Tahlia stood looking up at the strange design, a feeling of excitement rising inside her.

"I knew it!" she said. "I've tried playing with the ones I have come across before, but I could never get them to do anything."

She reached out a hand to touch one of the design's three concentric rings.

"Do not touch it!" snapped Dak, and the unfamiliar harshness in her friend's voice stilled Tahlia's hand instantly.

"Where does the door go?" asked Grifford, who had come to stand beside them.

"The access tunnels to the utility areas," said Dak, sounding far from enthusiastic. "It is where the workings of the fortress are. The pipe-work for the heating and water and sewers and such."

"And how far do these tunnels go?"

"They are going everywhere," said Dak.

Grifford stepped forward, looking up at the thing that Dak had called a moon lock.

"Sister?"

Tahlia understood Grifford's question instantly.

"Yes, there is one near our parent's rooms, near to where the servants take the laundry. It would have been easy for whoever took our brother to reach there without being seen."

"But that is an impossibility," said Dak, though there seemed to be little conviction in her voice.

"Why? It seems to make sense to me."

"It is an impossibility because these doors can only be opened by an Engineer."

"By an Engineer?"

"Yes."

"An Engineer?"

"That is what I am saying."

Tahlia frowned at the memory of a voice.

'That is an easy thing for you to be saying. It is me that is to be taking the risks.'

"Vlambra is an Engineer," said Tahlia as the realisation dawned.

"What!" said Dak and Grifford together.

"Vlambra is an Engineer."

"How are you knowing that?" said Dak, a look of shock on her face.

"Because of the way he was talking when he was in Merchant Dres' tent. It was different to when I saw him in the courtyard. He was talking just like you and your father talk. You know, 'I am thinking this,' and 'Do not be making me do that'. He was talking like an Engineer. And he was big. Not as big as your father, and nowhere as immense as a Forge-guard, but he was still big."

"No!" said Dak, her hands pressed to the side of her face. "An Engineer would never be having a complicit involvement in this!"

She was still staring at the wall in front of her.

Tahlia was also looking at the wall again. Another forgotten piece of memory had resurfaced in her brain.

'It is looking like an unnecessary risk if you are asking me. I am still not content at the thought of having to hide down in that stinking place for I do not know how long.'

"Did you say sewers?" she said to Dak.

"I did. Why?"

"Because I have just remembered that last night I heard Vlambra say something about hiding in some stinking place."

"Oh, but the sewers are not stinking," said Dak. "Well, excepting where the..."

Her voice trailed away.

"Where what?"

"I cannot be telling you anything more," said Dak. "I have said too much already."

"We cannot trust her," said Grifford.

"But I made a promise to my father!"

"That is convenient," said Grifford scornfully.

"Oh shush!" hissed Tahlia, before turning back to Dak. "Is a promise to your father worth more than my friendship?"

Dak swallowed.

"Tahlia, do not be saying that!"

"Well, is it?" said Tahlia, her voice rising. "If you know something that can help us find our brother, then you have to tell us."

"But it is not my business!" Dak moaned, but she was silenced by a sudden sound beyond the office door. It was the noise of wooden crates being moved.

"They are searching outside!" whispered Tahlia, looking at Dak, who herself was staring at the door back to the depot, her hands again pressed to the side of her face. "You have come this far Dak, so you're probably already in enough trouble if they find us. Tell us what you know."

Dak stood staring at the door for five seconds before turning back to the moon lock.

"The old sector that lies above the waste reservoir is having open sewerage pipes, and the Engineers are rarely going there."

"So?" said Grifford.

"So it is a perfect place for Vlambra to hide our brother while everyone searches in the wrong places," said Tahlia. "Kralmir is not in that merchant's cart, and he is not in the Encampment. He is here. In Klinberg."

Her brother's eyes were bright with indignation.

"We have to tell Master Hepskil about this," he said. "He can order them to be searched."

"I am thinking that is the best idea," added Dak.

Tahlia looked from her friend to her brother. What were they thinking! If they took this to Master Hepskil, things would take an eternity to get done. He would have to contact the Guild, and they would spend hours pontificating over protocol and precedent and authorisation. And if anything were ever agreed, they would doubtless send the Forge-guard in a lumbering search, and Vlambra would be aware of them before they even got close to his hiding place. With so many secret doors leading from these tunnels, she could hardly take that risk.

No, what was required here was a little stealth and a little cleverness. Dak would never see the sense in it, but Grifford...

"What if we were to rescue Kralmir ourselves?" she said.

"What!" said her brother.

"Just imagine, Grifford, if in the middle of all this chaos it were us who found our brother. Imagine what father would think of that!"

As she had anticipated, there was a sudden hungry look in her brother's eyes. He lifted his hand to the front of his tunic, to the demon-tooth pendant that she knew he always wore.

"You cannot be thinking this is a good thought," said Dak.

"Our father has failed to protect our brother, and the Order has failed to find him. Maybe my sister is right."

"Oh, no," sighed Dak, shaking her head.

"We can do this," said Tahlia. "It will be easy."

"But the door can only be opened by an Engineer," said Grifford. He stuck a thumb at Dak. "She said so herself."

"Dak?"

"Yes?"

"Can you open the door?"

"I am not supposed to be knowing how!"

"But you do know how."

Dak's hands dropped from her face, and her shoulders slumped.

"Yes," she admitted.

"So, open it," said Grifford. "Open it and let us go."

Dak looked at him, and Tahlia saw an unfamiliar expression on her friend's face.

"It would not be hurting to be asked," she said.

Grifford straightened himself up in front of her, his own eyes blazing.

"Could you open the door for us, Dak?" said Tahlia quickly, before her brother had the chance to say anything stupid.

