It was a grey afternoon, the kind where the sky looks like a big blanket of dull wool and the sun has hidden away who-knows-where. Lily, Ziggy, Skye and Dash were out in the garden, and they had built a boat out of three chairs, a broom and an old blue bedsheet.

"I'm the captain!" said Lily, who had blue eyes and a strong spirit. "A storm of stinky socks is coming over the horizon! Hold on tight, crew!"

Ziggy burst out laughing and jumped onto the chair, pretending to fall into the sea. Dash rowed super fast with a wooden spoon. Skye, calm as always, tucked a cushion behind them so nobody would get a boo-boo.

Then, all of a sudden, the wind stopped. The leaves stopped rustling. Everything went quiet, like when something special is about to happen.

And from far away came a sound the four friends knew very well: "CHOO-CHOOOO! CHOO-CHOOOO!"

It was Toby, the magic train, with colorful smoke puffing out in little clouds and wheels that sparkled like candy. He stopped right in front of the gate and said, as always, in rhyme:

"CHOO CHOO, all aboard, climb up without fear,
I'll carry you high where the soft clouds are near.
A castle of grey has gone dark, lost its light:
be kind, and the stars will shine back so bright."

The four friends looked at each other with shining eyes. A castle? A light to switch back on? They didn't need to be told twice, and they hopped aboard.

The little train set off gently, rising up through the clouds, higher and higher, past the little birds and over the rooftops. Through the windows they watched the world grow smaller and smaller. Then Toby slowed down and came to a gentle stop at the top of a hill.

When they climbed down, the children stood with their mouths wide open. In front of them, inside a thick white mist, stood an enormous castle: Mistlong Castle. It had towers so very tall, but it was all dark and silent. The crystal chandeliers were black, and the great door stood ajar, like a mouth waiting.

From far, far away came a sound: "Rrrumble... rrrumble..."

"What's that sound?" asked Dash, snuggling closer to Skye.

Lily pricked up her ears. "It's a thunderstorm," she said. "But it's far away, very far away. Hear it? It grumbles like a hungry tummy asking for a snack."

They all burst out laughing, and their fear shrank as small as a pea.

The mist, though, was so thick you couldn't see your hand in front of your nose. Skye reached her arms out into the white and found Lily's hand. "Let's stay close," she whispered.

All at once, out of the dark, came a thin little whisper, light as rustling leaves: "Shhh... quick... this way..."

The children froze, their hearts going thump-thump.

"Who are you?" asked Lily, plucking up her courage.

"I'm the castle's gardener," whispered the voice. "I take care of the flowers when twilight comes. But I can't let anyone see me."

"The twi... the what?" asked Ziggy, scratching his head, puzzled.

"Twilight," the voice repeated, sweet and patient. "It's that moment when the day says goodnight, and it isn't night yet but it isn't day anymore. The sky turns first pink and then deep blue."

"What a beautiful word!" said Skye, and she repeated it to herself, "twilight," so she'd never, ever forget it.

Touching the cold wall and following the voice, the children understood: the one speaking was a gnome with a pointy red hat, who smelled of wet earth and little mint leaves. He was so shy that he always stayed hidden in the shadows, afraid of being a bother.

"The castle has fallen asleep," whispered the gnome, sadly. "Up there, in the attic, all its memories have gotten mixed up. They're tiny magic lights, but now they roll around dark and confused. And if the storm arrives before they're back in their places... they'll go out forever."

"RRRUMBLE," went the thunder, a little bit closer than before.

Lily clenched her fists and took a deep breath. "Then there's not a minute to lose. We won't give up! Let's follow the voice, one step at a time."

"But we can't see a single thing!" said Dash.

"Then we'll use our ears and our hands instead of our eyes," Lily answered. "Close them almost all the way and trust me."

The gnome whispered: "Good... this way... touch the wooden handrail... and climb up slowly."

The children climbed a long spiral staircase, feeling the smooth wood with their fingertips. Lily was in front, then Skye, then Ziggy, and Dash came last in line. They all held hands, like a little train in the dark.

Halfway up, a step went "CREEEAK!" and shifted under Dash's foot.

"Help!" he cried, slipping backward.

But Ziggy, quick as a cat, grabbed him by the shirt. "I've got you, I won't let go!" And he pulled him up with all his might.

