Bo Delafort had just turned twelve when she pulled the moon from the mud of the Thames. She hadn’t been looking for it, and that made all the difference. You don’t find treasure, it finds you – that’s what riverfolk liked to say. All you do is keep your heart open.

It was August, early Saturday morning: lowest tide at Battersea Bridge. The exposed riverbed was gleaming in the sun, surprisingly empty of other people. High up against the cloudless sky, seagulls searched for fish guts. Bo saw a low arrow of geese, and felt the day’s heat on the rise, the river gentle at its edge. The Delaforts had been born by this river, and the Delaforts never left. She loved it down here. It made her feel peaceful and whole. The river was a vein in her body. Her other home.

Bo was a schoolgirl, but she was also a mudlark, sifting the riverbed for things to sell. The instant she had seen the glint in the mud, she’d known it was different. No tip of a sugar spoon, no edge of a threepence coin. The sun was beating through the flimsy boater her mother had insisted she wear, but she felt so cold. A breeze got up from nowhere, blowing her hair in her eyes, and slow waves of water sloshed the stones.

Now! the river suddenly said to her, in a deep and greenish voice, winnowing around the wind. Put your hands in now!

Goosebumps rose on Bo’s arms. She’d never heard the river speak. It felt like a blessing, the sign she’d been needing but hadn’t dared ask for. Older mudlarks along the banks had told her how one mudlark in a lifetime might hear the river – it was so rare and so strange that if it happened to her she might not believe her ears.

Some said it was foolish talk, that the river would never speak to the humans who trod its shores. Why would it? It had business of its own.

But Bo had always believed that beneath its shifting surface a strong power lay in wait, ready to talk. She had always hoped that one day it might let her know its secrets. So when it happened, this August morning by Battersea Bridge – the river’s words surging through her mind, making her heart beat harder – it didn’t feel as strange as perhaps it should.

Bo crouched down, her boots and the hem of her pinafore sinking into the mud. As the river had asked, gently she reached her trembling fingers through the cool of the clay.

The hairs on the back of her neck stood up. She’d touched something – the solid shape of an unknown object. It seemed to want to drag her into its own orbit, and she gasped but didn’t let go, feeling certain that she mustn’t leave this treasure in the dark.

It was stuck fast, but Bo held on, a sense of power beginning to bubble through her fingertips, up her arm, spreading like cold light into her entire body.

Don’t let go! said the river, its deep, green voice flooding her mind once more.

‘What the—?’ Bo managed to utter, as above her head the seagulls fell silent and the geese dipped out of sight. The morning sky turned dark and the city vanished.

A spiralling feeling began in her stomach, and she shut her eyes tight. No one had ever told her the river could do this. It was as if she were vanishing too, and she couldn’t stop. Bo’s body felt weightless, and somehow she was no longer on the foreshore but high above it, the wind in her hair, and she was flying like the flow of the river which was snaking far beneath.

Barely able to catch her breath, her eyes still closed, Bo could see a small room – winter it was, by the ice on the window and the fire in the hearth, full of the scents of herbs and spices.

How’s it winter? Bo wondered. Where have I gone?

Her attention was caught by an old man, standing in front of a table, upon which something was beaming silver and gold.

‘What’s that?’ Bo asked aloud – but the old man did not seem to hear.

‘Will they work?’ a voice said, from a corner out of Bo’s sight. It was a woman’s voice, desperate, insistent.

‘Tell me truly,’ the woman went on. ‘If I put them together, will they work?’

‘Lady, I make no promises,’ the old man replied, his voice tired. ‘I’ve done all I can.’

At these words, the vision began to vanish.

‘No!’ Bo cried, screwing her eyes tighter to keep the room there – but the vision was fading, the crackles of the fire dying to silent ash. The trapped wisps of warm sweet herbs evaporated. Bo heard the rush of the river, and the room disappeared. Once again she felt her stomach spiralling, the flow of the river coursing through her, the wind in her hair.

With a sudden sigh, the riverbed released the object from its mud and into Bo’s hand. She opened her eyes, staggering back. The dark sky was gone. The sun was up in the blue: the wharves and warehouses, the carriages and motor cars were all there on the opposite Chelsea bank, as if they’d never gone away.

It was an August morning again by Battersea Bridge. No winter, no room, no beams of gold and silver. She hadn’t moved.

‘What on earth?’ she whispered…