After being poked, prodded and ignored by their captors, Sarah was close to boiling point by the time they reached the place Euripides called the 'Temple of Life'.

It was, in fact, a large clearing in the heart of the forest, a nexus point for the paths through the trees. Here, the tribe had erected makeshift shelters from sheets of corrugated plastic and tempered steel. Sarah was surprised to see winking diodes and other signs of electrical power nestled amongst the foliage. Children gambolled around them, playing games of chase, and men were distributing food amongst themselves on tarnished metal dishes. Clearly, these tribes people weren't as primitive as she'd first imagined.

'In here,' said Euripides, gesturing to one of the huts. 'You shall face the judgement of the Elders.'

'How delightful,' said the Doctor. 'I do enjoy a good chinwag with an enlightened one,' he added, quietly.

'You're incorrigible,' said Sarah, unable to contain a laugh. She followed him into the hut, closely followed by Euripides, still wielding his spade.

Inside, two figures sat cross-legged on wooden plinths, at either side of another open doorway. One, on the left, was male. The other, on the right, was female. Both had their eyes closed, in meditative trance. They were similar in appearance to the rest of the tribes people, save for the fact they weren't carrying weapons.

Euripides gestured to a woven mat in the centre of the floor, and, with some reluctance, Sarah lowered herself onto it beside the Doctor.

'I am the left-tenant,' said the man, after a moment.

'And I am the right-tenant,' said the woman.

'You come before us as strangers,' went on the left-tenant, 'but I see you are not of the other tribe.'

'I'm Sarah Jane Smith, from Earth,' said Sarah.

'And I'm the Doctor.' He raised his hat in genial fashion, and then replaced it. 'I suppose you could say I'm a citizen of the galaxy.'

The right-tenant arched an eyebrow. 'You trespa.s.s within our borders. Were you sent by the pilot?'

'The pilot?' said the Doctor, his tone thoughtful. 'No, no. Not the pilot. We've only just arrived, you see.'

'Then you have come to plunder the crucible,' said the left-tenant, his voice growing in ire.

'No!' said Sarah. 'It's not like that at all. We're explorers. We're here to help.'

'To help?' said the right-tenant. She looked from Sarah to the Doctor. 'Then you come to protect us from the other tribe?'

'Yes, if you like,' said the Doctor. 'Tell us about them.'

'They hunger for the crucible,' said the left-tenant. 'Once, we were many, and our borders ranged for quadrants. Food was bountiful, and the wild beasts did not trouble us. Now, we have been forced to withdraw to protect the crucible. The other tribe grows strong, while we grow weak. Our people are diminished. The sheep and rats hunt us for food. The Temple of Life is our last remaining stronghold.'

'What do they hope to achieve, this other tribe, by capturing the crucible?' said the Doctor.

There was a collective intake of breath from the two Elders. They glanced at one another. 'They are heathens,' said the right-tenant. 'Unschooled in the Manual.'

'As surely as the pilot guides our way,' agreed the left-tenant.

'We told you, we've only just arrived,' said Sarah. She could hear the impatience in her own voice.

'The crucible is the source of all life,' said the left-tenant. 'The origin of the Uman species.'

'The well-spring from which we all flow,' added the right-tenant. 'When we reach Fall, the avatars will live anew.'

'And this other tribe want the glory for themselves,' said the Doctor. 'They want to be there when the avatars awaken. Oh, I've heard it all before. Usual story. And here I was hoping for something new.'

Sarah heard footsteps entering the hut behind her. She looked round. One of the tribes people was silhouetted in the doorway, but she could tell from the ragged breath and a glimpse of his troubled expression that something was wrong. 'They're here!' he said. 'They've come for the cruc-'

He broke off abruptly as the tip of a spear buried itself in his back, and he toppled forward, gasping for breath. Outside, Sarah could hear shouting. 'Doctor?' she said, panic rising.

The Doctor was already on his feet. He grabbed her hand and pulled her up beside him. 'In there,' he said, nodding toward the other doorway.

'You cannot enter the crucible,' said Euripides, raising his spade to block their path. 'No one but the Elders are permitted.'

Sarah watched as the two figures relinquished their positions on the pedestals and walked with some dignity into the adjoining chamber.

The Doctor, of course, was having none of it. He grabbed at the spade, wrenching it from Euripides' grasp and tossing it angrily to the floor. 'I told you we'd help, didn't I?'

Euripides hesitated and, with a quick 'Come on,' the Doctor led Sarah into the crucible.

It was not what she'd been expecting. Hearing the Elders go on about the place, Sarah had envisioned a grand temple, an ornate religious structure, but in the event the room was relatively small and underwhelming.

