When the road bent left, Fox bolted down the side street, then practically threw herself into the antiques shop. It was quiet inside and specks of dust hung in the air, suspended in the sunlight above the clutter of antiques like thousands of indoor stars.

Fox glanced around. The shop was full to bursting. Where tables laden with old-fashioned weighing scales, copper jugs and dusty cutlery ended, pianos, grandfather clocks, old trunks and wardrobes began. There were spinning globes perched on threadbare armchairs, ship wheels wedged into corners, jewellery boxes stacked one on top of the other and chandeliers hanging from the ceiling. Every nook and cranny was filled with junk. Fox blinked at it all in disgust.

Then she stiffened as she heard Fibber’s footsteps thundering closer. Would her brother shoot on down the road into another shop or had he sensed, as twins often manage to do, that Fox had turned in here? She shoved his briefcase under a piano to buy herself a little more time, then seconds later Fibber crashed into the shop, sending a pile of antiquarian books flying.

He turned on his sister. ‘Where is it?’

Fox could hear the panic in Fibber’s voice, but she noticed he’d hung back on unleashing the usual insults: squit-face, moth-brain, scum-breath. Fox wasn’t exactly a fan of these terms, but at least when Fibber used them she knew precisely where she stood. She had no idea what game he was currently playing with his recent silences and his lack of stamping all over other people. She plucked a rusty telescope from a shelf and turned it over in her hands.

‘I dumped your briefcase in a bin on the high street.’ She paused. ‘So, if I were you, I’d go and find it before it’s carted off for ever.’

Fibber snatched the telescope from Fox and hurled it over his shoulder. ‘I didn’t see you hovering round any bins.’ He straightened his tie to show that he meant business. ‘What did you do with it?’

‘If you keep pestering me,’ Fox muttered, ‘I’ll dump you in a bin.’

There was a cough from the back of the shop and the twins whirled round to see an old man emerging from behind a wardrobe. He had dark, wrinkled skin and a fuzz of grey hair and in his hand he held a duster.

‘I was once plunged head first into a bin,’ he said, ‘by a boy called Leopold Splattercash.’ He shuddered. ‘Dreadful human being. Used to chew his own toenails.’ The man tucked the duster into his apron. ‘So, what can I do for you two then? Bearing in mind that I don’t, regrettably, stock any bins in this shop.’

‘You should do,’ Fox mumbled. ‘You’ve got enough rubbish in here to fill hundreds.’

The old man picked his way through the antiques towards the twins.

‘Don’t even try to sell us any of this junk,’ Fox said haughtily. Then she kicked the old-fashioned writing desk beside the old man, which sent the inkpot that had been resting on top clattering to the floor, and added: ‘Petty-Squabbles never buy second-hand; we don’t need to when we can afford the best of the best.’

She glanced at her brother, expecting him to say something equally rude, but he was too busy rooting through the antiques for his briefcase.

The old man picked up the inkpot, set it back on the writing desk and blinked at the twins. He didn’t often come across children. His customers tended to be adults and he and his wife had never had a family. But he had remained optimistic about them nonetheless, because he knew, from personal experience, that when worlds and kingdoms needed saving it was children who stepped in to sort things out. But the two in front of him now didn’t seem the world-saving types. At all.

And so it was with a great deal of surprise that the old man noticed the blue glow coming from the half-open drawer of the writing desk the girl had just kicked. He bustled towards it and drew out a small velvet bag.

‘Impossible,’ he murmured, tipping a marble into his palm.

The sun had dipped behind the street now and in the gloom of the cluttered antiques shop the marble was sparkling with a fierce little light all of its own.

Fox plucked idly at her plait. ‘I suppose you’re going to try and claim that this marble is one of a kind and worth stupid amounts of money.’

Even Fibber, who was still worried about finding his briefcase, couldn’t help but look up at the glowing marble. ‘What’s it got inside it? Batteries? Miniature lights?’

‘Magic,’ the old man whispered.

Fibber plucked the marble from his wrinkled hand, turned it over in his palm, then rolled his eyes. He was too old to believe in magic. But, just as he was about to hand the marble back, the man reached out and grabbed Fibber’s wrist.

‘The world is not as you know it. But if I was to tell you the truth – that we only survive because of four unseen, unmapped, magical kingdoms that conjure weather for our world – you would laugh at me, just as I laughed years ago when I was told the same thing.’

Fibber tugged his arm free, but the old man kept talking, his voice low and urgent as if, perhaps, he had been waiting for this conversation for a very long time.

‘You’ll have learnt, of course, about the terrible hurricanes seventy years ago, which almost tore our world apart. Scientists have never understood why those hurricanes stopped as quickly as they started, but that’s because it had nothing to do with science… It was because of magic.’

Fibber shook his head. ‘This is madness.’

Fox, for once, was in agreement with her brother. ‘I detest old people,’ she muttered. ‘The cardigans and the slippers and the non-stop knitting are bad enough, but the nonsense that comes out of their mouths is unbearable. If I was Prime Minister, I’d pass a law saying that anyone over the age of fifty should have their mouth Sellotaped shut whenever they leave the house.’

