"Fifty feet," Tyson said. "Monster Donut shop—just over the hill!"

"This is bad," Annabeth muttered.

We were crouching behind a tree, staring at the donut shop in the middle of the woods. It looked brand new, with brightly lit windows, a parking area, and a little road leading off into the forest, but there was nothing else around, and no cars parked in the lot. We could see one employee reading a magazine behind the cash register. That was it. On the store's marquis, in huge black letters that even I could read, it said:

MONSTER DONUT

A cartoon ogre was taking a bite out of the O in MONSTER. The place smelled good, like fresh-baked chocolate donuts.

"This shouldn't be here," Annabeth whispered. "It's wrong."

"What?" I asked. "It's a donut shop."

"Shhh!"

"Why are we whispering? Tyson went in and bought a dozen. Nothing happened to him."

"He's a monster."

"Aw, c'mon, Annabeth. Monster Donut doesn't mean monsters! It's a chain. We've got them in New York."

"A chain," she agreed. "And don't you think it's strange that one appeared immediately after you told Tyson to get donuts? Right here in the middle of the woods?"

I thought about it. It did seem a little weird, but, I mean, donut shops weren't real high on my list of sinister forces.

"It could be a nest," Annabeth explained.

Tyson whimpered. I doubt he understood what Annabeth was saying any better than I did, but her tone was making him nervous. He'd plowed through half a dozen donuts from his box and was getting powdered sugar all over his face.

"A nest for what?" I asked.

"Haven't you ever wondered how franchise stores pop up so fast?" she asked. "One day there's nothing and then the next day—boom, there's a new burger place or a coffee shop or whatever? First a single store, then two, then four— exact replicas spreading across the country?"

"Um, no. Never thought about it."

"Percy, some of the chains multiply so fast because all their locations are magically linked to the life force of a monster. Some children of Hermes figured out how to do it back in the 1950s. They breed—"

She froze.

"What?" I demanded. "They breed what?"

"No—sudden—moves," Annabeth said, like her life depended on it. "Very slowly, turn around."

Then I heard it: a scraping noise, like something large dragging its belly through the leaves.

I turned and saw a rhino-size thing moving through the shadows of the trees. It was hissing, its front half writhing in all different directions. I couldn't understand what I was seeing at first. Then I realized the thing had multiple necks—at least seven, each topped with a hissing reptilian head. Its skin was leathery, and under each neck it wore a plastic bib that read: I'm A MONSTER DONUT KID!

I took out my ballpoint pen, but Annabeth locked eyes with me—a silent warning. Not yet.

I understood. A lot of monsters have terrible eyesight. It was possible the Hydra might pass us by. But if I uncapped my sword now, the bronze glow would certainly get its attention.

We waited.

The Hydra was only a few feet away. It seemed to be sniffing the ground and the trees like it was hunting for something. Then I noticed that two of the heads were ripping apart a piece of yellow canvas—one of our duffel bags. The thing had already been to our campsite. It was following our scent.

My heart pounded. I'd seen a stuffed Hydra-head trophy at camp before, but that did nothing to prepare me for the real thing. Each head was diamond-shaped, like a rattlesnake's, but the mouths were lined with jagged rows of sharklike teeth.

Tyson was trembling. He stepped back and accidentally snapped a twig. Immediately, all seven heads turned toward us and hissed.

"Scatter!" Annabeth yelled. She dove to the right.

I rolled to the left. One of the Hydra heads spat an arc of green liquid that shot past my shoulder and splashed against an elm. The trunk smoked and began to disintegrate. The whole tree toppled straight toward Tyson, who still hadn't moved, petrified by the monster that was now right in front of him.

"Tyson!" I tackled him with all my might, knocking him aside just as the Hydra lunged and the tree crashed on top of two of its heads.

The Hydra stumbled backward, yanking its heads free then wailing in outrage at the fallen tree. All seven heads shot acid, and the elm melted into a steaming pool of muck.

"Move!" I told Tyson. I ran to one side and uncapped Riptide, hoping to draw the monster's attention.

It worked.

The sight of celestial bronze is hateful to most monsters. As soon as my glowing blade appeared, the Hydra whipped toward it with all its heads, hissing and baring its teeth.

The good news: Tyson was momentarily out of danger. The bad news: I was about to be melted into a puddle of goo.

One of the heads snapped at me experimentally. Without thinking, I swung my sword.

"No!" Annabeth yelled.

Too late. I sliced the Hydra's head clean off. It rolled away into the grass, leaving a flailing stump, which immediately stopped bleeding and began to swell like a balloon.

In a matter of seconds the wounded neck split into two necks, each of which grew a full-size head. Now I was looking at an eight-headed Hydra.

"Percy!" Annabeth scolded. "You just opened another Monster Donut shop somewhere!"

I dodged a spray of acid. "I'm about to die and you're worried about that? How do we kill it?"

"Fire!" Annabeth said. "We have to have fire!"

As soon as she said that, I remembered the story. The Hydra's heads would only stop multiplying if we burned the stumps before they regrew. That's what Heracles had done, anyway. But we had no fire.

I backed up toward river. The Hydra followed.

Annabeth moved in on my left and tried to distract one of the heads, parrying its teeth with her knife, but another head swung sideways like a club and knocked her into the muck.

"No hitting my friends!" Tyson charged in, putting himself between the Hydra and Annabeth. As Annabeth got to her feet, Tyson started smashing at the monster heads with his fists so fast it reminded me of the whack-a-mole game at the arcade. But even Tyson couldn't fend off the Hydra forever.

We kept inching backward, dodging acid splashes and deflecting snapping heads without cutting them off, but I knew we were only postponing our deaths. Eventually, we would make a mistake and the thing would kill us.

Then I heard a strange sound—a chug-chug-chug that at first I thought was my heartbeat. It was so powerful it made the riverbank shake.

"What's that noise?" Annabeth shouted, keeping her eyes on the Hydra.

"Steam engine," Tyson said.

"What?" I ducked as the Hydra spat acid over my head.

Then from the river behind us, a familiar female voice shouted: "There! Prepare the thirty-two-pounder!"

I didn't dare look away from the Hydra, but if that was who I thought it was behind us, I figured we now had enemies on two fronts.

A gravelly male voice said, "They're too close, m'lady!"

"Damn the heroes!" the girl said. "Full steam ahead!"

"Aye, m'lady."

"Fire at will, Captain!"

Annabeth understood what was happening a split second before I did. She yelled, "Hit the dirt!" and we dove for the ground as an earth-shattering BOOM echoed from the river. There was a flash of light, a column of smoke, and the Hydra exploded right in front of us, showering us with nasty green slime that vaporized as soon as it hit, the way monster guts tend to do.

"Gross!" screamed Annabeth.

"Steamship!" yelled Tyson.

I stood, coughing from the cloud of gunpowder smoke that was rolling across the banks.

Chugging toward us down the river was the strangest ship I'd ever seen. It rode low in the water like a submarine, its deck plated with iron. In the middle was a trapezoid-shaped casemate with slats on each side for cannons. A flag waved from the top—a wild boar and spear on a bloodred field. Lining the deck were zombies in gray uniforms— dead soldiers with shimmering faces that only partially covered their skulls, like the ghouls I'd seen in the Underworld guarding Hades's palace.