Susan watched the Doctor's chin tighten. 'And who exactly are you calling an old codger, hmm? I am the Doctor, and this is my granddaughter Susan. We are visiting this planet and have as much right to be here, I suspect, as you do. Perhaps you should explain yourself before you start accusing innocent bystanders!'

'Sorry. I'm forgetting the procedure. That's not really my job. Still...' The woman fumbled in a pack attached to her belt, but her big-fingered gloves were too clumsy. The card she was trying to produce fluttered out of her grasp and landed in the mud. Susan scooped it up and studied the animated announcement.

A green planet swam up to fill the card and a formal logo overlaid it.

Kresta Survey Team: Prospecting for the Future

Project Co-ordinator Bethan Finch

'KST has legal survey rights to this planet with the first option for further development,' said Bethan.

The Doctor glanced at Susan. 'What sort of development?'

If Bethan tried to shrug, it was obscured by the rigid confines of her s.p.a.cesuit. 'That depends on what we find. Mineral deposits, opportunities for agricultural or colonial development. It's all up for grabs in this sector.'

Susan shook her head. 'But what about the people who already live here?'

'Indigenous inhabitants, you mean? If there are any.' She faltered. 'You're not with a protest group, are you? Jeeps, I hope they've given you decent decontamination facilities, because tramping around like that... in all this... well, it's irresponsible.'

'We are independent travellers,' said the Doctor. 'Although I always complain if I believe an injustice is being done. So what about the native population? Have you encountered anyone yet?'

'Only you, so far.' She seemed fl.u.s.tered and defensive. 'It all goes in the report. But if there are natives anyone above Cla.s.s D that's when they bring in the contact sociologists. They deal with the negotiations and compensation. Sometimes the locals get a reserve to live in or even full relocation to another planet.'

The flying camera hovered closer.

'Yes, yes, that's all very well,' said the Doctor. 'But you and your colleagues must be aware that you are not the first visitors to this world.' He stabbed a finger in the direction of the abandoned ship. 'Their expedition did not survive. Do you imagine yours will do better? And will you please call off that bug-eyed camera of yours!' He struck out at the little drone with his cane.

'Sorry,' said Bethan again. 'All encounters are recorded for evidence and training purposes.'

'What nonsense!' The Doctor ignored Susan's entreaties for him to stay calm. 'I demand to see your senior officer!'

'I'll tell him,' said Bethan, fiddling with her heavy gloves.

After a moment, the Doctor said. 'Well? Where is he? At your ship? And how far is that, hmm?'

Susan tugged at his arm. 'Grandfather, stop it. You're upsetting her.'

'I want to see him!'

'I don't know where he is!' Bethan lowered her face inside her suit to hide her tears. 'I haven't seen him for four days.'

The Doctor stepped towards her, but she stepped back, stumbled and sat down in the mud.

'My dear, I must apologise,' said the Doctor as he helped her up. 'Are you alone here?'

'It's just the two of us!' She flapped her arms in exasperation and poured out how this was their first prospecting trip, how it was meant to make their fortune. But Tino had been excited by the raw natural beauty of the planet.

'Tino?' Susan touched the Doctor's arm in recognition and he nodded grimly, but Bethan was in full flow by now: Tino had been surveying the habitat hundreds of new species and they were getting behind on the other elements of the survey. Then four mornings ago, he wasn't there. She thought he had wandered off on some hike of his own he did that too often but he'd never come back.

And all the time she waited, it felt as if the forest was watching. But she was sure Tino would be back soon. Of course he would. He had to be.

'Bethan, I fear there is something you must see.' The Doctor took her gloved hand and led her across the clearing to the line of tussocks.

'I know,' she said. 'They were the refugees. The records show that their ship went missing in this sector about fifty years ago, just after the Tarmusiac barrages went up. So no one came looking for them.' She stopped as she saw the final grave on the line the fresh grave with the helmet. Captain Tino Driscoll KST.

At first she was too stunned to react. 'But it wasn't here,' she kept saying. 'He wasn't here. Not two days ago. Did you do this? Did you find him? And bury him?'

Then she sat back down in the gra.s.s in her s.p.a.cesuit and cried quietly.

As the trees stirred in the wind again, it occurred to Susan that she felt no apparent breeze.

'Extraordinary,' said the Doctor. 'This place has such a sense of tranquillity. Can't you hear it?'

The trees were still again. There were no sounds of birds or insects. All Susan heard was the occasional crack of a twig or thump of falling fruit.

'It's growing, Susan. Can't you hear the forest growing?'

While he pottered off, picking up seed pods, marvelling at the delicacy of flowers that burst out between the tree roots, Susan poked at the muddy ground around the grave. There were footprints there bare prehensile feet with opposable digits, more like hands.

And she sensed the watching forest too as if the place was holding its breath.

They took the path Bethan indicated. It soon reached another glade where a small empty s.p.a.ceship was parked. While the Doctor sat out on the steps, Susan helped Bethan inside. 'I should never have come,' she said. 'Can't you hear it? The forest? Doesn't it ever shut up?'

