‘Look, it’s simple. I know the answer, but you don’t have to take my word for it. The triangles follow a simple sequence. If there are N dots in the lower row of the last triangle, the next one will have N plus one more. Add one to two and you’ve got three. Add one to two to three, and you’ve got six. One to two to three to four, and you’ve got ten. Then fifteen, then twenty-one…’ Celestine paused. ‘Look, it’s senseless taking my word for it. Graph up a chequerboard display on your suits — Forqueray, can you oblige? — and start arranging dots in triangular patterns.’
We did. It took quarter of an hour, but after that time we had all — Hirz included — convinced ourselves by brute force that Celestine was right. The only missing pattern was for the fifty-five-dot case, which happened to coincide with one of the deep grooves on the right side of the door.
It was obvious, then. That was the one to press.
‘I don’t like it,’ Hirz said. ‘I see it now… but I didn’t see it until it was pointed out to me. What if there’s another pattern none of us are seeing?’
Celestine looked at her coldly. ‘There isn’t.’
‘Look, there’s no point arguing,’ Childe said. ‘Celestine saw it first, but we always knew she would. Don’t feel bad about it, Hirz. You’re not here for your mathematical prowess. Nor’s Trintignant, nor’s Forqueray.’
‘Yeah, well remind me when I can do something useful,’ Hirz said.
Then she pushed forward and pressed the groove on the right side of the door.
Progress was smooth and steady for the next five chambers. The problems to be solved grew harder, but after consultation the solution was never so esoteric that we could not all agree on it. As the complexity of the tasks increased, so did the area taken up by the frames, but other than that there was no change in the basic nature of the challenges. We were never forced to proceed more quickly than we chose, and the Spire always provided a clear route back to the exit every time a doorway had been traversed. The door immediately behind us would seal only once we had all entered the room where the current problem lay, which meant that we were able to assess any given problem before committing ourselves to its solution. To convince ourselves that we were indeed able to leave, we had Hirz go back the way we had come in. She was able to return to the first room unimpeded — the rear-facing doors opened and closed in sequence to allow her to pass — and then make her way back to the rest of us by using the entry codes we had already discovered.
But something she said upon her return disturbed us.
‘I’m not sure if it’s my imagination or not…’
‘What?’ Childe snapped.
‘I think the doorways are getting narrower. And lower. There was definitely more headroom at the start than there is now. I guess we didn’t notice when we took so long to move from room to room.’
‘That doesn’t make much sense,’ Celestine said.
‘As I said, maybe I imagined it.’
But we all knew she had done no such thing. The last two times I had stepped across a door’s threshold my suit had bumped against the frame. I had thought nothing of it at the time — putting it down to carelessness — but that had evidently been wishful thinking.
‘I wondered about the doors already,’ I said. ‘Doesn’t it seem a little convenient that the first one we met was just the right size for us? It could have come from a human building.’
‘Then why are they getting smaller?’ Childe asked.
‘I don’t know. But I think Hirz is right. And it does worry me.’
‘Me too. But it’ll be a long time before it becomes a problem.’ Childe turned to the Ultra. ‘Forqueray — do the honours, will you?’
I turned and looked at the chamber ahead of us. The door was open now, but none of us had yet stepped across the threshold. As always, we waited for Forqueray to send his float-cam snooping ahead of us, establishing that the room contained no glaring pitfalls.
Forqueray tossed the float-cam through the open door.
We saw the usual red stutters as it swept the room in visible light. ‘No surprises,’ Forqueray said, in the usual slightly absent tone he adopted when reporting the cam’s findings. ‘Empty metallic chamber… only slightly smaller than the one we’re standing in now. A door at the far end with a frame that extends half a metre out on either side. Complex inscriptions this time, Celestine.’
‘I’ll cope, don’t you worry.’
Forqueray stepped a little closer to the door, one arm raised with his palm open. His expression remained calm as he waited for the drone to return to its master. We all watched, and then — as the moment elongated into seconds — began to suspect that something was wrong.
The room beyond was utterly dark; no stammering flashes now.
‘The cam—’ Forqueray said.
Childe’s gaze snapped to the Ultra’s face. ‘Yes?’
‘It isn’t transmitting any more. I can’t detect it.’
‘That isn’t possible.’
‘I’m telling you.’ The Ultra looked at us, his fear not well concealed. ‘It’s gone.’
Childe moved into the darkness, through the frame.
Just as I was admiring his bravery I felt the floor shudder. Out of the corner of my eye I saw a flicker of rapid motion, like an eyelid closing.
The rear door — the one that led out of the chamber in which we were standing — had just slammed shut.
Celestine fell forward. She had been standing in the gap.
‘No…’ she said, hitting the ground with a detectable thump.
‘Childe!’ I shouted, unnecessarily. ‘Stay where you are — something just happened.’
‘What?’
‘The door behind us closed on Celestine. She’s been injured…’
I was fearing the worst — that the door might have snipped off an arm or a leg as it closed — but it was, mercifully, not that serious. The door had damaged the thigh of her suit, grazing an inch of its armour away as it closed, but Celestine herself had not been injured. The damaged part was still airtight, and the suit’s mobility and critical systems remained unimpaired.
Already, in fact, the self-healing mechanisms were coming into play, repairing the wound.
She sat up on the ground. ‘I’m OK. The impact was hard, but I don’t think I’ve done any permanent damage.’
‘You sure?’ I said, offering her a hand.
‘Perfectly sure,’ she said, standing up without my assistance.
‘You were lucky,’ Trintignant said. ‘You were only partly blocking the door. Had that not been the case, I suspect your injuries would have been more interesting.’
‘What happened?’ Hirz asked.
‘Childe must have triggered it,’ Forqueray said. ‘As soon as he stepped into the other room, it closed the rear door.’ The Ultra stepped closer to the aperture. ‘What happened to my float-cam, Childe?’
‘I don’t know. It just isn’t here. There isn’t even a trace of debris, and there’s no sign of anything that could have destroyed it.’
The silence that followed was broken by Trintignant’s piping tones. ‘I believe this makes a queer kind of sense.’
‘You do, do you?’ I said.
‘Yes, my dear fellow. It is my suspicion that the Spire has been tolerating the drone until now — lulling us, if you will, into a false sense of security. Yet now the Spire has decreed that we must discard that particular mental crutch. It will no longer permit us to gain any knowledge of the contents of a room until one of us steps into it. And at that moment it will prevent any of us leaving until we have solved that problem.’
‘You mean it’s changing the rules as it goes along?’ Hirz asked.
The Doctor turned his exquisite silver mask towards her. ‘Which rules did you have in mind, Hirz?’