After a brief return to the surface for a breath of fresh air and a gulp of nectar, Frank turned into a sandy brown stingray and glided soundlessly over the seabed. If he was going to ambush Porky and Kate he'd better do it stealthily. He wondered if they'd already worked out what he'd done.

Triton had taken him to the top of a large building and showed him a mosaic, with shapes that shifted to show what was going on in the nearby ocean. He'd pointed out a certain area and told him how to get to it, explaining that this seemed to be where the disturbances were coming from. He wasn't looking forward to visiting the place he'd been told to. Pull yourself together, he thought. You're a praetor. You can handle anything. You can be anything.

Soon he came to a large, jagged cliff face. And before it was…

The mermaid graveyard was seriously creepy. It was clear why the pair had picked here for their hideout. It was conveniently close to the palace, and good at deterring unwanted visitors. Dark and eerie, with uneven graves covered in seaweed, moss, and barnacles, it was deserted apart from the odd fish or two skulking around.

Suddenly he saw something that didn't belong in a graveyard. It was a flash of light. If he listened carefully, he could hear loud disco music. How it worked under the ocean, Frank didn't know, but he supposed a god and goddess could do whatever they wanted. It transported him back to the aquarium in Georgia, where his ears had been assaulted by several different tunes blaring at once while he, Percy and Coach Hedge all made a run for it. It seemed to be coming from the cliff face, down at the bottom by a clump of seaweed hanging down. Upon getting closer, he could see through a slight gap that it concealed an entrance.

Frank turned into a hermit crab, the size of a human fist, so he could easily sneak in unnoticed. Then he scuttled inside. The cave entrance was only about a square foot wide. There was a dark, narrow tunnel leading down for a few metres, which then opened up into a large underground cave.

If there was one thing he remembered about the episode at the aquarium, it was the over-the-top décor, and this cave was no different. The disco balls had been replaced by anglerfish and glowing jellyfish, squids, and plankton which the pair seemed to have trapped and enslaved. Frank recognised the dazed expressions on those of them that had faces, and the lethargy with which they moved. Phosphorescent moss and glowing coral also covered the walls. The music was coming from an old-fashioned gramophone. And floating in the centre were his two old enemies.

Phorcys and Keto looked similar in some ways, but different in others. Phorcys was still wearing that hideous green wetsuit with Porky's follies written on it in sparkly silver. He obviously wanted to get some use out of the thing since he'd gone to the trouble of getting it personalised. It was looking worn out and had been patched up in multiple places. He still had pale, uneven blue eyes and held his body like a crab. The only real change was that his skin had taken on a sickly shade of green to match his suit.

Keto looked a lot more different. She'd ditched the Georgia Aquarium t-shirt, the glasses, and the pigtails. Her cheeks were no longer dimpled. She obviously wanted to embrace being back in the ocean. Her black hair flowed around her head in a dark halo, and she was wearing a green dress made from soft pieces of kelp woven together. She looked older, and angrier – more like one might imagine a goddess of sea monsters would look.

As he crept quietly down the wall, Keto spoke. Her voice was shrill.

'I don't understand what has happened to our baby! Why has it retreated?'

Frank shuddered at 'our baby'. He still hadn't got used to the whole siblings-having-babies thing, especially when the babies were vicious monsters.

Her brother opened his mouth to answer, but then paused and said:

'Wait. We're not alone.'

He turned his uneven gaze upon the small crab scuttling down the wall and his lips pulled back in a sneer.

'Well, look what the catfish dragged in! I told you I could smell a child of Ares anywhere, Frank Zhang.'

Realising he had been rumbled, Frank turned into the shark again and swam to face them.

The catfish didn't drag me anywhere. I just defeated it. And I told you, it's Mars, not Ares.

'Same difference!'

Keto snarled in rage.

'So that was you! I might have known. We thought Poseidon would send one of his own offspring to do it. We were expecting Percy Jackson. But you'll do instead.'

I'm one of his descendants, he pointed out. But his mind was racing. What did they mean? Had this whole thing been a trap for Percy…which he'd managed to swim into himself?

