all, I was okay with this. I can breathe underwater just fine, and my clothes don't even get wet unless I want them to. But it was still a bit of a shock when a hammerhead shark drifted through the bedroom window, regarded me, and then swam calmly out the opposite side of the room.

"Where—"

"Daddy's palace," Tyson said.

Under different circumstances, I would've been excited. I'd never visited Poseidon's realm, and I'd been dreaming about it for years. But my head hurt. My shirt was still speckled with burn marks from the explosion. My arm and leg wounds had healed—just being in the ocean can do that for me, given enough time—but I still felt like I'd been trampled by a Laistrygonian soccer team in cleats.

"How long—"

"We found you last night," Tyson said, "sinking through the water."

"The Princess Andromeda?"

"Went ka-boom," Tyson confirmed.

"Beckendorf was on board. Did you find . . ."

Tyson's face darkened. "No sign of him. I am sorry, brother."

I stared out the window into deep blue water. Beckendorf was supposed to go to college in the fall. He had a girlfriend, lots of friends, his whole life ahead of him. He couldn't be gone. Maybe he'd made it off the ship like I had. Maybe he'd jumped over the side . . . and what? He couldn't have survived a hundred-foot fall into the water like I could. He couldn't have put enough distance between himself and the explosion.

I knew in my gut he was dead. He'd sacrificed himself to take out the Princess Andromeda, and I had abandoned him.

I thought about my dream: the Titans discussing the explosion as if it didn't matter, Nico di Angelo warning me that I would never beat Kronos without following his plan—a dangerous idea I'd been avoiding for more than a year.

A distant blast shook the room. Green light blazed outside, turning the whole sea as bright as noon.

"What was that?" I asked.

Tyson looked worried. "Daddy will explain. Come, he is blowing up monsters."

The palace might have been the most amazing place I'd ever seen if it hadn't been in the process of getting destroyed. We swam to the end of a long hallway and shot upward on a geyser. As we rose over the rooftops I caught my breath—well, if you can catch your breath underwater.

The palace was as big as the city on Mount Olympus, with wide courtyards, gardens, and columned pavilions. The gardens were sculpted with coral colonies and glowing sea plants. Twenty or thirty buildings were made of abalone, white but gleaming with rainbow colors. Fish and octopi darted in and out of the windows. The paths were lined with glowing pearls like Christmas lights.

The main courtyard was filled with warriors—mermen with fish tails from the waist down and human bodies from the waist up, except their skin was blue, which I'd never known before. Some were tending the wounded. Some were sharpening spears and swords. One passed us, swimming in a hurry. His eyes were bright green, like that stuff they put in glo-sticks, and his teeth were shark teeth. They don't show you stuff like that in The Little Mermaid.

Outside the main courtyard stood large fortifications—towers, walls, and antisiege weapons—but most of these had been smashed to ruins. Others were blazing with a strange green light that I knew well—Greek fire, which can burn even underwater.