The old man ignored the twins’ comments. ‘When I was a child,’ he said, ‘I stumbled across a magical phoenix tear. And that tear transported me to Rumblestar, one of the four Unmapped Kingdoms, where a harpy called Morg was wreaking havoc with the magical winds that grew there.’ He shuddered as he recalled it. ‘But, with the help of some friends, I, Casper Tock, banished Morg and her followers from Rumblestar which, in turn, restored calm to our world’s weather.’

Fibber looked the antiques collector up and down. ‘You really think that you stopped the hurricanes because of something you did in a magical kingdom?’

Casper nodded. ‘Along with a girl called Utterly Thankless and a small dragon called Arlo, yes. Though I suppose we did have a bit of help from snow trolls and sun scamps, too. And Zip, a magical hot air balloon.’ He looked from Fox to Fibber. ‘I knew that one day Morg would hatch another plan to steal the Unmapped magic. She only needs to gain control of one of the four kingdoms for the rest to fall, so she won’t stop trying.’

Fox glanced round the shop. ‘I don’t suppose there’s any Sellotape kicking around in here?’

The old man ignored her again. ‘My wife, Sophie, and I have spent our lives trawling antiques fairs across the world, looking for another phoenix tear. Then we came across this shop for sale and I felt the pull of something familiar.’ Casper’s eyes shone. ‘It was the pull of magic. Tucked inside the drawer of this writing desk was that marble – a phoenix tear. I’ve been sure of it all along because, if you’ve encountered magic before, you know when it’s sitting in front of you again.’

The twins stared at the marble. The glow flickered mischievously in Fibber’s palm and for a second all thoughts about the briefcase, the Petty-Squabble fortune and Antarctica were forgotten.

‘Our planet is on its knees once again. If the rains don’t come soon, who knows what will happen? All of us are to blame for global warming. We could have done more sooner and stopped ignoring the signs around us. But it’s my bet there’s dark magic afoot, too.’ Casper paused. ‘It appears the phoenix tear’s magic is stirring and, for reasons far beyond me, I believe it has chosen you two as the ones to save us.’

For a moment, there was silence. Then Fibber snorted. ‘What a load of nonsense,’ he said.

‘And as for us sweeping in to save the planet,’ Fox added, ‘you can forget it. It’s not the Petty-Squabble way to start caring and helping and rescuing other people. What would be in it for us? No, it’s stamp or be stamped on – and we very much like to do the stamping.’

‘Exactly.’ Fibber dropped the marble back into Casper’s hand and glared at Fox. ‘So give me back my briefcase and maybe I won’t stamp on you.’

Casper tilted his head. ‘You mean the one under the piano?’

Fox stiffened as Fibber charged towards the piano and began rummaging beneath it. She looked from Casper to the marble, then back again at Casper. And in the old man’s eyes she saw something burning as brightly as the marble he held: hope.

He dipped his head at Fox. ‘Take the marble. Then run, girl, run headlong into this adventure. The Unmapped Kingdoms have chosen you and, when magic sets its sights on someone, it’s remarkably hard to wriggle free.’

Fox blinked. The old man was off his rocker – he had to be – but her plans lay in tatters, Fibber was on the brink of victory and there was something about this marble burning in the gloom. Something wild and hopeful. She grabbed it from Casper’s outstretched palm just as Fibber was raising his briefcase in triumph, then she turned and fled from the shop.

Fox tore back down the street. She couldn’t go back to the hotel because her parents had been very clear: come up with a plan or be posted to Antarctica. She had to get away from here. Immediately. And yet she had no idea where to go.

She hastened on down the street then the train station came into view once again, and Fox felt the marble tingle in her hand. Without thinking, she turned into the station, rushed past the empty ticket office and onto the echoing platform and there, like a gift – a glorious, hope-giving chance of a gift – was a train. And so strong was the pull of escape, of freedom, that Fox didn’t stop to consider that this train was a very old-fashioned steam train and that the steam pumping out of its chimney was, in fact, bright green.

She gripped the marble tightly, hurried along the platform and, though she didn’t know where the train was going to, leapt aboard. She turned to see Fibber dashing towards her. What was he doing? He had been desperate to find his briefcase and yet he wasn’t, it appeared, desperate to hurry back to their parents to reveal the business plan inside it. Had he been lying about the contents? What if his briefcase didn’t hold a genius business plan? Fox felt sure, though, that Fibber had something of value inside it, something he didn’t want to lose.

The train started chugging forward slowly and Fibber quickened his pace, throwing himself aboard just before the train gathered more speed. And Fox realised then that her world, which had seemed so ugly and unchanging before, now looked ever so slightly different. There were surprises and secrets bound up inside it: why on earth, for instance, had her brother followed her onto this train?

But it was only when the train doors snapped shut and Fox glanced down the carriage that she realised her world was filled to the brim with magic, too.