The stage Doctor said, 'I'm the man who's going to stop this before it starts. You are going to turn your fleet around, now, and leave.'

'You're a madman,' said the Megrati.

'If not, I will destroy your fleet.' The stage Doctor reached into his jacket and produced something that he pointed into the sky. 'Beginning with your Command Ship.'

The Megrati said, 'You are one man. Against an army. You cannot defeat us.'

'You wouldn't be the first to think so.' The stage Doctor looked up at the largest of the warships in the sky above. The Command Ship. Then he looked back at the Megrati.

'MCD engines, yes?' he asked.

The Megrati hesitated, then growled, 'Yes.'

The stage Doctor nodded. 'Megamonolithic Crystal Drive. The poor sentient being's answer to Hyperdrive. Terribly good at getting you from one side of a galaxy to another...'

The object in the stage Doctor's hand now emitted a shrill tone, and glowed a vivid emerald green.

Nyssa recognised the tone of a sonic screwdriver. Her heart leapt. The stage Doctor went on, 'But MCD crystals are highly susceptible to targeted sonic beams.'

There was an explosion at the rear of the Command Ship. Its engine casings erupted, shattered by eruptions of white hot power. The vessel pitched upwards, its hull straining, groaning like some great wounded animal.

'Your crew will have time to abandon ship,' said the stage Doctor.

He lowered the sonic screwdriver, and released his grip on the Megrati. It backed away from him, continuing to look up.

The Command Ship rose slowly in the upper atmosphere. Smaller explosions erupted sporadically along the hull. Escape pods appeared in swarms from openings along the vessel's underbelly and sped away towards other ships in the fleet.

'My ship!' the Megrati protested. 'How did you do that?!'

Nyssa heard her Doctor, next to her, say quietly, 'Sympathetic destruction.'

On stage, the Doctor in the frock coat said, 'Sympathetic destruction! The crystals in your engines are networked to the crystals that power the rest of the ship. I set up a resonance and they vibrated to destruction. Once that starts, there's nothing you people can do about it but get to the lifeboats.'

The Megrati fumed.

'Now,' said the stage Doctor, determinedly, 'give the order. Call off your attack and leave. Or I'll do the same with every ship you've got.'

For a moment, the Megrati didn't speak. It looked again to the heavens, where the Command Ship was now a lifeless, scarred hulk, drifting away.

Then it lifted a scaly wrist and spoke into a communicator band. 'Fleet... Stand down. Agrylus, you are now the Command Ship. Teleport me aboard.'

The Megrati snarled at the stage Doctor as it waited to leave.

The stage Doctor said, still loud enough for everyone to hear, 'Never return. If you do, there won't be any warnings next time. And you'll have no fleet left.'

The Megrati made no reply. A moment later it was enveloped in the same reddish yellow energy field in which it had arrived, and it faded away.

Everyone looked up. Sure enough, the Megrati ships were turning to leave.

A voice in the crowd cried out, 'Do it! Don't let them get away! Do the same to all their ships!'

There was immediate support from pockets of the audience.

'You've got them at your mercy!'

'Kill them!'

'Deal with them once and for all!'

The stage Doctor turned to the crowd with such a look that they fell quiet. Then he sighed. 'Is that what you want? Is that what you really want? Shall I kill them?'

No one answered.

'After all,' the Doctor continued, 'like you say, that would deal with them once and for all... Hey, maybe you're right.' He held out his sonic screwdriver. 'So come up here. Take this from me. And you can do it yourself. Well?'

No one moved. Then a movement right beside her made Nyssa jump. But it was only her Doctor, leaning forward, elbows on knees, hands crossed, as raptly interested in what would happen next as anyone.

After a long moment, the stage Doctor smiled. 'Well, then. They're leaving... and so am I.'

Nyssa's Doctor sat back with a broad grin on his face.

The stage Doctor went to the TARDIS and opened the doors. He paused on the threshold and said to his audience, 'I'll see them safely out of this star system... In the meantime, why not carry on with your party? You've got something to celebrate.'

