Annabeth didn't look convinced. "Just be careful. I don't want anything to happen to you. I mean, because we need you for the battle."

I grinned. "Back in a flash."

I clambered down the shoreline and waded into the water.

Just for you non-sea-god types out there, don't go swimming m New York Harbor. It may not be as filthy as it was in my mom's day, but that water will still probably make you grow a third eye or have mutant children when you grow up.

I dove into the murk and sank to the bottom. I tried to find the spot where the two rivers' currents seemed equal—where they met to form the bay. I figured that was the best place to get their attention.

"HEY!" I shouted in my best underwater voice. The sound echoed in the darkness. "I heard you guys are so polluted you're embarrassed to show your faces. Is that true?"

A cold current rippled through the bay, churning up plumes of garbage and silt.

"I heard the East River is more toxic," I continued, "but the Hudson smells worse. Or is it the other way around?"

The water shimmered. Something powerful and angry was watching me now. I could sense its presence . . . or maybe two presences.

I was afraid I'd miscalculated with the insults. What if they just blasted me without showing themselves? But these were New York river gods. I figured their instinct would be to get in my face.

Sure enough, two giant forms appeared in front of me. At first they were just dark brown columns of silt, denser than the water around them. Then they grew legs, arms, and scowling faces.

The creature on the left looked disturbingly like a telkhine. His face was wolfish. His body was vaguely like a seal's—sleek black with flipper hands and feet. His eyes glowed radiation green.

The dude on the right was more humanoid. He was dressed in rags and seaweed, with a chain-mail coat made of bottle caps and old plastic six-pack holders. His face was blotchy with algae, and his beard was overgrown. His deep blue eyes burned with anger.

The seal, who had to be the god of the East River, said, "Are you trying to get yourself killed, kid? Or are you just extra stupid?"

The bearded spirit of the Hudson scoffed. "You're the expert on stupid, East."

"Watch it, Hudson," East growled. "Stay on your side of the island and mind your business."

"Or what? You'll throw another garbage barge at me?"

They floated toward each other, ready to fight.

"Hold it!" I yelled. "We've got a bigger problem."

"The kid's right," East snarled. "Let's both kill him, then we'll fight each other."

"Sounds good," Hudson said.

Before I could protest, a thousand scraps of garbage surged off the bottom and flew straight at me from both directions: broken glass, rocks, cans, tires.

I was expecting it, though. The water in front of me thickened into a shield. The debris bounced off harmlessly. Only one piece got through—a big chunk of glass that hit my chest and probably should've killed me, but it shattered against my skin.

The two river gods stared at me.

"Son of Poseidon?" East asked.

I nodded.

"Took a dip in the Styx?" Hudson asked.

"Yep."

They both made disgusted sounds.

"Well, that's perfect," East said. "Now how do we kill him?"

"We could electrocute him," Hudson mused. "If I could just find some jumper cables—"

"Listen to me!" I said. "Kronos's army is invading Manhattan.'"

"Don't you think we know that?" East asked. "I can feel his boats right now. They're almost across."

"Yep," Hudson agreed. "I got some filthy monsters crossing my waters too."

"So stop them," I said. "Drown them. Sink their boats."

"Why should we?" Hudson grumbled. "So they invade Olympus. What do we care?"

"Because I can pay you." I took out the sand dollar my father had given me for my birthday.

The river gods' eyes widened.

"It's mine!" East said. "Give it here, kid, and I promise none of Kronos's scum are getting across the East River."

"Forget that," Hudson said. "That sand dollar's mine, unless you want me to let all those ships cross the Hudson."

"We'll compromise." I broke the sand dollar in half. A ripple of clean fresh water spread out from the break, as if all the pollution in the bay were being dissolved.

"You each get half," I said. "In exchange, you keep all of Kronos's forces away from Manhattan."

"Oh, man," Hudson whimpered, reaching out for the sand dollar. "It's been so long since I was clean."

"The power of Poseidon," East River murmured. "He's a jerk, but he sure knows how to sweep pollution away."

They looked at each other, then spoke as one: "It's a deal."

I gave them each a sand-dollar half, which they held reverently.

"Um, the invaders?" I prompted.

East flicked his hand. "They just got sunk."

Hudson snapped his fingers. "Bunch of hellhounds just took a dive."

"Thank you," I said. "Stay clean."

As I rose toward the surface, East called out, "Hey, kid, any time you got a sand dollar to spend, come on back. Assuming you live."

"Curse of Achilles," Hudson snorted. "They always think that'll save them, don't they?"

"If only he knew," East agreed. They both laughed, dissolving into the water.

Back on the shore, Annabeth was talking on her cell phone, but she hung up as soon as she saw me. She looked pretty shaken.

"It worked," I told her. "The rivers are safe."

"Good," she said. "Because we've got other problems. Michael Yew just called. Another army is marching over the Williamsburg Bridge. The Apollo cabin needs help. And Percy, the monster leading the enemy . . . it's the Minotaur."

Fortunately, Blackjack was on duty.

I did my best taxicab whistle, and within a few minutes two dark shapes circled out of the sky. They looked like hawks at first, but as they descended I could make out the long galloping legs of pegasi.

Yo, boss. Blackjack landed at a trot, his friend Porkpie right behind him. Man, I thought those wind gods were gonna knock us to Pennsylvania until we said we were with you!

"Thanks for coming," I told him. "Hey, why do pegasi gallop as they fly, anyway?"

Blackjack whinnied. Why do humans swing their arms as they walk? I dunno, boss. It just feels right. Where to?

"We need to get to the Williamsburg Bridge," I said.

Blackjack lowered his neck. You're darn right, boss. We flew over it on the way here, and it don't look good. Hop on!

On the way to the bridge, a knot formed in the pit of my stomach. The Minotaur was one of the first monsters I'd ever defeated. Four years ago he'd nearly killed my mother on Half-Blood Hill. I still had nightmares about that.

I'd been hoping he would stay dead for a few centuries, but I should've known my luck wouldn't hold.

We saw the battle before we were close enough to make out individual fighters. It was well after midnight now, but the bridge blazed with light. Cars were burning. Arcs of fire streamed in both directions as flaming arrows and spears sailed through the air.

We came in for a low pass, and I saw the Apollo campers retreating. They would hide behind cars and snipe at the approaching army, setting off explosive arrows and dropping caltrops in the road, building fiery barricades wherever they could, dragging sleeping drivers out of their cars to get them out of harm's way. But the enemy kept advancing. An entire phalanx of dracaenae marched in the lead, their shields locked together, spear tips bristling over the top. An occasional arrow would connect with their snaky trunks, or a neck, or a chink in their armor, and the unlucky snake woman would disintegrate, but most of the Apollo arrows glanced harmlessly off their shield wall. About a hundred more monsters marched behind them.