The boathouse is locked, so Robbo kicks the door in and we run up the winding staircase to the lookout tower. Looking down at The Wash, I see the central channel lit up like a neon streak and, across the lake, intermittent smaller flashes of blue.

“What are we supposed to do?” Ondine asks.

“Rufus said to hold back any sea beasts trying to come out of the water.”

“Will they really try, if their plan is to flood the town?”

The words are only just out of Robbo’s mouth when an enormous shark with a head like a giant’s saw leaps out of the water on the other side of the pier, cresting high enough in the air that it comes within a hair’s breadth of touching me. Robbo reaches his arm out, spanning my chest, and pulls me back.

“They’re coming for you,” he says.

He’s right. In the next few minutes – or it could be seconds, it all happens so fast – a parade of huge sea beasts leap and dive, curve and twist themselves, trying to reach me. I lob a series of water bombs at them, but it’s dark and it happens so quickly I don’t know if any make contact. The creatures are straight out of my childhood nightmares – huge snakes and eels, giant stingrays, dragonfish and fangtooth fish. It’s an onslaught of the ugliest and meanest creatures I’ve feared most of my life.

“Thom, look – down there!” Ondine says, pointing at the far side of the shore, where the moonlight illuminates the outline of what looks to be a giant agave plant crawling out of the water. “Is that – an upside-down octopus?” she asks.

Its tentacles move in disharmony, reaching out in all directions as though feeling for signs in the air.

“There’s more,” Robbo says, and we watch as more of the strange beasts climb out of the water, tentacles twitching like crazy.

“It’s like they’re taking up positions,” Ondine says, and I have the terrible feeling they’re strategically placing themselves to prevent the Sentinels from getting out of the lake.

I flick a few water bombs in their direction, but the strange beasts don’t appear fazed in the slightest as their tentacles continue their unnerving sky dance. I try to shape a vice out of the water, like I used on the giant eel, but the water just slides around between my hands, like I have no control over it.

And then I think it might be working, because two of the strange beasts begin to shift slightly to one side. But I’m not making them move. I see a huge, hulking thing, the height and width of ten men, shaking the ground as it emerges from the water. Taking one giant step forward, it turns its head and lifts its massive arms up like it’s waiting for someone to throw it a football.

But it’s worse than a football – so much worse. The thick blue neon stripe comes alive, sending out an arc of blinding blue light, which the giant catches in its hands. It lobs the blue ball at the nearest house, which erupts in flames.

“Let’s hope Lina’s been able to evacuate everyone,” Ondine says hoarsely.

We don’t have time to dwell on who might still be in that house, before the channel sends out another bolt of light, which the giant throws at the next house. My powers are as useless on the giant as they were on the strange beasts, the water bombs falling to the ground like they’re grapes, incapable of wounding him. Once again I’m unable to shape a vice with the water and soon the effort has me doubled over, panting from the exertion, powerless to do anything to stop what’s happening.

“It’s the electrical current,” Robbo says, resting his hand on my shoulder for just a second. “It’s interfering with your power. I’ve seen this before.”

“What can I do about it?” I ask.

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Robbo doesn’t have time to answer, before the tower is rocked by an impact that knocks us sideways. We roll to one side, sliding into each other before stumbling to our feet.

“Fire!” Ondine yells, as it becomes clear that the impact was a giant-lobbed fireball.

Flames rise quickly up the staircase, closing off our escape route.

“We’re going to have to jump,” Robbo says, craning his neck over the edge of the tower railing.

“Oh great – I’ve always wondered what two broken legs feel like,” Ondine says, before we jump.

I hit the dirt and roll, legs and ankles sore but mercifully unbroken. Ondine’s already up and on her feet, holding her hand out to help me up. Robbo lands with a thud and I hear a sickening snap, like the sound of a bone breaking. In an instant he’s up and on his feet, hurrying away from the fire, dragging his left ankle behind him.

