June 1935 Delphine walked from the powdery dunes down to the hard sand of the beach. Razor clams and whelks crunched beneath her feet. The full moon sat low in a cloudbank, burning.

She snagged a dry tangle of seaweed with her crab hook and hoisted it high above her head. It hung against the moon like a fright wig, gnats and bluebottles scribbling at its edges. She spun it once, twice, and hurled it back towards the dunes, where it crashed and slewed apart.

The distant sea was sleek, stippled with milk-bright flakes. Delphine imagined it surging inland, filling the crooked channels of the salt marshes, inundating the woods, flooding the secret chambers beneath the estate and finally seething through the corridors of Alderberen Hall itself, tearing down fine brocade and fittings and portraits, washing conspirators from their beds then receding, dragging the whole rotten edifice with it, leaving only mud miles of stark, honest mud.

On the dark sand left by the ebbing tide was a pair of black leather shoes. The beach was empty. She claimed salvage rights.

She sunk her crab hook into the sand, dragging it behind her as she began a loose circuit of the shoes. They were smart men's shoes. Indeed, they appeared to have been recently polished. They sat side by side on a folded newspaper.

Her crab hook scored a spiral winding inward. She stopped.

The laces were clean and black. She picked up the shoes. Apart from a few grains of sand in the stitching around the toe, they were immaculate. The soles looked brand new. It was as if a shoemaker had left his workshop door ajar and, spotting their chance, his latest creations had made a break for the seaside. The surrounding sand was packed into muscular ridges; there were scuffs that might have been footprints, but they petered out after a couple of yards. She was about to pick up the newspaper, imagining it might contain some vital clue, when she noticed a man in a grey suit, standing in the sea.

The tide was a long way out. She only spotted him because he reached up to steady his bowler hat. After that he stood still, one hand on his hat, the other by his side. He had his back to her. The water was up to his calves. His trousers were not rolled up.

Delphine watched. The man did not move. He was camouflaged, grey suit against grey sea.

She put down the shoe, then reached into her bag and retrieved the field glasses she had found lying around at the back of a locked chest of drawers in Dr Lansley's room. She put the cold metal to her eyes, twisting the eyepieces until the image focused.

His black hair glistened in the moonlight.

She watched. He stood, as if waiting. She considered shouting.

He took a step, wobbled; the slack hand went out for balance. He took another step. He began to walk.

The water was up to his knees. Delphine knew he would give in and turn back this early in the summer, the sea was perishing.

She stood on the flat, wide beach, the full moon blazing. Any moment, he would turn and spot her. He might think she was trying to steal his shoes. The water was up to his backside.

She had a crazy thought: He's not going to stop.

She tweaked the focus. His outline sharpened. Water was creaming round his waist. The fingers of his left hand trailed in the water.

Mr Garforth had warned her about the lethal riptides in this area. Breaks in the sandbars created currents that could drag even strong swimmers away from the shore in less than a minute. It was the kind of grisly warning that Mother came out with all the time.

The water was up to Mr Kung's belly. His left hand was submerged. The field glasses made it feel as if she were watching a scene in a film.

She lowered them. Mr Kung was still there.

'Hey!' she called, startling herself. The word came out small and hoarse. He did not react. 'Hey!'

A gust frilled the smooth water into dragon scales. Mr Kung waded deeper.

Delphine turned and ran.

She hammered on Mr Garforth's door and peered through the thick window. The cottage was dark.

Moonlit nights brought out poachers. Mr Garforth said the problem had got worse over the past few years, on account of jobs being scarce. He said they worked in gangs. He said he understood that a man must feed his family and that he felt no ill will. Then he told her about the time he had surprised a man setting snares for rabbits and winged the fellow as he ran away. When Mr Garforth finished the story, his eyes got a faraway look and he chuckled.

She hared across the marshes towards Prothero Wood, hoping to catch Mr Garforth patrolling the feed run. It had rained earlier and the ground was doughy. She vaulted trenches, silver ribbons of water flashing beneath her feet. The wind was picking up. Ahead, a belt of ash trees lapped at the damp air.

She took a shortcut between dense-packed trees and dropped onto the track bisecting the wood. She squinted against the darkness. Pussy willows curled and rose on either side, forming a tunnel that looked as if it had been left in the wake of a monstrous, slithering crocodile.

Since the incident with the bat, she had been wary of Prothero Wood. She steered clear of the tomb, but she could not avoid the wood entirely she had to walk through it each day to reach Mr Garforth's cottage. Her senses sharpened when she entered. It felt very much like enemy territory.

'Hello?' she shouted. 'Hello?'

Nothing. Cracks of sky glowed through the branches overhead. The moon was a headlamp in fog.

