Joe skidded to a stop, breaking from the shadow of the buildings to stand in wonder at the water’s edge. Ahead of him the filthy brown sea met the cloudless blue sky. In the distance a line of tower blocks jutted from the water, waves lapping at the middlemost storeys.

“Hey!” Kara called. “Don’t run ahead.” Her bare feet pounded on the wooden walkway as she came up behind him, her face flushed and her dirty yellow hair coming loose from its knot.

“I wanted to see the boats,” Joe said. “I like the boats.”

The Cut was busy today, the shipping lanes crowded with rusted tankers and boxy haulers, some creeping into port while others rattled off towards open water and destinations unknown. Local fishing sloops tacked between them, patchwork sails stiffening in the breeze. All of them were going somewhere. Somewhere that wasn’t here.

“It’s dangerous out this way, you know that.” Kara placed a protective arm round his shoulders. “We’re a long way from home.”

She was right, as usual. The Spur was the most notorious neighbourhood in the Shanties, an outflung tangle of wooden catwalks, shadowy towers and winding waterways miles from the heart of things. Only the very poor or very secretive made their homes out here.

“This ain’t a race, you know,” a voice called, and turning back Joe saw Mr Colpeper struggling behind them, his bald head scarlet and glistening. Far beyond him the Wall rose from the concrete and driftwood sprawl of the Shanties, its sloping side sparkling with a layer of crystal salt. At its base was the harbour, a busy ant’s nest prickling with cranes and masts and security towers.

Colpeper wheezed to a halt, his hands on his knees. “You kids’ll be the death of me, I swear.”

“You’re getting fat,” Kara said. “What? It’s true.”

Colpeper’s mouth tightened, then he barked a bitter laugh. “You don’t mince words, do you, sweetheart? One of these days that mouth’ll get you into trouble.”

He hitched up his jeans, staring out along the Spur. A short distance ahead the walkway tapered to its end, the blocks petering out into a rickety huddle of rafts and shacks. Beyond that there was nothing but open ocean.

“Past the pub then look for the spire, my dealer said.” Colpeper rubbed his hands. “He’s already got a buyer lined up in the City. This could be it for us, kids. Top dollar, right in our pockets.”

“Where have I heard that before?” Kara muttered sceptically. Colpeper was always sure this job was the big one, the chance of a lifetime. But somehow it never quite panned out.

Waves lapped at the concrete pilings as they moved along the walkway. An old man perched on the edge, clasping a homemade fishing rod. What’s he expecting to catch? Joe wondered. Rusty cans and bleached bones. Perhaps he was one of the ancient ones, who remembered London before the last barrier broke. Who’d lost everything to the waves.

Joe had tried to imagine a life on dry land, but he was never quite able to grasp it. Solid earth under your feet and all that green stuff – what did they call it, grass? The Shanties might be smelly and hectic and dangerous, but this was the only world he’d ever known.

Well, that wasn’t strictly true. He’d been to another world, one that few ever got to see. And it was almost time to go back.

“Keep up,” Kara called over her shoulder. “Honestly, first you run off, now you’re stood there daydreaming.”

She reached out and he took her hand, enjoying as he always did the sight of his little brown paw clasped in her big rosy-pink one. She tugged him forward, resentfully at first, then with a smile. “You are such a pain in the bum,” she laughed. “I don’t know why I let you hang around with me.”

“Because you lurve me,” Joe told her. “You lurve me so much, it’s disgusting.”

Kara lowered her head and kept dragging. “Horrid. Little. Brat,” she grunted with each tug.

“Quiet, you two,” Colpeper said abruptly. “This isn’t the place for games.”

A rusty shack loomed ahead, casting the walkway into shadow. The raft beneath it rocked in the wake of a passing ship and the whole structure creaked and groaned. Through the open door Joe could see figures moving through a fog of sweet smoke. He heard the clink of glasses and smelled the sour tang of a strong, locally brewed drink folks called Selkie.

“This is the Last Gasp,” Colpeper whispered. “Only the really bad crooks drink in here.”