Susan couldn't hear anything. She thought Bethan was feverish, and refusing to take off her environment suit didn't help. It filtered the air, Bethan said, and catered for all bodily functions. She could survive in it for eight days at least. It also made lying down impossible. Instead, she sat on a chair and fell asleep almost immediately.

The ship was spartan and functional like most s.p.a.ceships Susan had seen. Outside, the Doctor was in an unusually good mood. He had removed his jacket and undone the bow of his cravat. 'Remarkable. It's a powerhouse, this place. Most invigorating.' In his hand, the pods had split open and the newly germinating seeds had released little roots. Susan grimaced as they wriggled like hungry worms, curling around his fingers.

It was getting dark, so she went back in. It was cooler, less clammy, inside. She found some dry biscuits in a tin and sat on the bed, trying not to drop crumbs, waiting for Grandfather.

She woke with a start. Green daylight filtered in through the door. She heard a movement and guessed that Bethan had woken too. But Bethan was gone. Her environment suit lay empty and a bit ripe by the door. The floor of the cabin and chair where she had slept were strewn with scarlet petals.

Something stirred again. In a shadowy corner, a pair of round, honey-brown eyes blinked at Susan. Yet there was no shape behind them. Just shadow. She didn't scream; that would have been wrong. She wanted to reach out and touch their owner, but when she leaned forward, the eyes closed and were gone.

Vines that had either pulled themselves across the doorway or grown overnight, parted to let something, a half-traced shape or trick of the morning light, push a way out. Susan followed, but no one was outside. No Bethan, no Doctor.

She called for her Grandfather, but the forest swallowed her shout. It went nowhere.

Then the noise started a rustling in the trees, growing into a surging turmoil as branches swung to and fro in a fury. It overwhelmed her, swamped her head. She pulled back inside the ship. On a rack was another s.p.a.cesuit. She scrambled to put it on. It was voluminous, warm and best of all, once she clamped the helmet on, it shut out the forest.

Then she stepped back outside. She trudged along the path, heading towards the clearing by the refugee ship and almost tripped over Bethan Finch. The woman, dressed in loose fatigues, was crouching over Tino's grave. She was digging with her hands, sending the dark soil flying.

Susan tried to drag Bethan away, but an angry snarl was the only response. Already the grey casing of her partner's s.p.a.cesuit was emerging. Overhead, the trees went quiet. Through her visor, Susan saw many pairs of honey-coloured eyes gazing down. Then Bethan gave a cry of despair. She had unearthed the chrome neck of Tino's suit it was empty.

Above them, the trees erupted in a new frenzy. Susan glimpsed shapes capering along the branches. Foliage and figures in one.

Bethan swore loudly as a tall man stepped out of the undergrowth. He was dark-haired with a scrubby beard. 'Tino Driscoll, that isn't funny!' yelled Bethan. 'Where the heck have you been? I thought you were dead!'

He grinned. 'What's the fuss? I haven't been far.' He kicked at the burial mound with his bare feet. 'I've got a new life now. This old one's dead and buried.'

'You buried it?' Bethan said as he lifted her in a bear hug. 'You idiot!' she complained and hugged him back.

The shapes in the trees whooped. Branches swayed, raining flowers and leaves down in celebration.

But Susan stood helplessly alone. Where was her Grandfather?

The creatures above fell silent. They had gathered like a troop of ghosts, on top of the overgrown refugee ship and were watching her. She turned to run away as best she could, but she tripped and tumbled headlong to the mossy forest floor. Through her smudged helmet, she saw a familiar pair of brown patent leather shoes, neatly placed at the foot of a tree.

The Doctor sat on a branch above her, ten feet up, dangling his shoeless feet over the edge. He was gazing down with a bemused look of contentment.

Around him, the foliage twitched and rustled almost danced. Half-visible fingers reached down towards Susan. She struggled as they lifted her, suit and all, up into the tree and set her beside her Grandfather.

'Susan, really. Take that ridiculous contraption off.'

'No,' she said. 'I won't. It's not safe.'

'Oh, for goodness' sake, child! What are you scared of?' He leaned across, undid the seals on her helmet and lifted it off. 'Now look around you.'

Susan gripped his hand. Voices called from the leaves, chuckling and cooing what almost sounded like words. The forest spoke through them. She saw the half-human faces of the tree-people with their beautiful guileless eyes.

'Years ago,' the Doctor said, 'their forebears arrived here in that ramshackle transporter ship. They were fleeing from a war. And they probably planned cities and farms a proper colony, but they soon learned better. It's this planet. It's in a state of perfectly tuned balance. Every aspect of it is so completely at one with the rest, that it's hard to tell one ent.i.ty from another. Fascinating, absolutely fascinating. The people, the trees. Perfect harmony and perfect camouflage too.'

Susan thought for a moment. 'I know they've adapted to a new environment, Grandfather, because that's what people and life forms do... but surely this is devolution? And so fast! They're changing backwards!'

'Really, Susan?' He looked surprised. 'I'd have said they were much better off, wouldn't you, hmm? In many ways, I could be quite envious.'