"Thank you, Ziggy," said Dash, out of breath. "You saved me."

They started climbing again, while the thunder kept grumbling: "RRRUMBLE."

At last they reached the attic. And there, even in the dark, they saw something wonderful: lots of tiny glowing spheres, like sleeping fireflies, scattered everywhere. Some rolled lazily across the floor, others flickered in the corners.

"These are the castle's memories," whispered the gnome. "Each one has to go back into its own little nook, in the wall. But all by myself, I can never do it. And besides, I'm afraid of making a mistake."

Lily scooped up the first little light in her hands. It was warm and glowed faintly, like a tired little star. She slipped it into a nook in the wall, and right away the little light switched on, strong and bright.

"It works!" shouted Ziggy, clapping his hands.

But there were so, so many little lights, and the storm was coming closer and closer: "RRRUMBLE! RRRUMBLE!"

"We're too slow," said Lily, looking around. "We'll never make it on our own."

That's when an idea lit up inside her, bright as a memory. "Let's make a chain! I'll pick up the little lights and pass them to Skye. Skye passes them to Dash. Dash passes them to Ziggy. And Ziggy puts them in the nooks. That way we'll be super fast!"

And the relay began. Lily gathered, Skye passed them along gently, Dash gave two little hops and handed them over, Ziggy tucked them in. One little light, then another, and another still. Every time a memory went back to its place, it switched on, and its light made it easier to see the one next to it.

"Let's pass one to the gnome too!" said Skye. "He shouldn't have to stay all alone in the shadows."

The gnome, shy, reached a trembling little hand into the dark. Lily set a little light in it and said: "Thank you for everything. Without you we would never have made it all the way up here."

The gnome was left speechless. In all those years, no one had ever thanked him. A happy tear slid down onto his red hat. And then, slowly, he placed his little light in the nook, just like the others.

And here something incredible happened. The castle's little stone statues, the ones that had always stood still in the corners, began to move. They had seen so much kindness, and kindness, as everyone knows, is catching: one statue passed a little light to another statue, and that one to another still, laughing with joy.

Now there were ever so many of them passing the memories along, hand to hand, faster and faster than the storm.

"RRRUMBLE!" The thunder was almost right over the castle now. Just one little light was left, the very last one, and it was rolling straight toward a dark crack in the floor.

"We're losing it!" shouted Dash.

Lily didn't think twice. She dove forward, stretched her arm out as far as she could... and caught the little light with her fingertips, a moment before it vanished into the crack.

"Quick, into the nook!" whispered the gnome, holding his breath.

Lily, with her heart pounding hard, got back up and put the last little light in its place.

The thunder grumbled one last time, "rrrumble"... and then, all at once, it went quiet.

For a moment there was only silence. Then, all together, the crystal chandeliers in the attic lit up with a sound of tinkling bells. The light slid down the spiral staircase, ran along the corridors and reached all the way to the great Throne Room, where the golden throne began to shine again and the red carpets seemed to smile with happiness.

The mist melted away like sugar in warm water. And from the castle windows the children saw that the storm had passed on by, beyond the hills, without doing any harm at all. In its place, up in the sky, a whole sea of stars had opened, shining like a thousand little lights all switched on.

The four friends burst out laughing and hugged each other tight.

And then, slowly, the gnome with the red hat stepped out of the shadows. He was tiny, with a soft white beard and two eyes as sweet as honey. "Thank you," he said, and this time he wasn't whispering anymore. "You didn't just light up the castle. You lit me up too. Now I know I don't have to hide anymore, because I have friends."

"We'll come and visit you," promised Skye, and she gave him a little kiss on his hat.

Outside, under the stars, Toby was waiting for them, with his colorful smoke tickling their noses.

The four friends climbed aboard and waved goodbye to the gnome, who, from the garden that was glowing now, waved back, swishing his red hat.

Toby whistled softly and said, in rhyme:

"CHOO CHOO, come home now, little hearts so true,
the kindness you gave is a treasure for you.
One kind little act set the next one alight,
and the dark turned to stars in a single night."

The little train glided gently among the stars, while the four friends, tired and happy, held hands. Skye, as always when she was happy and a little drowsy, dozed off sweetly, whispering her new word: "Twilight..."

And up there, at the top of the hill, Mistlong Castle shone brighter than it had ever shone before.