‘You’ve got a hunch, though.’

‘It could be that it was never meant to be,’ said the Doctor. ‘It could be that Hereafter was too difficult a world to convert. Or a fault might have developed in the main atmospheric processors. Or…’

‘Or?’

‘Nothing,’ he said quietly.

‘Or what?’ she snapped.

‘Really, nothing.’

She gave him one of her looks.

‘All right,’ he said. ‘It could be… that there’s some kind of influence at work here. I’ve seen… something like it before, once or twice.’

‘What sort of influence?’ asked Amy.

‘Let’s not worry about it until I’m sure,’ said the Doctor. He began to stride along the snowy path between the trees with great purpose. ‘Let’s hope it’s a glitch. A processing glitch that I can fix.’

‘A glitch?’ asked Amy, narrowing her eyes to look at him.

‘Not even a glitch.’

‘No?’

‘Less than a glitch. Smaller than a glitch.’

‘Glitch- ish?’ she asked

‘Exactly!’ declared the Doctor. He looked back at Bel. ‘Which way from here?’ he called brightly.

Rory doubted he could run much further. His legs and lungs were hurting from the effort, and his heart was pounding. He could barely draw a deep enough breath.

There was cold sweat on his spine under his clothes.

This was certainly not the way he’d have chosen to spend Christmas.

He smelled something suddenly. It wasn’t a strong smell at all, but against the clear, pure atmosphere of the woodland it stood out sharply. It was a warm smell, wet and metallic, like the linty steam of a laundrette, or the outflow of the industrial washing machines at the back of Leadworth hospital. What was that? What could possibly be warm and wet in a place so locked in by ice and snow?

He came through a stand of trees to a lip of rock. A bank fell away below him, thick with slithered snow.

Below him was a river. It was quite broad, falling steeply down the rocky throat of a gorge to his right into the steep cut basin below him. The far side, steeper than the one he stood on, was densely packed with trees.

The river, once it had cleared the jumbled, snowcovered undergrowth of the gorge, was ten or twelve metres across. It had frozen over with a thick crust of ice that had been overlaid with the previous night’s snow. It looked like a broad stretch of pale concrete. The gorge clearly trapped cold air over the open stretch of water.

Rory glanced over his shoulder. The green thing was still in pursuit, trudging through the trees, occasionally raising a clamp-hand to snap branches out of its way. It was thirty metres behind him and closing.

Rory had speed on his side, but not stamina. The thing just kept going. Rory knew he’d have to rest soon. He was exhausted, as if he’d run a marathon only to find there was no one waiting for him with a tinfoil blanket and a bottle of orange squash.

He made a snap decision. The density of trees on the far side of the river looked as though it offered the best chance of hiding he’d seen all day. The river was an added plus too. The green thing was big and evidently heavy. Rory doubted the ice, even though it looked like bullet-proof plate glass, would support a weight like that.

He hurtled down the bank, almost falling and rolling. He kept his footing, skidding down the snow like a downhill skier. He picked his way through the snowy rocks and boulders by the river’s edge, sliding on small, trapped puddles of ice, and reached the river.

Rory knelt down, reached out, and tested the ice with his hand. He applied firm pressure. It felt rock solid. As a nurse, he’d seen plenty of people brought into casualty with hypothermia and worse after falling through ice into ponds and lakes. Going out on ice was a stupid, stupid risk to take. Then again, none of the accident victims he’d seen being wheeled in on stretchers had taken to the ice because they were being chased by a two-metre tall, bipedal crocodile with baleful red eyes and a ray gun.

On cue, the green thing appeared at the top of the slope behind him. The afternoon sunlight flared off its pie-segment red lenses as it turned its ridged head to look at him.

Rory got up. He put a foot out onto the ice, let it take his weight, and then gingerly stepped clear of the bank. It was slippery, despite the dusting of snow. It felt like a window lubricated with washing up liquid under his feet. He took one step, and then another, arms wide for balance, teetering. The ice beneath him creaked. It made the sorts of popping, squealing protests that polystyrene packing made when you got a new TV or microwave out of its box.

He wobbled. He took another step. Another.

Another.

He glanced back. The green thing was coming down the slope after him, surefootedly negotiating the deep snow. It had a clear view of him. It could shoot him now. He was an open target.

He took another step. He took another. He was almost halfway across.

The ice gave out under him.

He plunged straight down into the river as though a trapdoor had slammed open underneath him. The moment he went, he knew he was done for. Even if the shock of the freezing water didn’t actually kill him stone dead, he was miles from help and medical attention. His body temperature would drop sharply, and never recover. He would seize up and die.

He went under, right under the water. He was braced for the terrible cold. It was so cold, it seemed to burn him. Then he realised it wasn’t cold at all.

The water under the crust of ice, fast-flowing and brisk, was warm. The water was warm.

Rory floundered, baffled. He struggled for the surface. Above him, he saw daylight. The ice had given way in several places, its disintegration prompted by the hole he had caused. The warm water was eating away at the edges of the plunge-hole, like a corrosive agent at work, broadening it and creating a channel.

He struck up towards it, arms churning, weighed down by his waterlogged parka. He broke the surface and took in a lungful of air. The cold stung his face.

The warmth of the water was almost like a blessed relief from the gnawing ache of winter.

Spluttering, he started to tread water, the motion of the water rotated him in the ragged space he’d made in the ice cover.

He saw his monstrous pursuer. It had reached the bank and was staring out at him. He was right there in front of it, but it didn’t seem to register him properly.

Heat, he thought. Heat. It was following my heat.

Now I’m in hot water, I’m harder to detect. It can still see me, but my thermal image is more difficult to isolate.

Rory took a deep breath and went under. He didn’t want to be visible at all. He wanted the water to mask him entirely.

‘In hot water’ indeed.

He wanted the river to carry him along and hide his trail from the creature.

For a moment, almost jubilant, he considered his luck. Falling through the ice had seemed to represent certain death, until he discovered the water beneath was warm. Being cornered in the water seemed to represent a second certain death, until it became apparent that the surrounding heat was confusing his implacable pursuer.

Then Rory realised there was a downside after all.

He swam underwater, borne along by the current, intending to surface for another lungful of air further downstream.

But he was under the ice again. He struck it from below, expecting it to splinter and give, but it was solid. It was as hard and firm as an oak lid on the top of the river. There was no air. There was no gap. There was no space for him to grab a breath.

He wasn’t going to die of hypothermia or temperature shock. He wasn’t going to be broken or blasted by a giant green monster.

He was simply going to drown.