As the majority of the audience burst into applause, Nyssa smiled. Whatever face the Doctor wore, she thought, he is always the Doctor.

The stage Doctor gave a deep, theatrical bow.

His white wig dropped to the stage, revealing a tidy, shorter crop of dusty-grey hair underneath.

He straightened, a bit awkwardly, and backed towards the police box.

With just a quick smile towards the rear seats, he ducked inside and closed the door.

Adric said, 'He's left his granddaughter behind.'

Tegan sighed and gave him a shove. 'Adric, you dill.'

Then the air was filled with an all too familiar groaning, rasping sound. And the TARDIS faded from the stage.

'However scared you are, Clara, the man you are with right now, the man I hope you are with... Believe me, he is more scared than anything you can imagine right now and he he needs you.'

The Eleventh Doctor, Deep Breath (2014)

In Deep Breath, Clara struggles to come to terms with the Doctor's latest regeneration. Although she's seen the Doctor's previous incarnations, it's still a shock that the young-looking man she knew and loved is dead, and in his place stands an older-looking man with angry eyebrows and a Scottish accent.

In Doctor Who, regeneration involves a change of appearance and the healing of wounds and diseases. Apparently, it can sometimes involve a change of gender, but seems always to involve a change of personality yet the regenerated person keeps the same memories. They are a different person and yet they are the same, which is why regeneration is such a strange and unsettling concept.

Yet it's not a wholly alien concept. Regeneration happens on Earth, too. Perhaps fittingly, one of the first scientists to really investigate regeneration on Earth was himself a Scottish doctor.

In 1763, the Scottish surgeon John Hunter arrived in London after three years working for the British army in France and Portugal. Struggling to find work in the capital as a surgeon, Hunter worked in a dentistry practice instead. At the time, sugar was very cheap largely because of the huge number of unpaid slaves involved in its production and lots of people in London had bad teeth that caused them pain. Hunter's dental practice didn't just remove people's bad teeth, it offered to replace them.

If you were poor but had apparently healthy teeth, Hunter's dental practice would offer good money to buy them. The healthy teeth would then be extracted and transplanted into the toothless mouth of a rich person, who would pay handsomely for the service.

As accurate medical records were not kept at the time, it's difficult to know how well the transplants worked, or how often they led to the spread of other diseases. But Hunter, as a surgeon, was interested in the principles of transplanting, and conducted a number of experiments. He successfully transplanted the claw from a c.o.c.kerel onto its comb, and even tried implanting a human tooth into another c.o.c.kerel to see if it was possible to do a transplant between different species of animal. Today, we might find those experiments crude, not to mention cruel, but Hunter learnt valuable lessons about the importance of transplanted tissue being fresh and of matching the size of transplanted organs.

This research paid off for him. In 1767, he was elected a fellow of the Royal Society which remains to this day an academy of the most distinguished scientists and the following year he was made a surgeon at St George's Hospital in London. He even became surgeon to King George III. But Hunter continued to study animals and had access to the King's menagerie of interesting creatures. When one of these died in 1776, Hunter became the first person ever to dissect an elephant.

Since his days in France and Portugal, Hunter had collected specimens of animals. As a distinguished surgeon, he was sent more including rare specimens of kangaroos collected from James Cook's voyage to Australia in 17681771. He also collected specimens of unusual humans bits of bone that showed examples of particular diseases or medical complications, and even the whole skeletons of people who were very short or very tall.

Hunter used his collection to teach both the public and the next generation of surgeons. After his death, his collection was given to the Royal College of Surgeons where, as the Hunterian Museum, it continues to be used to teach surgeons. The public can visit, too, and the c.o.c.kerels on which he experimented with transplants are still on display.

Importantly, while other doctors of the time taught human anatomy, Hunter was keen on comparing the structure and function of animal bodies, too, and the different ways that bodies can adapt to or compensate for damage done to them. This comparative work led to major scientific advances: one of Hunter's students, Edward Jenner, investigated a mild disease called cowpox, which dairymaids caught from their cows. Jenner's interest was that these dairymaids seemed not to catch a more common and deadly disease called smallpox. He gave people cowpox and they were then immune to smallpox. His discovery vaccination takes its name from 'vacca', Latin for cow.