She hesitated. Snug in the belly of the wood, listening to the slow shhhhhh of the wind, she started to doubt herself. Maybe she had misunderstood. Maybe she had imagined it. The thought calmed her.

But she had seen Mr Kung, standing there. She had watched him through binoculars. What if she did nothing, and he died?

Delphine fumbled for a plan. Mr Garforth could be anywhere on the estate. She could run round till sunrise and still not find him. By the time she got to the Hall, it would be too late. It might already be too late.

She sprinted along the track, not sure where she was going. The track eased left then lurched right, dipping through slush and then flattening out. A shrew scurried across her path and she had to leap to avoid it; she skidded and when she looked up she saw Daddy.

He stood side-on to her, in the middle of the track, breathing smoke. He had no coat. His back was bent and his unbuttoned shirt cuffs hung like tattered bandages. He lifted an open palm to his mouth as if yawning; a red point of light sharpened between the first two knuckles. He lowered his hand, sighed smoke. His eyes were closed.

'Daddy.'

He snapped upright. He cast around in the darkness, then found her. His face was pale. He tossed his cigarette to the ground and stomped it out.

'Hello.'

'Mr Kung is in the sea.'

Daddy squinted. 'Sorry?' He shook his head. 'What do you want me to come and see?'

'Mr Kung is in the sea. With his clothes on.'

'Oh, right. Oh, well then.'

'You have to come quickly.'

'Right.' Daddy raised his arms slightly and glanced around.

'Now!' Delphine swiped at the air, then turned and began running again. When she glanced back, he was following with clumsy, flat-footed strides. She slowed to let him close the distance then scrambled up the bank into the shortcut. She heard the gasp and crash as he beat his way through a holly bush.

'Where are we going?' he said.

'The beach!'

Once they were out on the salt marshes Daddy found momentum, pumping his arms. Delphine accelerated to two-thirds her normal speed and led him through the easy route firm ground and plank bridges, no jumps. He kept pace. A crosswind kept trying to spin her clockwise, ruffling the reeds. The moon was out from behind the clouds, turning the dunes to caster sugar.

She pictured reaching the crest to see Mr Kung in a stripy bathing suit, perkily towelling himself off. Perhaps nocturnal bathing was normal in China. Perhaps he had just got overexcited from what she remembered from her atlas, Inner Mongolia was a long way from the seaside. Or what if there was no trace of him at all? Daddy seemed to be in one of his placid moods tonight, but if word got back to Mother that she had faked an emergency and worse, that she had exerted Daddy unnecessarily the showdown would be apocalyptic.

Delphine fell onto her hands and knees as she hit the summit. The shoes were still there. She peered at the sea. The water was choppy it was hard to pick out a figure amongst the slump and crunch. She moved to take out the field glasses then, remembering Daddy, angled her body to hide the bag as she removed them.

She saw Mr Kung. He was up to his shoulders. He still wore his bowler.

'He's there! He's there!' called Delphine, pointing frantically.

Daddy staggered up towards her. He had a scratch on his forehead and his ankles were painted with mud.

'Where?'

'There!' said Delphine. She looked. Mr Kung had gone. 'Oh my God.'

'Where?'

'He's gone under! He's gone under!' She was dashing down the dune, windmilling her arms. As the sand levelled out she broke into a sprint, focusing on the point where she had last seen him, but the sea was swelling, shifting, and she began to worry she was running towards the wrong spot. She slowed, searching for landmarks she could triangulate by, then something crashed into her shoulder from behind and she hit the sand.

She landed face down. When she lifted her head she saw Daddy running faster than she had ever seen him, stampeding towards the waterline.

'Where is he?' yelled Daddy.

Delphine scrambled to her feet. 'I don't know! He went under!'

Daddy did not hesitate; he ploughed into the sea with a chain of splashes like a Vickers gun strafing a pond. He lurched forward as the water dragged at his ankles, then drove himself onwards with sweeps of his lean forearms.

'Where is he?'

Delphine halted at the water's edge. 'Somewhere here, I think!' She waved her arm back and forth, indicating a wide cone.

Daddy was waist-deep, paddling with his arms. He looked around.

'I can't see him!'

'He was deeper! He went under!'

A blast of wind lifted the water into spikes. Daddy dived.

Delphine could not believe it. She stood at the water's edge, stunned and alone.

Daddy surfaced, mouth wide, filling his lungs. He dived. He came up again, swum a yard deeper, plunged. When he rose a third time he was gasping, tendrils of hair whipping droplets as he cast about.

'I can't see him! It's too dark!'

A stammer pinned her tongue to the roof of her mouth: 'I duh . . . ' She clenched, took a breath. 'I duh-duh . . . '

Under again. Water rolled fizzing to her feet. She willed herself to step forward, to run in and help him, but her legs did not move. Another four seconds passed. Daddy did not surface. He burst up, spluttering, wiping hair from his eyes. He was treading water. He dived.

