The two of them started to make their way towards the city, following an ancient path that zigzagged down the mountainside. The path had obviously not been used for some considerable time, and there were several occasions where they had to detour around huge boulders that blocked their way, or find alternative routes where the track had eroded to nothing.

Eventually they stepped down onto the valley floor. At ground level the city was even more impressive. Ace hadn’t appreciated the sheer scale of the place. Even in their ruined state, the curved buttresses that she had spotted from the mountainside towered over them.

‘Who built this, Professor?’ she asked, running a hand over the wind-scoured surface. ‘And what happened to them?’

‘Ah …’ He gave a deep sigh, a pained look creasing his features. ‘This was the home of the Astingir; an ancient race of poets, explorers and inventors. This city was their proudest achievement. It was also the final battleground in their war with the Wraiths.’

‘A war that they lost, I’m guessing?’ said Ace grimly, staring at the devastation around her.

Before the Doctor could reply, there was a low, grating rumble and the ground beneath their feet started to shake. Ace stepped back from the towering buttress in alarm as dust and rocks clattered down around her and from somewhere in the distance a shattering roar heralded the final collapse of one of the already precarious buildings.

Ace looked to the Doctor for reassurance but the little Time Lord was hurrying off along one of the wide, rubble-strewn avenues, peering at the digital readout on his pocket watch and muttering worriedly to himself, oblivious to the masonry crashing down around him.

She jogged after him, suddenly suspicious that their arrival on this planet was, as usual, not as random as she had initially thought. She caught up with him at the doorway of a low, domed building; more intact than most of the others around it. The earth tremors had started to subside now and the Doctor was peering down a steep flight of steps that led down into the gloom of the building’s interior.

‘All right, Professor. What’s going on?’ demanded Ace. ‘We’re here for a reason, aren’t we?’

‘Unfinished business, Ace,’ said the Doctor, his piercing blue-grey eyes regarding her.

‘It always is with you,’ said Ace accusingly. ‘It’s something to do with that war, isn’t it?’

He nodded. ‘The Wraiths had all but destroyed the Astingir; driven them to the very edge of extinction. But those who had fought so hard had done so for a reason. They were giving the people in this building a chance to finish creating something. Something that would end the war. Something terrible.’

Ace stared down the steps into the darkness, unexpectedly feeling the skin on the back of her neck prickle. There was a power here. She could feel it. ‘It’s still down there, isn’t it?’