Prologue

The river first ran uphill on a Tuesday, which was unfair, because Tuesdays were already full of spelling tests and fish pie. Ruairi Faolan saw it happen from the records-room window of Harbour House School, one hand wrapped round a cracked blue mug and the other pressed over the red ledger he had been told never to open. Outside, the water of the little harbour stream lifted itself from the culvert, shivered like a horse refusing a gate, and began climbing back towards the hill.

Nobody screamed at first. That was the thing Ruairi remembered later, when grown-ups tried to make the story neater. The gulls complained, naturally, because gulls complained at weddings, funerals, lunch boxes, and the existence of closed bins. Mrs Ward dropped a stack of permission slips. Cormac Vale said a word that made the younger children gasp with delight and the older children pretend they had known it since birth. But for three whole breaths, everyone simply watched the impossible do its work.

Ruairi was twelve years old, nearly thirteen if you believed birthdays should be counted in advance, and old enough to know that water was meant to have manners. It went down. It found the lowest place. It accepted defeat with a shine on it. This water did not. This water climbed stone steps, crossed the yard drain, and curled round the iron boot scraper like it had an appointment indoors.

The red ledger knocked once beneath Ruairi's palm. Not a sound exactly. More a thud felt through bone. On its cover, where the leather had cracked in a shape like a crooked smile, the stamped gold letters glimmered: RAVENMERE ESTATE OFFICE — RIVER TOLLS, LOST PROPERTY, AND UNUSUAL PASSAGES. The book had no business being in a school records room. Ruairi had no business touching it. Then again, the river had no business climbing a hill, so the day had clearly resigned from ordinary behaviour.

'Put that down,' said Niall Keane behind him. The caretaker's voice was quiet, which made it worse. Mr Keane could shout over a hall full of tin whistles and still sound less dangerous than he did when he spoke softly. 'Ruairi. Step away from the book.'

Ruairi did not step away. He had learnt, from years of being corrected for things that were not quite lies and not quite truths, that adults often used a child's name when they meant obedience rather than listening. He slid his fingers beneath the ledger's front cover instead. The brass clasp was warm. Not sun-warm. Pocket-warm. Like something had been held for a long time by a person who had been running.

'You told us the Ravenmere records were burned,' Ruairi said. His voice came out smaller than he wanted, but it came out. That mattered. 'You told Eileen Ward there was nothing left.'

Mr Keane's eyes flicked to the window. The uphill river had reached the yard wall. Water should have fallen back. It rose instead, trembling up the mortar seams, silver threads finding old cracks. Children pressed against classroom glass. Somewhere below, the school bell began to ring, though no hand had pulled the rope.

The bell had been silent for forty years. Ruairi knew because everything in Harbour House School had a story attached to it, and half the stories were used to stop children doing interesting things. Do not touch the east-wing door; it swells in damp. Do not climb the coal steps; they remember ankles. Do not ring the old bell; it is only there for show. Do not ask why Ravenmere pays for the roof repairs. Do not ask why the river is named on every old map but not on any new one.

That last one had been Ruairi's favourite question until people stopped answering it. The harbour stream was called the Finnow if you asked the library map, the school drain if you asked Mr Keane, and that nuisance if you asked anyone who had slipped in it. But in the old photograph tucked into the ledger's first page, a line of children stood on the same yard stones beneath a hand-painted banner: FIRST CROSSING OF THE UPWARD RIVER, 1912.

Ruairi stared at the photograph. The children wore stiff collars, patched boots, and the solemn faces of people told to stand still by a photographer with no patience for personality. One child near the left edge held a storm lantern even though the sky above them was pale. Beside the lantern, a girl had tied a black ribbon round her wrist. On the back, in brown ink, someone had written: When the river climbs, bring back what was borrowed.

'You are not to read that,' Mr Keane said. His hand closed round Ruairi's shoulder, not hurting, but firm enough to claim the room. 'This is old adult business.'

'It's in a school,' Ruairi said. 'That makes it child business as well.'

A smile twitched at the edge of Mrs Ward's mouth before fear swallowed it. She was the school secretary, a woman who could find a lost inhaler, a missing violin, or a parent who owed trip money with equal mercy. She had once told Ruairi that rules were ladders: useful for reaching things, dangerous if rotten. Now she looked at the ledger as though she had been waiting years for it to bite.

'Niall,' she said, 'if the river is moving, we cannot pretend the book is not here.'

'We can if the boy leaves it alone.'

The boy. Ruairi hated the phrase. It made him sound like a chair someone had forgotten in the way. He lifted the ledger fully from the shelf. Dust bloomed round it and made the room smell of old paper, salt, and the inside of a cave. A folded ferry ticket slipped from between the pages and skated across the floorboards to rest against his shoe.

The ticket was cream-coloured, punched twice, and marked for a route that no ferry had sailed in living memory: NORTH QUAY TO RAVENMERE LANDING — ONE CHILD, RETURN NOT GUARANTEED. Someone had written Ruairi's name across it in ink still wet enough to shine.

For a moment the world narrowed to that impossible black line of letters. Ruairi Faolan. Not Rory, as substitute teachers tried when they got lazy. Not Master Faolan, as old Mrs Bell from the shop said when she wanted him to carry boxes. Ruairi, written by a hand he did not know on a ticket older than his grandparents' wedding photograph.

The school bell rang again. This time the sound came through the floor, through the walls, through Ruairi's teeth. Children began shouting downstairs. The river slid under the records-room door in one perfect ribbon, dry as glass at the edges and wet only in the middle, as if it had decided not to ruin the carpet out of politeness.