'Curse that Jackson brat!' Phorcys began ranting. 'And that Bill! He took away everything we had left! Our nereids, our telkhines, our hippocampi! And all our sponsorships – gone!'

Frank had no idea who Bill was or what he'd done, but he had to admire the guy.

Bill was right to free those poor creatures. They didn't deserve to be locked up just because you were. And they clearly weren't happy. All the nereids wanted to do was play Go Fish. That wasn't entertaining at all. And your sponsorships were totally lame. Monster Doughnut and Bargain Mart? Seriously?

'Good thing there is no need for anything like that in the ocean,' Keto said. 'We no longer need the approval of mortals now that we are in our rightful home and have regained our powers.'

Frank scoffed.

And what powers would those be? You're too cowardly to show your faces so you send your monsters to attack Atlantis.

To attack Atlantis…and lure unsuspecting demigods, apparently.

'You seem very sure of yourself. Did you know that when it reaches adulthood, the male flabby whalefish fuses its mouth shut and ceases to feed? It gets rid of its stomach and oesophagus and uses all its previous meals to grow a huge liver which it has for the rest of its life.'

She waited. There was an awkward silence.

Erm…no, I didn't. But I guess I do now.

'Hmm. You used to be more squeamish.'

That was the old me. I've been through a lot since we last met.

'As fascinating as this conversation is,' Phorcys interrupted, 'Can we get on with it already?'

'Yes, yes, you old windbag.'

Get on with what, exactly? Frank asked, although he wasn't sure he wanted the answer. Especially since they both exchanged a satisfied smirk.

'We're so glad you asked,' Keto said. 'You see…since the Skolopendra failed to kill you…perhaps you will be of some use to us.'

'We're actually glad you showed up instead of Percy. He was somehow able to overcome my power. But you…your powers won't help you now.'

Frank had a horrible feeling they were right. It was Percy who'd gotten them out of that tank, while he'd just floated there as a useless goldfish.

Suddenly, the water began to close in on him, holding him prisoner. He struggled against it, but it felt like jelly, sending him bouncing back every time.

'Try and get out of this one! You remember what I told you. Mother said I get to keep anything I caught!'

Frank could have come up with a clever comeback concerning his mother, but insulting the pair hadn't helped so far – in fact, it had only gotten him into trouble. He couldn't see a way out of this one. Not on his own.

'You'll be a useful V.I.P. – very important prisoner, remember? Think of the damage you could do for us with those powers of yours,' Keto taunted. 'And the fact that you're a child of the war god makes it even better.'

Frank struggled against the water that was suffocating him, threatening to drown him, refusing to give up. He'd defeated Shrimpzilla so that it wouldn't cause damage anymore – he wasn't going to end up being its replacement.

Phorcys and Keto's laughter began to fill the cave, clashing horribly with the loud music that was still playing. He felt like the enslaved sea creatures – trapped, unable to leave. How had everything gone so wrong? If only Percy was here. Percy could fix this. But Percy couldn't hear him now.

This, however, gave him an idea. It was desperate, but it was the only thing he could think of to do. He closed his eyes and prayed hard.

Poseidon…if you can hear me…I could use a little help…please…like, now!

The water now felt like it was crushing him. Soon he'd have to give in. Maybe if he agreed to help those two, they would release him. But he would never do that. Not in a million years.

I've never asked you for anything. I helped you, now please help me!

Just when he couldn't take it anymore, when he was willing to say or do just about anything to get out of this, the water released him. Instead of making him feel dazed and tired, it suddenly filled him with a burst of energy, leaving his senses tingling. He spun round in a circle, flapping his fins. He could swim again!

'WHAT?!' Phorcys's yell filled the cave, accompanied by his sister's shriek. 'How…why?'

Sucks for you! Frank kicked his tail hard, and swam as fast as he could to the small tunnel so he could make his escape. Just before he reached the cave exit, he turned into a sardine, the first small fish he could think of, and swam for his life, back through the mermaid graveyard and toward Atlantis once more, repeating a silent thank you as he went.