We now know that when we get the mild disease cowpox, our bodies easily develop antibodies to neutralise it. Cowpox is similar in structure if not in effect to smallpox, so antibodies developed to neutralise cowpox will neutralise smallpox, too. Jenner's vaccination and the improved versions that followed it led to the World Health Organization announcing in 1980 that smallpox was the first disease to be eradicated, via a vaccination programme carried out worldwide.

Comparative anatomy had other uses, too. In 1824, the doctor and geologist Gideon Mantell visited Hunter's collection, and found that the teeth of a specimen of iguana there looked like tiny versions of giant fossilised teeth Mantell had acquired some years earlier. This led Mantell to name the fossilised creature he'd discovered Iguanadon 'Iguana-tooth'. Along with the fossilised remains of two other creatures, Iguanadon would later be used to define a new kind of animal: the dinosaur.

But Hunter had been interested in iguana because of a remarkable feature of its tail. Like many lizards, if an iguana is caught by its tail for example, by a predator it can let its tail break off and so escape. Eventually, the tail will grow back in a process called regeneration. That process always fascinated Hunter. In fact, one of his earliest animal specimens, collected while he was in Portugal in the early 1760s, is still on display as object 2222 in the Hunterian Museum.

The specimen is of a lizard called a Lacerta and it has two tails. Hunter's own notes describe how if a lizard's original tail is only partly detached, or if a new tail is split while early in its growth, two tails will be formed. There, in a gla.s.s jar in the museum, is something rather like what happens to the newly regenerated Tenth Doctor in The Christmas Invasion (2005), when he regrows his severed hand.

'Quite by chance I'm still within the first fifteen hours of my regeneration cycle, which means I've got just enough residual cellular energy to do this...'

The Tenth Doctor, The Christmas Invasion (2005)

We now know that all life on Earth is able to regenerate to some degree. Segmented worms such as the earthworms you might find in a garden can regenerate their bodies if they are cut in half crossways. In fact, both halves will regenerate into complete worms, creating two genetically identical worms where once there was just one. (Something like this happens in Journey's End (2008), when the Tenth Doctor siphons off excess regeneration energy into the hand severed in The Christmas Invasion which, as a result, grows into a second, separate version of himself.) The reason earthworms can regenerate like this is because their bodies contain cl.u.s.ters of stem cells. Stem cells are useful because, through a process called cellular differentiation, they can become more specialised for different roles in the body. It's a bit like the Doctor's sonic screwdriver a single tool that can do lots of very different jobs. In human adults, stem cells can differentiate in ways that help repair different kinds of damage to the body. But in human embryos, stem cells have a much greater range of differentiation: they can become specialised in any of the ways the body needs.

Earthworms retain these kind of embryonic stem cells into adulthood, which is why they can wholly regenerate when they've been cut in half. Some larger animals retain stem cells like this, too. It's how iguanas and lizards can regrow their tails, while many amphibians can regrow lost limbs.

This kind of regeneration doesn't always happen immediately. For example, as crabs grow they get too big for their hard, protective sh.e.l.ls. As a result, they must periodically shed their sh.e.l.ls and hiding away because they are vulnerable grow new and roomier ones. This process called 'moulting' is somehow linked to regeneration, because while it is happening, the crabs can also regrow any limbs or claws they might have lost.

One Earth creature is even known to be able to regenerate the whole of its body. Turritopsis dohrnii are found in the Mediterranean Sea and also in the waters round j.a.pan. They start their lives as tiny larvae called planula, which then settle on the seafloor as tube-shaped polyps, and from these polyps grow jellyfish. But when Turritopsis dohrnii jellyfish get old or sick or face danger, they can become polyps again in effect, going back to their childhood state in a form of rejuvenation.

'Ben, do you remember what [the Doctor] said in the tracking room? Something about, "This old body of mine is wearing a bit thin."'

'So he gets himself a new one?'

Polly and Ben Jackson, The Power of the Daleks (1966)