Mr Keane swore properly then. Mrs Ward snatched the emergency folder from the desk. Ruairi bent and picked up the ferry ticket. The ink did not smudge. His name stayed fixed, bold and accusing, like a dare printed by someone who had known he would be standing there before he was born.

'Give me that,' Mr Keane said.

Ruairi put the ticket inside the cracked mug. It was a ridiculous hiding place. It was also the only one he had. The mug had belonged to his father, or so his mother said on the rare occasions she spoke of him without looking over Ruairi's shoulder for an escape route. A crack ran from rim to base, mended with a line of gold glue that made it look wounded and proud. It had survived three house moves, one dishwasher incident, and Ruairi's attempt at making cocoa with salt instead of sugar. It could survive a ferry ticket.

The river reached his shoes and stopped. Not because it had run out of strength. Because it was waiting.

That was when the locked door at the end of the corridor opened by itself.

The east-wing door had been painted shut in a colour adults called heritage green and children called swamp. Every September, new pupils were told the same three reasons for ignoring it: the floor beyond was unsafe, the roof beyond was rotten, and the staircase beyond led nowhere worth visiting. Ruairi had believed the first two because grown-ups loved rotten roofs. He had never believed the third. Now the old handle turned with a sigh, and a smell came out that did not belong to any locked room: river mud, candle smoke, apples, and cold iron.

A smaller child began crying downstairs. Not the loud performance kind of crying that came with grazed knees and injustice over biscuits, but the thin panicked kind that made older children go still. Ruairi's stomach folded. Whatever the ledger wanted, whatever the river thought it was doing, the school was full of people who had not asked to be part of an old secret. That was the first proper lesson of the day: mysteries were not games just because they arrived with interesting props.

Mrs Ward moved first. She kicked off her sensible shoes, lifted her skirt clear of the ribbon of water, and marched to the corridor as if impossible rivers were another form to be filed before lunch. 'Everyone stays calm,' she called. 'Niall, get the infants to the hall. Ruairi, with me.'

'Absolutely not,' said Mr Keane.

'He is already named,' Mrs Ward snapped, and the words struck the air like the bell. 'You know what that means.'

Ruairi did not know what that meant, which seemed rude, considering he was the named party. He clutched the cracked mug against his jumper and followed Mrs Ward. The river parted around his ankles and left his socks dry. He decided not to think about that yet. A person could only have so many impossible facts before breakfast settled badly.

Beyond the east-wing door lay a passage that should have been dark. Instead, pale green light trembled along the walls. Children's drawings from years ago curled on noticeboards: boats with square sails, a hill with a blue line climbing it, a girl holding a lantern, a boy with a key in his mouth. Ruairi stopped at the last one. The boy in the drawing had black hair sticking up in every direction and ears too large for his head. It might have been any boy. It looked horribly like him.

'Do not touch the walls,' Mrs Ward said.

'I was not planning to lick them.'

'Good. Keep standards high.'

Her dry reply steadied him. That was another thing stories got wrong: courage did not always roar. Sometimes courage sounded like a school secretary refusing to let terror ruin her sarcasm.

At the end of the passage, a narrow stair curved down instead of up. The school had been built on a hill above the harbour; there should have been no room for a stair descending under the east wing. Ruairi knew the building's shape the way bored pupils knew ceilings. He had counted tiles, cracks, and radiator pipes during assemblies. This staircase was an insult to geometry.

On the first step sat a brass key.

It was not large. That disappointed him slightly. Important keys, in books, were usually grand enough to open castles or threaten pockets. This one was plain, tarnished at the bow, with three teeth cut unevenly along the bit. A black ribbon had been tied through it, the knot stiff with age. The key lay in a perfect dry circle while the river streamed around the step.

Mrs Ward put a hand to her mouth. 'No.'

'Is it yours?' Ruairi asked.

'It was my grandmother's.'

'Your grandmother's key is sitting in a school stair that wasn't here yesterday.'

'Yes.'

'Right.' Ruairi breathed out. 'Good to have facts.'

She laughed once, a cracked sound. Then the laugh broke into something closer to grief. 'She told me the river would come back if anyone tried to erase the children. I thought she meant remember them. Light a candle. Say a prayer. Old people speak in riddles when they are tired.'

The ledger thudded again under Ruairi's arm. He opened it despite Mr Keane shouting from the records room. The first page was not a list of tolls. It was a roll call. Names marched down the paper in faded ink, each with an object beside it. A cracked mug. A red ledger. A locked door. A ferry ticket. A black ribbon. A brass key. A storm lantern. A school bell. An old photograph. A salt-stained map. Ten objects. Ten promises. Ten children borrowed from the river and never properly returned.

Ruairi swallowed. 'Borrowed children sounds bad.'

'It was,' Mrs Ward said.

'Are they dead?'

She looked at the green-lit stair. 'That depends who kept the ledger.'

From below came a sound like oars knocking against stone.

Every sensible part of Ruairi wanted to go back to the corridor, find his class, and complain loudly that nobody had completed a risk assessment. Another part, the part that had always noticed missing names and doors painted shut and adults turning quiet at the same places in stories, leaned towards the stair. If the river had climbed all this way to ask a question, it would not leave because Ruairi pretended not to hear.

He picked up the brass key. It warmed instantly in his palm, and somewhere deep beneath Harbour House School, a child began to sing his name.