The floor was formed from a series of interlocking metal grids, while the walls and ceiling were bland metal plating, now tarnished and worn. Around the room were arranged a dozen lozenge-shaped pods, standing upright against the walls, each of them protected by a shattered gla.s.s dome. There were no plants in the room, save for the questing vines spilling from the front of the broken pods.

'It reminds me of s.p.a.ce Station Nerva,' said Sarah.

'Stasis pods,' said the Doctor as he circled the room, taking it in. 'They've all failed.'

'Then the people in them...' began Sarah. But it was quite clear. The people inside the pods had long ago decomposed, and now they were reduced to nothing but compost, giving sustenance to the plants.

'We must protect the crucible at all costs,' said the left-tenant. Sarah had almost forgotten he was there.

'Oh, I think we're long past that,' said the Doctor. He was pacing back and forth, a crooked finger held to his chin, looking thoughtful. 'The crucible... the pilot... the Fall...Yes, of course!' he exclaimed, evidently pleased with himself.

'They're colonists from Earth,' said Sarah. 'They've just forgotten they came here by s.p.a.ceship.'

'What?' said the Doctor, in astonishment. 'No, of course not. We're still on the s.p.a.ceship.'

'But...we can't be!' said Sarah. 'Wooded glades, oak trees, tribes of indigenous people...'

'Ah!' said the Doctor.

'Now what?'

'So you admit they're oak trees?'

Sarah harrumphed.

'Think about it, Sarah,' said the Doctor. 'Trails through the woods that seem like corridors. Blinking diodes in the walls. Farm animals that have evolved into killers. Stasis chambers that have given up the ghost. Then there's the devolution of language "left-tenants", the "Uman" race.' The Doctor glanced at the Elders, who were regarding him placidly. 'These people have been on this ship for a very long time. Or rather, their forefathers have. Long enough for their physiology to adapt. It's a generation ark ship, heading out to the stars. You see? The technology is the same as on the Ark.'

'But what about the plants?' said Sarah.

'Brought on board to generate oxygen,' said the Doctor. 'At some point they must have spread from their biodomes, slowly taking over. Behind all of that forest is a gleaming s.p.a.ceship interior.'

'And the people?' said Sarah.

'What remains of the colonists,' said the Doctor. 'And the original crew.' He glanced at the failed stasis pods to underline his point.

'Those are dangerous truths, Doctor,' said a woman's voice from the doorway. It was crisp and clear, and somewhat bizarrely, had an East European accent.

Sarah saw the Elders freeze. She turned to watch the Doctor approaching the doorway, just as a gleaming figure emerged into the dim light. It was humanoid, but made no pretence at replicating human form. It was smaller than the tribes people, about the height and build of Sarah herself, and appeared to be made from the same base metal as the walls. It was s.e.xless, with a moulded human face, eyes that shone a brilliant, electric blue, and an open slit where its mouth should have been. It was carrying a gun in both hands.

'The pilot, I presume?' said the Doctor.

'In a manner of speaking,' said the woman's voice. 'I'm operating this drone, along with a handful of others, which are currently pacifying the colonists.'

'How very timely,' said the Doctor. 'May I a.s.sume that we are cordially invited to the bridge?'

'Too right,' said the woman. 'You're going to explain exactly how you came to be walking about my ship.' The drone hefted its gun, and indicated for them to follow.

'Tally-ho!' said the Doctor.

The bridge, the pilot explained as they walked, was on an upper deck, accessible via a lift shaft that bisected the vessel through its midsection.

The Doctor walked ahead of her, deep in conversation with the drone that had found them in the crucible or rather, with the female pilot controlling it. Sarah couldn't make out much of what was being said, but she gathered the Doctor was explaining about the TARDIS and how he and Sarah came to be on the vessel in the first place, while no doubt doing his best to charm any relevant information from her.

After a while the forest began to thin, and the canopy gave way to brightly lit corridors, of a kind so familiar to Sarah. They were functional and clean, with luminous panels, sliding metal doors and ancient, dusty computer terminals installed at regular intervals. The place felt deserted, as if it hadn't seen life in decades. The drone guided them along, until, after what seemed like hours, they finally arrived at the lift shaft.

'After you,' said the Doctor, waving her through the portal and into the small metal box that would transport them up to the bridge. She noted the drone wasn't joining them, instead retreating to an alcove in the opposite wall, watching them with its eerie, expressionless face.

'Are you sure you know what you're doing?' said Sarah.

'Sarah,' said the Doctor, with mock chiding. 'Don't I always?'

