LIA SHOT AHEAD ON HER PET SWORDFISH, SPIKE, trailing bubbles after them. The swordfish did a 360-degree spin, showing off. Max gunned the aquabike’s engines with a turn of his wrist and roared after her.

“Keep up, Riv!” he called back.

His robodog was nosing through a school of fish. He wagged his metal tail at Max’s voice. “Crab, Max! Crab!” he barked.

“Not now,” Max said.

Rivet lifted his head, and his leg propellers whirred as he zoomed toward them. Rivet wasn’t the very latest technology when it came to dogbots, but Max’s modifications had made him one of the fastest.

Lia dipped Spike’s nose, and the swordfish cut through the water toward the seabed, darting around barnacle-covered boulders.

Anything you can do … thought Max, leaning with his body to follow her. He’d always dreamed of owning an aquabike, and this was a Lightning X4. One of the best! he thought.

Spike thumped his tail into the seabed, throwing up a cloud of sand. As Max pulled back on the bike’s handlebars to climb above it, he gently squeezed the brakes and looked over his shoulder for one last glimpse of Sumara.

Yep, I’m definitely not dreaming, he thought. That place is real. The great coral columns of the Merryn city rose through the water, shaped into towers and buildings. Stone walkways stretched between them, and colorful flags of seaweed fluttered in the currents. Glowing plants lit up the underwater streets like torches. Max shook his head in wonder. He couldn’t think of anything less like the metal skyscrapers of Aquora. Until yesterday, that island city had been his home.

So much had changed in a single day.

Max had spent his whole life above the water. His dad hated the sea — feared it, even — ever since his wife, Max’s mom, had vanished with her brother while exploring. Dad told him that before Max’s mom set out, people had made fun of her. They said she was crazy for searching out the Merryn — a legendary underwater people. But she’d been right all along. Max had found that out for himself.

Still, Max would rather she had been wrong, if it meant he could have her back.

His mom’s sub, the Leaping Dolphin, had never been recovered after it had gone missing. He’d found a scrap of it in the Merryn city of Sumara, but he still had no idea what had happened to her or her brother.

I won’t lose my dad as well, he thought.

He felt a squirm in his stomach. His father, Callum North, Chief Defense Engineer of Aquora, had been kidnapped by an evil scientist known to the Merryn people only as “the Professor.” He could control the great creatures of the sea with advanced technology. Max didn’t know why the Professor had snatched his dad, but he wasn’t going to stop searching the oceans until he’d found him again.

The Professor was no friend to the Merryn, either. He had stolen their most treasured possession, the Skull of Thallos, which gave them the power to control the sea. So far, Max and Lia had recovered one part of the skull, but there were three other sections to rescue. Without the skull, the Merryn’s aqua powers had been weakened. They couldn’t do battle with the Professor’s Robobeasts themselves, so Max had agreed to help them.

Max set his jaw as he stared back at the beautiful city. They need me, he thought. And I won’t return until the Skull of Thallos is complete.

Max lifted his hand to the scars on his neck. The new gills still felt strange, but breathing through them was as natural as … well, breathing. His skin, which should have wrinkled like he’d spent too long in the bath, was smooth as silk. All thanks to receiving the mysterious Merryn touch!

Rivet arrived at his side, red eyes flashing. “Race, Max!”

“Oh, you think you can beat me, do you?” he replied.

Rivet barked and his tail wagged faster, then he streaked off toward a clump of tall, swaying weeds. Max tipped the handlebars and shot after him. A host of crabs scuttled out of his way as he wove between the tendrils of seaweed. Ahead of him, a squid pulsed out of sight behind some sharp-looking rocks, and turned an angry orange color. Max was gaining on Rivet, and twisted the throttle to full speed.

“Almost got you …” he muttered.

A shape swept in front of him, causing him to brake hard. Max somersaulted over the handlebars. Would the jagged rocks tear the flesh from his back? Thankfully, he bounced over them and onto soft sand, though he was dizzy and confused. He turned and saw Lia sitting on top of Spike, her hands on her hips.

“What in all the seven seas do you think you’re doing?” she snapped.

Even underwater, Max felt a blush rise to his cheeks. “I was just —”

“Terrifying innocent creatures?” Lia said.

“Sorry,” said Max sheepishly. “It’s all so new to me.”

Lia’s face softened. “This isn’t a Breather playground,” she said. “We need some ground rules.”

“Don’t you mean water rules?” Max said.

Lia didn’t smile at the joke. “Number One: Respect the ocean. Don’t cause damage, and don’t harm the creatures that live here. Number Two: The ocean might be beautiful, but it holds many dangers — so be careful around every plant and creature, however innocent they look. And Number Three: Never, ever go into the undertow.”

“What’s the undertow?” asked Max. He’d just seen Rivet digging in the seabed with his nose. No bones in there, boy, he thought.

“You Breathers don’t know much, do you?” said Lia. “An undertow is an extremely fast current flowing near the seabed. It’s almost invisible, but if you get caught in one, it can be deadly. Are you listening?”

Max’s attention snapped back. “What’s that? Oh, yes … One: Respect. Two: Dangers. Three … er … What was three?”

“Undertow,” Lia said.

“Got it,” Max muttered. Rivet was pulling a sparkling pearly shell out of the sand.

“Now, I’m hungry,” said Lia, taking off her netted knapsack and feeling inside. She pulled out a disk-shaped green object, broke it in two, and offered half to Max.

“What is it?” he said.

“Seaweed cake,” she replied, taking a bite. “Delicious!”

Max brought the cake to his lips and took a nibble. It tasted like salty cardboard. “Mmm,” he said, but his voice gave him away.

Lia laughed. “You’ll get used to it.” She fed a piece of the cake to Spike. “Which way are we supposed to be going?”

Max called to Rivet. “Here, boy!”

His dogbot dropped the shell and swam over. Max opened the storage compartment in the dog’s side and took out the long white jawbone of the Skull of Thallos. The surface glowed, almost translucent.

Max let go of the jawbone, allowing it to float in the water. It slowly rotated, as if guided by invisible currents, until it pointed off to the left. “It worked!” he said. “We’ll find the next piece, no problem.”

He reached out to take the jawbone when — whoosh! — a large silver fish swooped in from nowhere and snatched the white piece. It darted away before Max even had time to gasp.

The jawbone of the Skull of Thallos had been stolen!