Grabbing the centre of the string, he began to bend it in half, and moved Clara so that she was standing next to Mae. ‘You can cheat by using something called a wormhole.’

‘And that’s my sock?’ asked Warren.

‘Exactly!’ said the Doctor, slipping one end of the argyle patterned tube over the nectarine and the other end over the satsuma. ‘Think of it like a short cut through space and time. A quick way to travel vast distances without all the hassle.’

‘So is that how the TARDIS travels through time and space?’ asked Clara.

‘Sort of, yes,’ said the Doctor. ‘Well, no actually. The TARDIS travels through the Vortex, which is – in layman’s terms – a huge, complicated, multidimensional, trans-temporal … thingy. And like Warren’s sock, it comes in two lovely colours.’

‘Well, I’m glad we got that cleared up,’ said Clara.

The Doctor whipped out his sonic again and went back to scanning the wall beside the hospital entrance. ‘So now you know what a wormhole is,’ he said. ‘You can keep the fruit, by the way.’

Clara and Mae smiled and pocketed their treats. Warren took what was left of his sock back, scowled at it and dumped it in a nearby bin.

‘The wormhole surrounds the entire world,’ said the Doctor. ‘But this hospital is a weak point – lots of grief played out on a daily basis and yesterday in particular – so it will be the easiest spot to breach. All I have to do is find the right frequency to reverse the polarity of the neutron flow …’ He flicked from one sonic setting to the next until, suddenly, the wall began to shimmer, as though a vertical expanse of water.

‘Amazing!’ breathed Clara, stepping towards the entrance of the wormhole, arm outstretched. The Doctor quickly slapped it away. ‘Ow!’ she cried.

‘What did you do that for?’

‘You were about to stick your hand through into another world!’ the Doctor said. ‘Apart from the fact that the wormhole is likely to vaporise human flesh and zap your fingers into a billion different directions – you don’t know what’s on the other side if your hand does make it intact. You could be reaching into somebody’s bathroom, and that’s not very good manners.’

Clara pulled her hand back, scowling. ‘So how do we get through this thing if it will vaporise us on contact?’

‘We use a vehicle,’ explained the Doctor. ‘The metal frame will act like a faraday cage, absorbing all the zappy bits and keeping the gooey stuff inside safe from harm.’ Then he added: ‘We’re the gooey bits.’

‘So, back to the ambulance then?’ said Mae.

‘Bingo!’

They ran across the parking lot to where they had left the ambulance earlier, and were climbing in when the Doctor spotted something in a small clump of trees that separated the hospital grounds from the main road. ‘Oh, no …’ he said, and hurried over.

Dr Mairi Ellison was sitting beneath one of the trees, her cheeks wet with tears. She was holding hands with one of the Shroud. The Doctor approached and made to lift the woman’s veil, only for Dr Ellison to turn and glare at him.

‘Get it away from me now!’ she screeched. ‘Get it away!’

‘The anger stage,’ said the Doctor, sadly. ‘The Shroud’s grip is intensifying. I will get it away from you Mairi, you just have to trust me.’ He reached over and carefully lifted the woman’s blue veil to reveal the eyes of an elderly man.

‘It could be her father, perhaps,’ suggested a voice. The Doctor looked up to find Clara standing behind him. He let the veil drop.

‘Come on,’ he said, turning to stride back towards the ambulance. He climbed into the driver’s seat and fastened his seatbelt. ‘Let’s do this,’ he said, firing up the engine with his sonic.

‘Wait a minute,’ said Warren, leaning through from the back section of the ambulance and resting a hand on the Doctor’s shoulder. ‘We’re seriously about to leave Earth and go to another planet?’

The Doctor beamed. ‘Yep!’

‘And we’ll be able to breathe there? It’ll have oxygen?’

Clara and Mae turned to the Doctor in alarm.

‘I was hoping nobody would think to ask that,’ said the Doctor. ‘But you did, Warren. Well done. Go to the top of the class and have a gold star in putting the shivers up everyone.’

‘So?’ said Clara. ‘What’s the answer? Will we be able to breathe?’

‘Almost certainly,’ replied the Doctor. ‘Make that probably. A strong probably.’

‘How can you possibly know?’

‘Away from the mental tentacle thing, the Shroud have a basic humanoid biology,’ the Doctor explained. ‘That means they’ll need the same sort of atmosphere as we do.’

‘You’re guessing, aren’t you?’

‘Have been since we got here,’ said the Doctor. ‘But don’t let that put you off. I’m sure there’s a plan forming in my head somewhere. It’s bound to let me know when it’s ready. Now, if nobody has any further questions, I suggest we drive at full pelt into that brick wall over there and hope we come out the other side on an alien planet.’

Revving the engine, the Doctor stamped down on the accelerator and drove at the shimmering wall. Everyone in the ambulance closed their eyes and found something to grip on to.’

‘Geronimo!’

They emerged inside a long tunnel that looked as though it was made of solid, grey thunderclouds. Like concrete that had been disturbed before it could set properly. The Doctor brought the ambulance to a halt, switched on the headlights and whistled appreciatively.

‘Wow!’ he said. ‘That is new.’

‘What is?’ asked Mae.

‘The wormhole,’ said the Doctor. ‘It’s like nothing I’ve ever seen before.’

‘Yeah, cos usually you’ve seen one wormhole, you’ve seen them all,’ said Clara.

‘Precisely,’ said the Doctor, missing the sarcastic tone. ‘I’ve not been through that many, but they’ve always been instantaneous before. Arrive at the other end at the exact moment you pass through – but this is incredible.’

He reached for the door handle, only for Clara to stop him. ‘What are you doing?’

‘Going outside for a better look,’ said the Doctor.

‘But you’ll let all the air out!’

The Doctor’s eyes narrowed. ‘We’re in an ambulance,’ he said. ‘It’s not airtight. Air started getting out the second we arrived. As we’re not rolling around with skin the colour of the TARDIS, I’d say there’s air seeping in as well.’ Then he opened the door and hopped out.

Warren and Mae climbed out of the back of the ambulance and, after a few seconds of pouting, Clara joined the group.

‘It’s tricky to walk,’ said Warren. ‘Uneven underfoot.’

The Doctor stooped to run his fingers over the ground. ‘It looks like the Vortex,’ he said. ‘Only dead.’

‘Maybe this is what happens to wormholes after they die?’ suggested Clara.

The Doctor shook his head. ‘They can’t die,’ he said. ‘They’re not alive to begin with. They’re just passageways. Tunnels from one time and place to another.’ He pulled out a pair of ornate opera glasses and peered down the wormhole through them. ‘And can you see? There’s a light at the other end.’

He passed the glasses to Warren.

‘I can see it,’ he said. ‘It’s shimmering – just like the wall of the hospital was.’

‘Then that must be where it comes out,’ said Mae.

‘The exit.’

‘But it can only be a couple of miles away,’ said Warren, handing the opera glasses back to the Doctor. ‘Not even as far as the Moon.’

‘You’re forgetting,’ said the Doctor. ‘The actual wormhole may only be a few miles long, but it may cover several thousand light years, or even more.’

‘So we’ve still no idea where it will take us,’ said Mae.

‘Only one way to find out,’ said Clara, opening the driver’s door for the Doctor. ‘All aboard. Next stop, the mysterious world!’

They climbed back into the ambulance and continued driving. The journey was hard going, with the Doctor having to swerve to avoid some of the larger protrusions jutting up from the tunnel floor. More than once, they heard a metallic rasp as the side of the ambulance scraped along the sides of the wormhole.