Sep looked at tall, strong Lamb – shoulders back, her mother’s headscarf tied round the plaster cast on her wrist. She met his stare evenly, and he saw Arkle nod and Hadley smile, the trees behind them split by the recent lightning.

Barnaby had been a gift from his mum. Sep had tucked the bear beside her in the hospital bed when she was sleeping after the operation – and he’d wondered, on the way home in his granda’s car, whether her dying would mean he’d get to stay on the mainland forever.

He’d never forgotten the way the thought had felt in his head. Hot. And guilty.

He reached out to the box’s cold stone, squeezed Barnaby one last time – then tossed him after the other sacrifices and stepped towards the others.

‘Finally!’ said Arkle. Mack raised his sacrifice.

‘You’re putting in your watch?’ said Lamb.

‘Why not?’ said Mack, shrugging. ‘There, I’ve stopped the hands. It’ll always show the time of this perfect moment, when we did this together.’

‘It doesn’t matter what you put in,’ said Arkle. ‘Right? It’s just a thing.’

‘It does so matter,’ said Sep. ‘We’re doing this for each other – whatever you sacrifice has to mean something.’

‘All right, Seppy. So now what do we do?’ ‘Close it,’ said Sep.

Mack’s dark brows furrowed low as he heaved on the box’s stone lid. Sep looked for Barnaby, but the box was deep, and there was only darkness inside.

He thought about the broken wrist that had kept Lamb from hockey camp, the holidays abroad that had isolated Hadley, Mack and Arkle from their usual friends; the way they’d bumped into one another on the beach, and how they’d drifted into such perfect happiness over the last couple of weeks.

‘You know,’ he said, ‘I’ve loved this summer. I’ve never really had anyone to . . . I mean –’

The lid slipped. Mack swore and snatched his hand away, gripping his fingers.

‘Are you OK?’ said Hadley. ‘Are you bleeding?’

‘No, it’s fine.’ Mack flexed his fingers and swore again.

Then he laughed. ‘I think it bit me.’

They stood back and looked at the sacrifice box. As the lid darkened in the rain it disappeared into the forest’s skin, seeming like nothing more than a small, turf-sunk boulder. The world smelled bright and fresh and green.

‘It’s kind of amazing we found this,’ said Lamb.

‘It was the storm,’ said Sep, twisting his headphones. ‘The box wasn’t here last week. The rain must have washed it out.’

‘How old do you think it is?’ said Hadley. ‘Hundreds of years. Maybe thousands.’ ‘Maybe millions?’ said Arkle.

‘You’re an idiot,’ Lamb muttered, then flicked his ear. Arkle grinned.

‘I still think we should’ve made it into a fire pit,’ he said, flipping the lid on his lighter up and down.

Sep saw Hadley watching him from under her fringe. Her tormentors, Sonya and Chantelle, had chased her to the forest and along the edge of the ravine, and her eyes were still red. He looked up through the trees at the clouds – great towers of pearl that flattened to anvils on the sky and spread out over the island. The tide crashed on to the rocks, and he thought of his mum listening to the waves through the open window of the living room.

‘Now what?’ asked Arkle.

‘We say Sep’s words,’ said Mack.

‘They’re not really mine,’ said Sep, remembering how the words had come to him, like a knife driven into his skull – a waking dream so vivid he’d cried out in the bright sunshine.

‘What do you mean?’ said Lamb.

‘I kind of dreamed them. They’re just . . . they’re the box’s rules.’

‘All right, so we say them. Then what?’ ‘Then we’ll always be friends,’ said Sep. ‘How does that work?’ said Arkle.

‘Because we’re making a promise to each other.’

‘And it’ll be our secret,’ said Hadley, gripping her inhaler. ‘We can’t tell anyone.’

‘Oh, I don’t think we should talk about this at school,’ said Arkle. ‘I mean, it is kind of lame.’

The rain started again, cold drops on their hot skin. ‘Let’s say the words then,’ Sep said.

‘Bagsy not stand next to you, poo-legs,’ said Arkle.

They arranged themselves around the box as the rain grew heavier, draping the clearing’s edge in grey sheets and closing them in.

Sep tried to peer through it.

Something was moving in the shadows between the trees.

He narrowed his eyes, focused on a shifting speck.

There was a sound like someone whispering – or shouting from far away – and a long moment hung in the clearing. Sep felt his skin crawl as he sensed other figures around the box, their shadows closing in.

Then Hadley said, ‘Sep?’ and the moment lifted, and they were alone again.

Two crows spun through the rain. They settled on a branch high above them, shaking the drops from their feathers and shuffling their feet.

‘I’m fine,’ said Sep.

Hadley took his hand in hers and lifted it over the box, joined their palms to the others’. Sep felt how warm her skin was, caught her soft scent on the forest’s breath, and closed his eyes.

‘Ready?’ said Mack. ‘Remember what to say?’ Lamb nodded, her jaw muscles tight.

Something’s really happening, Sep thought – then Hadley squeezed his hand, and he forgot about everything that wasn’t her.

‘Now,’ she said. ‘Before Roxburgh finds us.’

They spoke the words – the rules of the sacrifice. ‘Never come to the box alone,’ they said, hands unmoving.

‘Never open it after dark,’ they said, fingers joined together. ‘Never take back your sacrifice,’ they finished – then let go.