This time he came up quickly.

'I can't . . . see him!' Daddy was weakening she heard it in his voice. At night, the sea was freezing; dipping your head underwater felt like clamping it in a vice.

He dived. He came up breathing raggedly. He slopped hair out of his eyes, screamed. A wave lifted him up. He looked around. He was alone. He took a breath and ducked back under.

Delphine had her cardigan balled up in her fists. The tight, trapped feeling had spread from her tongue, down her jaw to her shoulders and chest. She wanted to yell at him to give up. He surfaced, coughing, slapping about for purchase. He let out a wounded cry. When he went under again, Delphine could not tell if he had meant to. His head tipped back and he sank.

A wave hid the place where he had gone under. When it had passed, he was still missing. She stared. Her vision narrowed. It was crazy to believe this sucking, plunging sea held two living men. They were gone. Her bladder tingled. She was going to collapse.

Daddy broke gasping, went under, surfaced, found his footing. Water streamed from his hair and nostrils. He was walking. He was yelling. She thought he was calling to her, but she could not make out words. His mouth was wide; his teeth glowed. He held something in his arms: driftwood wrapped in black canvas. As the water got shallower, his burden pulled him down to a stoop.

'Gah . . . ah . . . gah . . . ah . . . ' Every breath was a hoarse roar. The tide was dragging at his legs. He shook water out of his eyes, looked around. He spotted her. 'Help me!'

She ran into the sea. It was scalding. She gasped at the pain. Daddy staggered; the driftwood fell from his arms. She ran for him, steadied him. She looked down at the thing floating at his feet.

It was Mr Kung.

He was face down in the shallow water. She grabbed him under one of his arms. Daddy was huffing, shivering. He caught hold of Mr Kung by his other arm and, together, he and Delphine pulled Mr Kung out of the waves and onto the sand.

They laid him on his back. His spectacles were gone. His eyes were open and crusted with sand, cataracts of froth purling in the corners. Blood and foam ran from his nostrils. Bloody water flowed from his mouth. His skin was the colour of tallow. He was not breathing.

'Help me pick him up,' said Daddy. Delphine caught hold of the sodden lapels and heaved. Once they had lifted Mr Kung upright, Daddy swung him round and began squeezing him in a bear hug, letting his head loll.

Lots of water came out of Mr Kung's mouth. It splattered brightly against the firm sand. Daddy squeezed again. More water came out. Mr Kung's lips were slack and mauve. Phlegm hung in a silver beard. His eyes stared blindly. Daddy hugged him again and again. Water came out and each time Mr Kung shrugged as if to say it is no good, I am doing the best that I can.

Delphine wrung her hands in wretched, trembling prayer. Daddy lowered Mr Kung onto the sand, rolled him onto his back and shook him.

'Hello!' Daddy said. 'Hello!' He slapped Mr Kung across a shining cheek gently, at first, but then harder, and harder, as if interrogating a spy, left, right, left, right, and Mr Kung shook his head, no, no, no, no. Daddy thumped Mr Kung in the chest and water arced from his mouth.

Mr Kung spluttered.

Delphine shot a look at Daddy. Daddy's eyes were wide. He thumped Mr Kung again. A little more water came out. Delphine looked down at Mr Kung's pursed, purpled lips. Something black hung from the corner of his mouth.

'In his mouth!' she said.

Daddy grabbed Mr Kung's chin and tilted his face upwards. He squeezed the cheeks so Mr Kung's lips popped open in a prim 'oh'. His teeth were bad, ranging from cream to mahogany. Daddy plunged two fingers into Mr Kung's throat; he hooked out a thick, dark clot of seaweed with a long tail that kept coming. The strand snapped; Daddy cast it aside and dug in again. This time, he pulled carefully, pinching thumb and forefinger to tease out the delicate flukes.

Mr Kung coughed. His face tightened. Bloody water welled up in his mouth. He inhaled it, gargled.

Daddy scraped the last of the weed from Mr Kung's mouth. Mr Kung tried to breathe, and again he choked, froth streaming down his face.

Delphine looked around for some way to help. She ran to the sea, scooped up some water, then knelt over Mr Kung's head. She tilted her hands and poured a little water onto either eye, washing away the foam and sand. It made him look worse. His naked eyes were pink and sightless, pupils rolled back.

Mr Kung made a strangling noise. Daddy gripped his shoulders with pale, tendon-mapped hands. He looked at Delphine.

'I can't save him,' he said. He gazed around at the empty beach. Something brought him smartly to attention. 'You have to go to the house and get help.'

'But it's over a mile away.'

'I can't save him.'

'But what do I say?'

Daddy pounded a fist against the sand. 'Get Dr Lansley! Bring him here now!'