The Doctor nodded. ‘Shall we go and take a look?’

He pulled a box of his everlasting matches from his pocket, struck one, and the two of them made their way down the steps, the sound of their footfalls echoing off the walls. The entire building was of a different construction to the rest of the city, hewn from some crystalline substance that seemed to suck in the light from the match, sending strange lightning-like patterns flashing through the walls.

‘The place looks completely untouched,’ said Ace, her voice reverberating eerily. ‘How come this building escaped when everything else has been destroyed?’

‘Because the Wraiths did not dare touch this place,’ said the Doctor grimly. ‘Partly because once they learned of what it contained they wanted it for themselves, partly because they feared what might happen if it was damaged.’

‘But what was it, Professor?’ Ace was beginning to lose patience with him. ‘What did this building contain?’

‘That.’ The Doctor pointed with his umbrella.

The stairs had ended at the entranceway to a vast, circular chamber ringed with columns made from the same glassy material as the corridor. In the centre was a pit, and from the pit rose a twisting, spitting column of blazing, luminous energy.

The Doctor entered the chamber, hurrying to one of six, tall, cylindrical control consoles that stood like sentinels around the edge of the pit. Ace followed him, staring up in awe at the energy column, feeling her skin prickle all over as she neared it, but getting no sensation of heat despite the brilliance of the light. As she stepped forward to get an even closer look, she felt a gentle pressure holding her back. She looked down to find the crook of the Doctor’s umbrella hooked around her arm.

‘Let’s not get too close,’ he murmured. ‘Not until I’ve made a few checks at any rate.’

Ace returned her gaze to the swirling vortex. ‘What is it, Professor?’

‘Incredibly dangerous,’ said the Doctor, studying the readouts that flickered on the control console. ‘The Astingir called it the Quantum Anvil.’

‘And this is what they used to win the war?’

‘Yes. A kind of temporal possibility engine.’ The Doctor nodded. ‘The Astingir drew the Wraiths to this planet and then used the Anvil to fracture the time stream of each and every one of them into infinite potential outcomes, leaving them unable to interact with the real world. They truly are wraiths now; ghosts created by the machine.’

Ace couldn’t tear her eyes from the madly thrashing maelstrom. There were strange shadows dancing through the ribbons of energy. ‘And you knew it was here?’