Susan sensed the warm air, damp and rich, pressing in around her. The branches seemed to reach down to enfold her. How could she not have noticed? The s.p.a.cesuit was stifling her. She tried to pull it off and nearly fell.

Gentle hands pulled her back up to her perch. This was where she belonged, where she had always longed to belong. She was enticed and entranced. How she and Grandfather had arrived here and their means of departure, were becoming irrelevant slipping away. Was this journey's end at last?

They sat dreamlike for hours or so it seemed. Down below, the other arboreal denizens of the forest laid flowers on the line of overgrown graves and sat around watching the flickering holographic images of their ancestors.

'Delightful,' said the Doctor at last. He swung out his legs and noticed that one of his socks had a hole in its heel. So much for perfection. 'Down, please,' he said and the hands lowered them to the forest floor.

Susan knew that he would soon be bored. She took his arm and they walked back along the line of tussocks. She wanted to say goodbye to the tree creatures, but they seemed to have lost interest and had wandered away. Finally she and the Doctor strolled through the forest towards the tree-stump-shaped ship that she remembered as their real home.

'But what about the developers, Grandfather? If they arrive, they'll destroy this place.'

The Doctor nodded. 'Perhaps. Or perhaps this world has its own ways to survive.' And he fumbled through the mossy bark for the TARDIS keyhole.

Bethan Finch, former Project Co-ordinator, struggled to remember how she used to send a planetary report. She had tapped the words in on the screen and, at the third attempt managed to send the report off to New Projects Hub at Earth Central 483 light years away.

The words were DON'T BOTHER.

Then she took Tino's hand and they ran out into the forest to play.

'You have a primary and secondary reproductive cycle. It is an inefficient system, you should change it.'

Commander Linx, The Time Warrior (19731974)

We first met the potato-headed Sontarans in The Time Warrior, when Commander Linx crash lands on Earth in the thirteenth century and threatens to change history by giving people guns and robot soldiers. The Time Warrior also introduces a new companion Sarah Jane Smith who, at the end of the story, sees Linx killed in an explosion.

As a result, Sarah is amazed to see Linx alive and well several thousand years in the future in The Sontaran Experiment (1975). But this isn't Linx: it's another Sontaran who just happens to look very like him. Later in the story we glimpse a third Sontaran and he looks just the same, too.

Asked in The Sontaran Strategem (2008) how Sontarans tell each other apart, one replies tersely, 'We say the same of humans.' But there's another explanation for why so many Sontarans look alike. 'They're a cloned species,' explains the Fourth Doctor in The Invasion of Time (1978). 'They can multiply at the rate of a million every four minutes.'

Cloning is a way of reproducing living organisms, resulting in copies that are genetically identical. We'll discuss exactly what 'genetically identical' means shortly, but the word clone comes from the ancient Greek for 'twig'. It's been known for thousands of years that it's possible to take a twig or other cutting from a healthy plant and from it grow another, almost identical plant. That's useful because if we find a plant that produces food or flowers we like, we can make lots of copies of that plant and grow more of the things we want or need.

Fittingly for the potato-like Sontarans, the potato is an easy plant to clone. Take a potato and locate the small 'eyes' of new growth on it a largish potato will have seven to 10 eyes. Cut the potato into rough cubes of 2.5 centimetres (or 1 inch) each, with one eye per cube. Leave the cubes to dry overnight called 'chitting', which encourages growth. Then lay your cubes out on well-drained soil, some 50 centimetres (20 inches) apart, pressing each cube into the soil with the eye facing upward. Cover the cube in 15 centimetres (6 inches) of mulch and water until evenly moist. Keep the mulch moist over the coming 16 to 20 weeks as the potatoes grow.

Each cube should produce its own potato plant, and each plant ought to produce at least five potatoes. So if you begin with a potato with seven eyes, you could expect to harvest 35 potatoes. If you then cloned each of those 35 potatoes, you'd see a harvest of 1,225 potatoes the cloned 'grandchildren' of the original.

It's not quite the astonishing rate of reproduction that the Sontarans manage, but it means we can produce large quant.i.ties of potatoes quickly, easily and cheaply. Potatoes are also tasty and can be cooked in many different ways, so we can see why they became a staple food of the fast-growing population of Europe in the eighteenth and nineteenth centuries.

But in the 1840s this essential food crop was struck by a microscopic organism called Phytophthora infestans literally 'plant-ruining destruction' more commonly known as 'late blight'. Potatoes infected with blight quickly rotted away to foul-smelling mush and couldn't be eaten (or cloned). Blight infection spread rapidly, the potatoes people depended on for food were lost, and there were terrible famines in Ireland, Scotland and other European countries. In Ireland alone, a million people died and another million fled the country in total about a quarter of the whole population. It has been argued that this huge human tragedy permanently altered the county's politics and culture: a microscopic organism changed the history of an entire nation.

Why were potatoes so vulnerable to blight? In fact, it turns out that cloned plants and animals are more vulnerable to disease and pests. It took some years for scientists to work out why, but perhaps the most important single step in getting to the answer was made at about the same time as the potato blight was causing such devastation.