'Now I know we're in trouble,' she muttered.

The lift shot up like a bullet in the barrel of a gun, almost knocking the Doctor sideways and forcing him to grab Sarah's arm for support. He grinned that ridiculous grin.

Moments later, the doors opened onto a small antechamber, devoid of any furniture or other signs of life. Stencilled on the wall, in bright red, foot-high letters, was the word 'PROSPERITY'.

'Of course,' said the Doctor. 'Now everything makes sense.'

'Care to enlighten me?' prompted Sarah.

'All in good time,' he said. He waved quiet any further protest as he crossed the room, heading for the hatchway that, she presumed, led through to the bridge.

The pilot must have been aware of them, as Sarah heard her voice, booming out from the console. 'Come in. And please accept my apologies. The place must be a state.'

Sarah ducked her head and stepped through the hatchway onto the bridge.

The first thing that hit her was the thick, cloying odour of decay. The second was the sight of the corpse in the pilot's chair. It was partially skeletal, but dry, leathery flesh still clung to the bones, encasing much of the chest and skull. The eyes had dried and shrivelled away, and the lips had curled back, exposing a gap-toothed smile. Wires erupted from the back of the skull, snaking away into a hatch in the ground. Sarah could see fluids bubbling in them.

'Oh... goodness...' she said, trying not to balk at the sight.

'Please don't be alarmed,' said the voice from the console. 'I may look half-decomposed, but I'm still in there. The machines are keeping me alive. They won't allow me to die until my mission is complete.' She paused. 'The name's Ana, by the way.'

'How long...?' started Sarah, but found she couldn't finish the question.

'Three and a half thousand years,' said Ana, 'give or take a few.' She laughed. 'It's been a long shift.'

'Does it hurt?'

The laughter stopped abruptly. 'That doesn't matter,' said Ana. 'I have a job to do. I promised I'd get Prosperity to landfall, and that's precisely what I'll do.'

'Prosperity,' said the Doctor. 'The first of the great Ark ships to leave the Earth. You were declared lost after only three hundred years, a triumphant failure. The transmissions stopped. People mourned for you.'

'We were never lost,' said Ana. 'Our mission continues to this day. Things might have gone a little awry...'

'A devolved population who've forgotten the purpose of their mission, and live out their days according to complex superst.i.tions derived from it. A crew that's been dead for centuries. Farm animals grown into monstrous predators.' The Doctor sighed. 'Awry might be something of an understatement.'

'But they're still people,' said Ana, 'and they deserve to be saved. There's a colony waiting for them. The first of its kind.'

'Things have changed, Ana, while you've been navigating a path through the stars,' said the Doctor. 'Relatively speaking, millennia have pa.s.sed on Earth. The human race has developed faster-than-light technology. They've populated the stars, forged immense empires, and encountered other sentient species. People pa.s.s unhindered between populated worlds. It's really quite wonderful.'

'Then it's all been for nothing,' said Ana. 'The mission has failed.'

'Oh no,' said the Doctor. 'Where would the universe be without pioneers to blaze a trail? Your mission inspired thousands of people to build more and better. It's because of you that they succeeded. Don't you see? It was a marvellous success.'

'And yet those people down there in the lower levels I've failed them,' said Ana. 'Their antecedents gave their lives to the stars. I owe it to them all to continue.'

The Doctor crossed to the console. His fingers danced over the controls. 'I could end it, Ana, if you asked me to,' he said. 'I could override the engines, boost them to unimaginable speeds. You'd reach landfall in a day or two. You could complete your mission, give those people a new world, where they can prosper.' He turned to look at her. 'But you know what that would mean.'

There was a moment's pause. 'Yes, Doctor.'

'What?' said Sarah. 'What is it?'

The Doctor turned to her. 'I'm sorry, Sarah. There'd be no going back. The engines will be destroyed in the process.'

'And she'll die,' said Sarah. 'That's the choice you're giving her.'

The Doctor nodded. 'That's the choice I'm giving her.'

'But there has to be a way!'

'Will they be all right?' asked Ana gently. 'I mean, the planet you've chosen. Will they prosper there?'

The Doctor turned his head away from her, closing his eyes as if in sorrow, thought Sarah. No, as if he was listening to something, tuning in to infinity...

Then he opened his eyes. 'I don't know,' he said at last. 'I hope so.'

'Well then,' said Ana. 'A leap into the dark. But that's all this ever was anyway...'

From the safety of the TARDIS, they watched Prosperity breach the atmosphere of the unnamed planet, lighting up like a silent firework with the heat of entry. Sarah studied the scanner until it was gone. She felt maudlin.