Amy reached past the Doctor and pressed her hand against the plate. A neon glow travelled up the metal under her hand. There was a click, and then a hiss, and then the hatch clanged shut in the faces of the Ice Warriors, locking them out.

The Doctor looked at Amy. They were nose to nose.

‘How did you know that would work?’ he asked.

‘I didn’t,’ she said.

‘Good thing it did, though, eh?’ he pointed out.

‘I would think so,’ she replied.

They both jerked back a step as several echoing blows were delivered to the other side of the hatch.

‘That’ll keep them out for a moment,’ said the Doctor.

‘What if they cut through the lock like they cut the other ones?’ Amy asked.

‘Oh, they will,’ said the Doctor. ‘But now we have a head start. And it’s because you’re human.’

‘What?’ asked Amy.

‘It’s because you’re human,’ the Doctor repeated.

‘And for those of us who aren’t fluent in non-sequitur?’ Amy asked.

‘You could operate the hatch,’ explained the Doctor,

‘because the palm-checker recognised your genetic code as human. It’s the same reason that Bel could open the door. Humans built this, so human gene-codes work the locks.’

‘Even human genes as old as mine?’ asked Amy.

‘Gene-code is gene-code,’ replied the Doctor.

‘Wait,’ said Arabel, flustered and looking at the Doctor, ‘if you can’t open the lock, doesn’t that mean you’re not human?’

The Doctor glanced awkwardly at Amy. ‘Ah, yes.

How do I explain that best?’ he began.

Before he could answer, Amy shrieked, ‘Samewell!’

Samewell’s outstretched hand was hovering above the palm checker. At Amy’s protest, he snatched it back.

‘I just wanted to see if it would work for me too,’ he said. He pouted.

Several more dull but herculean blows landed on the other side of the hatch and made him recoil.

‘I’d best try it on another door, though, eh?’

Samewell added.

‘I think that would be healthier for all of us,’ the Doctor said.

He looked around, and took stock of where they had ended up. It was a service room, full of metal shelves and hoppers. The shelves were racked with tools and equipment that looked part surgical and part horticultural. The hoppers were packed with mechanical spare parts wrapped in plastic. He picked up a few items and examined them.

‘We can’t really stay here, Doctor,’ Amy said.

‘No, we can’t,’ said the Doctor.

To reinforce her point, the banging on the hatch ceased, and was replaced by a high-pitched and very unpleasant wailing noise. It was like a dentist’s drill with the volume turned up.

‘No, we really can’t,’ said the Doctor. ‘That’s a focused sonic drill. They’ll be through that hatch in two shakes. Two shakes if we’re lucky. Probably more like one shake. Let’s get going.’

They hurried away from the painful noise to the end of the service room. There was another hatch, shut tight.

‘Now you can try it, Samewell,’ said the Doctor.

Samewell put his hand on the plate. The hatch opened.

Samewell looked extremely pleased with himself.

Passing through the hatch, they entered a gloomy corridor lit by a line of blue overhead lights. It stretched away in both directions. From the left came the roar of heavy turbine machinery. The Doctor took them to the right. He got Samewell to close the hatch behind them.

‘That’s two barriers they’ve got to get through,’ the Doctor remarked.

Feeling a little more secure, they walked briskly down the corridor.

‘What was that you were saying to the Ice Warriors before we slammed the door in their faces?’ Amy asked.

‘Oh, you know. Saying hello.’

‘How do you say hello to an Ice Warrior?’ she asked.

‘Um, “Hail, Ice Warrior”?’

‘You’re not as funny as you think you are,’ said Amy.

They reached another hatch. This time, Amy opened it.

The room on the other side was dark, but quickly woke up as automatic lights flickered on. The air smelled stale and slightly dusty. It was a large room, lined in pale white shipskin, with a wide, flat floor covered in odd patterns. The patterns were circles and spirals, inlaid in a fine, contrasting metal filament.

There was another hatch at the opposite end of the chamber, and one side of the space was fitted with complicated workstations and consoles. There were also two chairs facing the console station. The chairs had high, padded backs and raised armrests. The area looked like the cockpit of a spaceship.

The Doctor walked over to the workstation area. He seemed intrigued by the control systems. ‘Go and see what’s behind the next door,’ he said to the others.

‘Don’t go too far.’

‘What are you going to be up to?’ Amy asked.

‘I’m going to look at these,’ he said, leaning over the consoles. He ran a speculative finger along the fascia above a line of touch-sensitive pads. Dust came away on his fingertip. ‘I think I know what this is,’ he said.

‘In fact, I’m sure I know what this is.’

‘Really?’ asked Amy.

‘Give me a moment,’ said the Doctor, investigating further. He gestured over his shoulder. ‘Look at the floor, Pond. Look at the patterns on the floor. Where have we seen that before?’

‘Uh, I don’t know?’ Amy said.

‘Think about it. We’ve seen it recently.’

‘Really? I don’t know.’

‘Then hang on a minute,’ he said. He sat down in one of the high-backed chairs, laced his fingers together, and cracked his knuckles. He had already pressed a few switches at random. Several indicator lights had come on. The consoles began to hum with power. ‘Allow me to show you what this is.’

‘Have we really got time to stop and play around, Doctor?’ Amy asked.

‘We’ve got time to stop and play around with this,’

said the Doctor. ‘If,’ he added, ‘this is what I think it is.

And, as I believe we established, it is.’

Arabel and Samewell returned from their examination of the next door.

‘It’s another hallway,’ said Bel, ‘and then some rooms beyond that. We didn’t go too far.’

‘Good,’ said the Doctor. He adjusted some more controls.

‘What are you doing?’ asked Samewell.

‘He’s showing off,’ said Amy.

‘I’m not,’ said the Doctor. ‘I’m bringing these long-dormant systems back online, and juicing them up to operational power.’

‘Yeah, but he’s not telling us why he’s doing that, or what it is he’s doing it to,’ Amy said to Bel and Samewell, ‘and the reason is because that way it’ll be more impressive when it finally does whatever it’s going to do.’

‘No harm ever came from a bit of dramatic anticipation,’ said the Doctor. ‘There is an art to the building up of suspense. A prince from Denmark told me that.’ He gently tinkered with a few more settings, and then picked up a chunky remote-control device that slotted into a socket in one of the consoles. He stood up.

‘Come on,’ the Doctor said to them. ‘Come over here. Into the centre of the room. Hurry now.’

Power was building. They could all hear the ambient tone. Light levels in the room were starting to increase too.

‘What have you done?’ Amy asked.

‘It’s safe, I swear,’ said the Doctor. He made a tiny adjustment via the remote control.

The hum of the mounting power levels turned into a lazier throb, like a slowly cycling energy pulse.

‘OK,’ he said. ‘Ready? Hold on to your hats.’

‘I don’t have a hat,’ said Samewell.

‘You should get one,’ replied the Doctor. ‘Hats are cool.’

The Doctor pressed an activator on the control pad.

The light in the room around them altered quite dramatically. Not only did it shimmer and dim, the actual quality of the light seemed to change, becoming softer and less intense. It was like a scene change in a West End play. The effect was so odd, Arabel, Samewell and Amy all murmured in surprise.