She hurried into the police box in the corner of the laboratory. Yaz turned and beckoned to him. ‘Come on in.’

Will did as he was told. He’d glimpsed the interior up on the station, he’d watched five people walk inside the thing, but the impossibility of the space inside it still went against everything he’d been taught.

The Doctor started flicking controls on the TARDIS console, and a vista of deep-field space glowed into life on the 360-degree scanner that shrouded the crystalline rafters – the gaseous smudges and spirals of galaxies were scattered all around, little bigger than stars, and just as numerous.

‘Is that the whole universe?’ Yaz whispered with a thrill.

‘A tiny portion of it,’ the Doctor answered.

‘The observable universe is made up of maybe two trillion galaxies,’ Will added.

‘And we’re not just looking from Earth. With these 4D charts we’re looking out from galactic zero.’ Glowing frames appeared over certain areas as the Doctor worked, enlarging the galaxies mapped within those borders, zooming in to inspect key areas than vanishing as she moved onto the next starscape. ‘We know that Ace was time-stormed away to Svartos. That’s in the Ninth Galaxy. Well, I say Ninth but it depends where you’re counting from: it’s the Seven-Billion-and-Third Galaxy if you’re standing on Hastus Major.’ She was circling the console, stabbing here and there with her fingers like a heron poking waters for fish.

A dark-shrouded planet crowned with a colossal crystal citadel came into view.

‘Svartos,’ the Doctor announced, scrutinising the controls. ‘In 1987, Svartos would’ve been … round about here. And, yep, there’s a good strong space-trail showing.’

Yaz watched as a glittering line snaked across the virtual sky, as if Earth and Svartos were just two points on some enormously complicated join-the-dots.

‘So what?’ said Will. ‘Dorothy’s not on Svartos now, is she?’

‘No,’ the Doctor agreed, ‘but checking the various radiation signatures tells us this: that last night, Ace, Chantelle and Kim and the others were time-stormed somewhere else, at far lower power.’

‘In which case, they should be ten times closer than Svartos is,’ Yaz reasoned. ‘That’s good, isn’t it? You have the energy trace from Ace’s bat, and we have an area to search!’

‘Trouble is, the TARDIS has searched that area,’ the Doctor informed her. ‘And the bat’s not there.’

Yaz felt her shoulders sag. ‘Are you certain?’

‘Watch. Another broad-range scan …’ The Doctor yanked on a glowing lever and a criss-cross grid swept over the holographic universe above them. ‘Yeah. Certain. No trace from the bat, and no trace of time-storm trails converging.’

‘But if those time-storm trails are less prominent,’ Yaz argued, ‘they’ll just be harder to spot.’

‘Yes! Way, way harder to spot,’ the Doctor said. ‘And why do you think that might be the case, eh?’