* * *

Dak turned to Tahlia and saw the look of honest entreaty on her face. She knew the look to be well practised and probably insincere, but she had done as Dak had requested and asked. How could she now say no?

'That will teach you to be prideful, Dakskansia.'

She turned to the door and examined the moon lock.

This was not her business. This should not be Engineer business at all. But the mechanism on the secret passage through the shield-bastion had not repaired itself, and the doors that it led too had been purposefully unblocked. Those things could only have been done by the hand of an Engineer. Someone had made this into Engineer business.

But she had promised her father.

'Your father is not here. He is probably still drunk in his bed.' That unfamiliar feeling rose inside her again. 'How many promises has he broken to you, Dakskansia?'

"Dak!"

Tahlia's voice broke her from her thoughts.

"Yes?"

"Can you open the door?"

"Yes," said Dak. "I can."

She looked back up at the door. Despite her sudden resolve, the realisation of what she was about to do stoked itself inside her, making her stomach boil. She took a deep breath and tried to calm herself. Then she realised that she did not have the numbers she needed to run the cypher that would open the door, and her heart gave a disturbing thud in her chest.

But she also felt relief. She could not open the door, so the decision was out of her hands. Then her brain betrayed her. It presented her with a memory of her father's words on the day he had first shown her the moon lock. She remembered how he had described the source of the numbers.

They were linked to the door's location.

She knew the supply-depot was on the twenty eighth level of the fortress, so that would be her first number.

Twenty eight. She pictured the number in her head.

They were outside the keep, but still within sphere two of the fortress, so two was her second number.

Twenty eight and two. The second number joined the first.

"Well!" snapped Grifford. "Are you going to open it or not?"

"I have to concentrate," said Dak, her focus on the task she was attempting somehow shielding her from the boy's harsh words.

"Be quiet, brother," said Tahlia. "She has to concentrate."

Dak looked up again at the moon lock.

They were in the section of the north-west shield bastion, which was section six.

Twenty eight, two and six. The three numbers sat clearly in her brain.

Gear ratio! What was the day's gear ratio?

Without it she was unable to calculate the cypher, and she would look a fool in front of Grifford and Tahlia. She turned to see them both staring at her, Tahlia with a look of expectation and Grifford with one of scorn, and suddenly she remembered. She had seen the gear ratio that very morning as they had passed through the lower guild-yard, when the Engineer's clock had whistled the hour and nearly made her jump from her boots. She had looked up at the clock with the ratio sphere in its centre without really seeing it, but the image was somehow emblazoned in her brain.

Eight to twenty four!

Twenty eight, two and six, with a gear ratio of eight to twenty four.

"This will be taking a few moments," she said, turning back to the door and closing her eyes as she always did when she calculated the cypher. As always, she ran it through her head twice, and both times the answer was the same.

Two hundred and twenty seven.

So she was looking for the two hundred and twenty seventh day, which lay in the second week of the ninth month. She found the correct panel on the middle ring, then began to trace the maze of grooves around the design, turning the rings in her head.

Everything else in the room seemed to fade as her concentration locked onto the task in front of her, and she did not even notice Grifford and Tahlia withdraw from her shoulder and return to the room's far end.

* * *

Tahlia crouched and pressed her ear against the office door. She could hear the hollow wooden sound of crates being moved about in the room beyond, and then an echoing bang as something heavy was dropped onto the floor.

"Careful with that!" said an angry voice.

"Watch your tongue, clerk!" said a voice tinged with equivalent anger. "Your superior has left us with no time to be careful."

Tahlia heard the other voice muttering something, but she could not make out the words.

"Where are you going, now?" came the Executive-officer's voice.

"To the records office," said the clerk, whose own voice was suddenly close by the door.

Tahlia stiffened, but did not move, except to glance over her shoulder at Dak, who was still standing still and staring at the wall with the strange design. She pressed her ear back to the door.

"We have already searched the office."

"Well you left the glow-light on!"

The handle beside Tahlia's ear turned, and she leapt away behind the door. Grifford stepped back quickly, and his hand dropped to his sword.

"Leave it!" snapped the Executive Offices. "What's in those crates back there?"

The door opened a fraction, and Tahlia could see the clerk, his hand on the handle. His face was turned away, looking back into the Depot.

"They are empty, just as these are," he said.

"Then they must be opened, so come with me. The Pantler insisted that his staff be present while I search."

The clerk paused for a few more seconds before pulling the door closed with a slam.

Tahlia put her ear back against it, and she heard the clerk go, again muttering something inaudible to himself. When all she could hear was the banging and thumping of the soldiers continuing their search of the crates, she left the door and went back to Dak, who seemed unaware of the disturbance.

"Well?" she said.

"It is fine," said Dak. "I have it."

* * *

Dak reached out and began turning the three rings, until she was happy with the configuration, then she moved the two symbols from the edge around the new track she had created until they joined together on the correct panel. Then she moved the round dial in the centre of the design to the correct segment.

She studied the arrangement of all the pieces and knew instinctively that she was right. She reached out to the lock's central circle, and after taking a deep breath, she pushed it. It moved in smoothly, and there came a familiar deep dull thud from within the door, followed by the whirring of gears. The door swung open a fraction. She reached out and pulled it fully open, revealing the steady blue light from the tunnel beyond.

Tahlia pushed past her and peered through the door, her hands resting on the raised lower sill.

"By the light of Fortak's sun, I was not expecting that!"

She was staring at the clean metal corridor, with its uniform pipework and regular blue lights.

"No, I did not think that you would be," said Dak, the enormity of what she had just done sitting heavy in her chest once more.

Tahlia stayed crouching where she was. She looked left down the corridor, and then right, and Dak saw her body suddenly stiffen.

"What is it?" asked Grifford.

"You had better both come and see this."