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.

Beyond this, the sea floor stretched into gloom. I could see battles raging—flashes of energy, explosions, the glint of armies clashing. A regular human would've found it too dark to see. Heck, a regular human would've been crushed by the pressure and frozen by the cold. Even my heat-sensitive eyes couldn't make out exactly what was going on.

At the edge of the palace complex, a temple with a red coral roof exploded, sending fire and debris streaming in slow motion across the farthest gardens. Out of the darkness above, an enormous form appeared—a squid larger than any skyscraper. It was surrounded by a glittering cloud of dust—at least I thought it was dust, until I realized it was a swarm of mermen trying to attack the monster. The squid descended on the palace and swatted its tentacles, smashing a whole column of warriors. Then a brilliant arc of blue light shot from the rooftop of one of the tallest buildings. The light hit the giant squid, and the monster dissolved like food coloring in water.

"Daddy," Tyson said, pointing to where the light had come from.

"He did that?" I suddenly felt more hopeful. My dad had unbelievable powers. He was the god of the sea. He could deal with this attack, right? Maybe he'd let me help.

"Have you been in the fight?" I asked Tyson in awe. "Like bashing heads with your awesome Cyclops strength and stuff?"

Tyson pouted, and immediately I knew I'd asked a bad question, "I have been . . . fixing weapons," he mumbled. "Come. Let's go find Daddy."

I know this might sound weird to people with, like, regular parents, but I'd only seen my dad four or five times in my life, and never for more than a few minutes. The Greek gods don't exactly show up for their kids' basketball games. Still, I thought I would recognize Poseidon on sight.

I was wrong.

The roof of the temple was a big open deck that had been set up as a command center. A mosaic on the floor showed an exact map of the palace grounds and the surrounding ocean, but the mosaic moved. Colored stone tiles representing different armies and sea monsters shifted around as the forces changed position. Buildings that collapsed in real life also collapsed in the picture.

Standing around the mosaic, grimly studying the battle, was a strange assortment of warriors, but none of them looked like my dad. I was searching for a big guy with a good tan and a black beard, wearing Bermuda shorts and a Hawaiian shirt.

There was nobody like that. One guy was a merman with two fish tails instead of one. His skin was green, his armor studded with pearls. His black hair was tied in a ponytail, and he looked young—though it's hard to tell with non-humans. They could be a thousand years old or three. Standing next to him was an old man with a bushy white beard and gray hair. His battle armor seemed to weigh him down. He had green eyes and smile wrinkles around his eyes, but he wasn't smiling now. He was studying the map and leaning on a large metal staff. To his right stood a beautiful woman in green armor with flowing black hair and strange little horns like crab claws. And there was a dolphin—just a regular dolphin, but it was staring at the map intently.

"Delphin," the old man said. "Send Palaemon and his legion of sharks to the western front. We have to neutralize those leviathans."

The dolphin spoke in a chattering voice, but I could understand it in my mind: Yes, lord! It sped away.

I looked in dismay at Tyson, then back at the old man.

It didn't seem possible, but . . . "Dad?" I asked.

The old man looked up. I recognized the twinkle in his eyes, but his face . . . he looked like he'd aged forty years.

"Hello, Percy."

"What—what happened to you?"

Tyson nudged me. He was shaking his head so hard I was afraid it would fall off, but Poseidon didn't look offended.

"It's all right, Tyson," he said. "Percy, excuse my appearance. The war has been hard on me."

"But you're immortal," I said quietly. "You can look . . . any way you want."

"I reflect the state of my realm," he said. "And right now that state is quite grim. Percy, I should introduce you—I'm afraid you just missed my lieutenant Delphin, God of the Dolphins. This is my, er, wife, Amphitrite. My dear—"

The lady in green armor stared at me coldly, then crossed her arms and said, "Excuse me, my lord. I am needed in the battle."

She swam away.

I felt pretty awkward, but I guess I couldn't blame her. I'd never thought about it much, but my dad had an immortal wife. All his romances with mortals, including with my mom . . . well, Amphitrite probably didn't like that much.

Poseidon cleared his throat. "Yes, well . . . and this is my son Triton. Er, my other son."

"Your son and heir," the green dude corrected. His double fish tails swished back and forth. He smiled at me, but there was no friendliness in his eyes. "Hello, Perseus Jackson. Come to help at last?"

He acted like I was late or lazy. If you can blush underwater, I probably did.

"Tell me what to do," I said.

Triton smiled like that was a cute suggestion—like I was a slightly amusing dog that had barked for him or something. He turned to Poseidon. "I will see to the front line, Father. Don't worry. I will not fail."

He nodded politely to Tyson. How come I didn't get that much respect? Then he shot off into the water.

Poseidon sighed. He raised his staff, and it changed into his regular weapon—a huge three-pointed trident. The tip glowed with blue light, and the water around it boiled with energy.

"I'm sorry about that," he told me.

A huge sea serpent appeared from above us and spiraled down toward the roof. It was bright orange with a fanged mouth big enough to swallow a gymnasium.

Hardly looking up, Poseidon pointed his trident at the beast and zapped it with blue energy. Ka-boom! The monster burst into a million goldfish, which all swam off in terror.

"My family is anxious," Poseidon continued as if nothing had happened. "The battle against Oceanus is going poorly."

He pointed to the edge of the mosaic. With the butt of his trident he tapped the image of a merman larger than the rest, with the horns of a bull. He appeared to be riding a chariot pulled by crawfish, and instead of a sword he wielded a live serpent.

"Oceanus," I said, trying to remember. "The Titan of the sea?"

Poseidon nodded. "He was neutral in the first war of gods and Titans. But Kronos has convinced him to fight. This is . . . well, it's not a good sign. Oceanus would not commit unless he was sure he could pick the winning side."

"He looks stupid," I said, trying to sound upbeat. "I mean, who fights with a snake?"

"Daddy will tie it in knots," Tyson said firmly.

Poseidon smiled, but he looked weary. "I appreciate your faith. We have been at war almost a year now. My powers are taxed. And still he finds new forces to throw at me—sea monsters so ancient I had forgotten about them."