Saffie stared at the number. It couldn’t have been a phone number; it was too short, and it was unlikely to be a computer password, which would probably have needed to be a combination of letters and numbers, never mind special characters and capital letters. It had to be the combination for some kind of lock. But where was it?

Saffie made to close the book, intending to search the rest of the apartment blindly, but as the paper moved, she noticed a shimmering smudge on its leftmost corner.

Acorn seemed to have noticed it too, as he cautiously moved his nose towards it, sniffed it, then reeled back in disgust.

“What is that stuff?” Saffie asked him, but Acorn wasn’t listening. Instead, he pounced off Dax’s computer desk and began sniffing the carpet before following an invisible trail to Dax’s boiler cupboard.

“Oh, good thinking Saffie,” Holly called as Saffie followed Acorn and reached for the boiler’s door. “Can you switch off both day and night heaters? He’s not going to be needing hot water for a while.”

Saffie murmured an absentminded “Sure,” and opened the door. Perched on a shelf next to the boiler was a combination safe with a digital five number input. The number in the sketchbook had to be the combination for it, which meant the Onyx had to be inside.

With sweating palms, Saffie entered the numbers 67209.

There was click, and the door popped open.

Without hesitation, Saffie grabbed the handle, swung the heavy door outwards, and peered inside.

Her elation deflated immediately. It was empty.

Whatever the Onyx was, it had been stolen.

“He had ten packs of marshmallows in his flat,” was the first thing Holly said when they got back home. “Can you believe that, Pete?”

Saffie’s dad was too engrossed in his Sunday newspaper to respond.

“A bunch of druids have been hanging around an old chapel in Hackney,” he said, ignoring Holly and showing them a picture of the chapel, which looked abandoned and derelict. “Druids! They’re all on drugs and they pay no taxes! Listen to this - the council are letting them squat there! Disgusting.”

“Pete, will you listen to me?” Holly snapped.

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Saffie’s dad continued to ignore her mum by reading another article out loud: “Restaurant owner Jade Sakata, of Sakata Sushi in Soho, has come under fire from investors after failing to develop her proposed multi-million pound restaurant and entertainment empire in London’s newest skyscraper, The Trellis.”

“Pete, will you put that bloody paper down?!” Holly raged. “Ten packs of marshmallows!”

“Dax likes marshmallows,” Saffie said in frustration. All she’d heard the entire morning was her mum criticising Dax and the way he chose to live his life. She was sick of it.

Holly turned to her sharply.

“Yes, well I like almond butter, it doesn’t mean I should stockpile it in preparation for a nuclear fallout!”

Despite the comment, Holly did stop banging on about the state of Dax’s apartment for the next hour, at least.

Lunch was a quinoa and beetroot salad that Holly had pre-prepared the previous day.

“What time can we go to the hospital?” Saffie asked through a mouth full of grains.

“Honey, you don’t need to visit him every day,” said Peter. “He must have friends.”

“Friends?” Holly laughed cruelly. “Pete, this is Dax we’re talking about. He doesn’t have any friends.”

“Dax DOES have friends,” Saffie defended him.

“Really?” Holly scoffed, raising an eyebrow. “Who? Please tell me.”

Saffie tried to remember those who were part of his clan in the game League of Lore.

“Well, there’s AngelDestroyer995, LucyIsAlwaysLate, oh, and Sir_Fartsalot.”

Holly closed her eyes and muttered something under her breath.

“Angel Destroyer?” said Peter. “That is NOT a real name.”

“No,” said Saffie, “it’s his username. I think his real name is Mohinder.”

“So how come none of these so called friends have paid him a visit in hospital?” Holly said, folding her arms.

“None of them live in the UK,” said Saffie.

Peter scoffed. “Don’t tell me, they live in Timbuktu!” and let out a bellowing laugh.

“Close,” said Saffie. “One lives in Sierra Leone.”

Peter cut his laugh short and frowned.

“Well, Lord Fluffbottom can’t be that good of a friend if he hasn’t travelled to visit him in hospital,” Holly said spitefully.

“It’s Sir_Fartsalot,” Saffie said flatly. “And besides, his best friend will be visiting him every day.”

“And who is that?” Holly demanded.

“Me,” said Saffie.

Peter, seemingly having had enough of the conversation, grabbed his newspaper from the living room coffee table and returned to the dining table with it, much to Holly’s annoyance.

As he shook it open, an unopened letter fell from it. It was the prettiest letter Saffie had ever seen - the paper was old, worn and yellowed with shimmering gold leaves patterned around its edges, and it was sealed with a blue wax stamp that had dribbled a little. The engraving in the stamp was that of a stag’s head encased in a flaming sun.

“Dad, there’s a letter,” Saffie said, seeing as though he hadn’t even noticed that it had almost landed in his mug of tea.

“What, darling?”

“A letter just came out of the paper. It must have got mixed in with it by acciden-”

Saffie cut herself short as she saw the name at the top of the address.

Sapphire Sparkes

Saffie ran to it and snatched it up.

“It’s for me!” she said.

Her dad lowered the newspaper and pulled his glasses to the tip of his nose.

“What are you looking at, dear?” he said.

“This awesome looking letter!” Saffie enthused.

She shook it, but her movements slowed as she saw the growing confusion on both of her parents’ faces.

They couldn’t see it.

The letter was in Overworld.