We take cover behind a Cadillac parked in the lot, no longer able to see the strange beasts and the giant, but we do hear the sound of the blue light missiles making contact with more buildings.

“Are you okay?” Ondine asks Robbo.

“I’ll be fine,” he says, ripping the sleeve off of his shirt and winding it around his injured ankle.

“What’s Plan B?” Ondine asks.

“What’s the south bank like?” Robbo asks me.

“In what way?”

“The north and east banks rise up, and the west bank also rises up to the city centre. But what about the south bank?”

“It’s flat – almost like a plain. It gets flooded every year during the spring thaw.”

Robbo nods. “We need to get to the south bank.” He’s up on his feet in a flash, and Ondine and I follow, rushing along beside him. “It’s just a theory, but I’m guessing the electrical current is strongest here at the north end, because they’re torching this part first. If it’s weaker at the other end, you might be able to control the water.”

He stumbles and winces, and suddenly I understand his plan. “If we flood the plain, there’s a chance the tsunami will have less of an impact.”

Robbo nods. “It’s a small chance, but it might work. Your dad said the water level’s at its lowest level in years, so maybe they’ve been holding water back, planning this attack. If we can drain the lake – even a little bit – we might have a chance of stopping the town flooding.”

“We’d better run,” Ondine says, taking off at a sprint.

“Go with her,” Robbo says. “I’ll catch up.”

Ondine’s fast, and I have to work to keep up with her. As we race along the western bank, it appears Robbo’s right – we see none of the strange beasts here, there’s no giant lobbing fire bombs. Mercifully, we’re able to run unimpeded along the bank. There’s a multitude of faint green flashes of light under the water, evidence of the movement of scores of sea beasts. There’s a blue shimmer down the centre of the lake, and I hear the thrum of electricity like a low echo in the night sky.

We reach the south bank and I stop, bracing myself against the energy crackling in the air. I lift my hands up and flick my fingers backwards, towards me once, twice, three times – and then it happens. I feel the water coming towards me, gently at first like I’m pulling on a heavy tablecloth. Soon the force of it bends my fingers so far back they’re at an obscene angle but, grunting and pushing back with all of my strength, I keep control of the water. It rushes onto the plain in a steady flow, as I yell at Ondine to stay behind me.

I’m concentrating so hard at steering the water in two channels, either side of us, that I’m completely unprepared for the voice in my head: Thom, it’s time. It’s happening. Run like hell.

New plan, I tell Rufus. I’m diverting the water to the plain on the south bank.

Get out of there, Thom. It’s too risky.

“Rufus says it’s time,” I tell Ondine. “You and Robbo should get out of here. Run in a straight line behind me. I’ll try to keep a path through the water for you.”

“I’m not leaving you here on your own. And where is Robbo?”

In that second, my fingers bend back so far they’re like wet noodles stuck to the ends of my hands. The water surges forward, giant walls of it to either side of us. I realize it’s too late for Ondine to run – she’ll never make it down the narrow channel if the force of the water keeps up like this. I should be happy that Robbo’s plan is working, but all I can think is that I’ve brought Ondine here to die. She’s going to drown here, and I’ll be powerless to stop it from happening.

Just as I’m picturing Ondine spinning down to her watery grave my noodle-fingers regain a bit of strength and I flex them, pointing them up at the night sky. They keep on moving until they’re pointing at the huge waves and then the rush and whoosh of water changes from deafening to almost completely quiet. Instead of moving past us onto the plain, the water changes direction.

“What’s happening?” Ondine asks as the water on the plain begins to make its way back to where it came from. “Why is it moving the other way?”

There’s a loud rumble and then the earth beneath our feet starts to shake. We both fall forward onto our knees, watching helplessly as the water flows in the wrong direction.

“It’s not – could it be - ?” Ondine asks, a look of horror on her face.

“I think it is,” I say, in a voice that sounds nothing like mine.