“Favourite of yours, then?” Kara smirked.

Colpeper nodded. “It was, back in the day. Before I went legit. You’re lucky you didn’t know me then, I wasn’t half so good-natured.”

His voice was almost wistful, and Joe wondered how much Mr Colpeper had really changed. He was decent enough most of the time, but when he got in one of his tempers no Beef would dare stand up to him.

They passed the pub, approaching the tip of the Spur. Squinting ahead, Joe realised this might be the furthest from home he’d ever been. Across the water he could make out a smudge on the horizon – the hazy mainland shores of Wycombe and the Chilterns, so distant and unattainable he might as well have been gazing at the moon.

“That must be the spire,” the big man said, gesturing south into the waters of the Cut. The peak of a building broke the surface, topped with the outline of a black bird. Joe peeled off his shirt and Colpeper unzipped his pack, taking out a rubber mask and a steel canister. “Just take a look for now. If the goods are intact, we’ll see about proper salvage.”

Joe nodded, clipping the tank to his cargo shorts and biting down on the mouthpiece. The oxygen tasted of old rust.

“And don’t take any stupid risks,” Kara warned him. “It’s not worth it.”

Joe frowned. “I hab dud dis befaw, oo no,” he told her, the mouthpiece garbling his words. It was good that she worried, but sometimes he wished she’d put a bit more trust in him.

He dropped to the boards, flippers dangling over the edge. Scum glistened on the water, a seagull carcass grinning from a nest of seaweed. The Stain, they called it, a festering vortex of garbage and human waste that spread from the Shanties for miles in every direction. But he had no choice; they’d come all this way. So he braced himself and took the plunge.

Joe trod water, taking a good look left and right. He wasn’t close enough to the sea lanes to worry about the big ships, but it’d be just his luck if some idiot on a jetski came clipping round the corner. Then he kicked out, beating a path through the muck, keeping his arms and legs tucked in to limit the risk of touching anything unpleasant. He kept his scalp shaved for the same reason; there was nothing worse than washing someone else’s poo out of your hair.

He reached the spire, the sun baking on his back. He gave a thumbs up, receiving an answering nod from Colpeper. Then he kicked off, angling down into the dark. The Stain cleared, a shaft of sunlight broke through and the world below was revealed.

The houses here were low, a maze of narrow terraces and algae-stained roofs. Joe saw shattered windows, rotting curtains waving in the current. But there were no cars – they must’ve been dragged up for scrap years ago, along with anything else the early Beefs had seen fit to scavenge. This whole area had been picked clean.

He thought of the old fisherman. Had he grown up in a street like this, before the water came? Decades had passed since then, but time had no meaning down here.

Joe turned, treading water and looking up at the building looming over him. A church, Colpeper had called it. He knew what the word meant; the Shanties were full of shacks where the faithful gathered to sing and pray. Kara had always cautioned him to steer clear – if there is a god, she said, he’s probably not someone you want to make friends with. I mean, look at the world.

But this church was different and rather grand. From the corners of the steeple sprouted four stone carvings, horned figures with spread wings. They looked oddly at home down here, watching over their sunken kingdom.

Joe scanned the nearby buildings for the word Colpeper had made him memorise. A sign said POST OFFICE, another SUPERMARKET – a large flat structure with a line of rusty carts anchored outside. Then he saw it. Letters were missing so that the sign now read “R XY C EMA”, but this had to be the place. It was a squat brick building, the entrance just a gaping rust-edged hole. Joe swam closer, taking hold of the steel frame. He peered inside.

The carpets, once red, were almost black with silt. Joe tugged the torch from his pocket, winding the crank five times, then flicking the switch. Shapes emerged from the gloom: rotted chairs and a smooth fibreglass counter. The walls were lined with pictures sealed in grimy frames. Joe wiped one clean and saw a woman wearing next to nothing holding a gun in her hand. He wondered what kind of place it had been, this cinema.

Silver winked as a school of sprats darted out of the light. Doors branched left and right, blocked with fallen debris. But in the far...