‘I should have killed you first, instead of Lester,’ Mitchell replied. ‘I can see that now: my stupid mistake. I had to find some way to stop you interfering. But I couldn’t take a chance on Lester and Amy sending out a distress signal, if I went for you first. But I screwed up, didn’t I?’

‘What’s too important f me to interfere in?’

Mitchell just laughed. ‘You wouldn’t understand.’

‘Try me.’

‘Would it make you turn back? See things my way?’

There had developed a subtle change in the tone of Mitchell’s voice. There was a touch of echo to it, as if he were no longer wearing his helmet. Saul searched for local networks, and came across a public map of the area that included the locations of several emergency access entrances just inside the mouth of that tunnel. It now yawned ahead of him, just a hundred metres or so away.

‘I said try me.’

‘I’m trying to save the human race,’ replied Mitchell. ‘Is that good enough?’

Saul nearly stopped dead in his tracks. ‘What?’

‘Some of the Founder species wanted to destroy the network, because they came to realize that, once they’d created a wormhole leading into the very far future, as a result time between the two mouths became fixed, immutable. Do you understand?’

Saul saw he was now very nearly at the tunnel entrance. He’d hoped that, if he could just keep Mitchell talking, he had a better chance of catching up. But his muscles were starting to protest, and Mitchell meanwhile sounded like he hadn’t done anything more strenuous than take a short jog.

‘That doesn’t make one damn bit of sense to me,’ Saul gasped.

‘You’ve heard of the observer effect in physics, right? Once you observe an event, an infinite range of possible outcomes collapses to just one. It’s the same with time. Whenever you create a wormhole, and move one mouth through space at relativistic speeds, you create a path into the future – but the objective time that passes outside the wormhole becomes fixed into a single, unchangeable destiny. Do you see? It’s the death of free will. That’s why the Founders fought each other for control of the networks. Some of the artefacts we found had been weapons in that war.’

Saul finally reached the mouth of the tunnel, his lungs burning from overexertion. He knew he had to keep going but, in truth, he wasn’t sure he could. He glanced back the way he’d come, but by now the lander was lost amidst the boulders and dust.

‘The growths?’ he panted, his back resting against the tunnel wall. ‘They were one of those weapons?’

Mitchell laughed. ‘You’ve got it all wrong. The growths aren’t trying to kill us,’ he said. ‘They’re saving us.’