'Not unless you're a patch of nice tasty gra.s.s,' he said, 'or you're planning on having one for dinner. They get a bit tetchy about that.'

'And what's that?' She indicated further down the valley, where the river disappeared behind a forested ridge of hill. All she could make out was a flicker of colour, and sunlight glinting sharply off a reflective surface.

The Doctor frowned, and, with a faint sense of satisfaction, Clara realised that he hadn't noticed it until she'd pointed it out. 'Romans,' he said at last. 'A military camp.'

'A Roman encampment,' said a voice behind them. 'And we don't take kindly to spies.'

Clara and the Doctor turned to see two gleaming short-swords held by two grim-faced men wearing chainmail and legionary helmets.

She raised her hands and attempted a friendly smile. 'Don't worry,' she said, 'we won't hurt you.'

'So much for Roman law and order,' muttered Clara. 'Are you going to kill us just for walking around a forest?'

The soldiers said nothing as they marched the Doctor and Clara through the fortified army camp. From their vantage point on the hill, she had seen only the very tip of it but, past the walls and ditch, the full camp held dozens of rows of tents, thousands of soldiers. As their little party moved along the painstakingly straight dirt roads, soldiers paused in their duties to watch the new arrivals.

In the centre of the camp, they were ordered to stop before a large wooden building and were left with a single guard for a few minutes, before being escorted inside.

They were brought to a richly furnished room where two men and a woman looked over a large, finely detailed map. The woman straightened as they entered, her dark brown eyes inscrutable as she looked them over. 'Ah, our spies. What have you to say for yourselves?'

'How about we're not spies?' said the Doctor.

'Well, of course,' said the woman, with a slight nod of her head. 'Who are you, then?'

'I'm the Doctor, and this is Clara Oswald.'

'Doctor?' She glanced to one of the men beside her and raised an eyebrow. 'Senator?'

The man shook his head. 'Foolish,' he said. 'Be rid of them and let us return to the matter at hand.'

'And who, exactly, are you?' the Doctor asked her.

The woman took a step towards him, moving with an imperious grace. 'I am Ulpia Severina, Caesar and First Citizen of the Senate and People of Rome, wife of the Restorer of the World.'

'You're Caesar?' said Clara, unable to hide her surprise.

'Yes,' she said sharply. 'My husband is dead, murdered by traitors. I rule now, in my own name.' She turned her attention back to the senator. 'Well, I doubt they're Germanic spies: this one sounds as though she's from Britannia, and his voice has the barbaric pitch of some Scotti tribe.'

'You want to say that again?' said the Doctor.

Ulpia gave him an a.s.sessing gaze. 'Or perhaps the Picts,' she said. 'It makes little difference.' She tapped her fingers on the table, considering. 'Perhaps you can be of some use; we're short of doctors, and many of my soldiers are injured. I need-'

Ulpia's head snapped up, and Clara became aware of a buzzing noise, like a thousand tiny insects, coming from outside. Ulpia strode out of the tent followed by her guards and the senator. The Doctor and Clara exchanged an uneasy glance and followed.

Outside, the legionnaires were in a flurry of activity as orders were shouted, and archers and slingers lined up.

'Perhaps it would be better if you were to stay inside,' the senator said to Ulpia.

'I won't cower like some frightened child,' she told him, her eyes scanning the camp. 'Where are those wretched things?'

As she spoke, Clara saw a shining ma.s.s fly steadily down the road towards them. As someone shouted an order to fire, Clara's eyes began to make sense of what she was looking at, and she realised it wasn't some strange silvery cloud but a vast swarm of insects. It swept through the gathered soldiers, unaffected by their missiles and arrows, and moved towards the central square.

'Caesar, please,' the senator said, backing into the building behind him. 'What are they?' asked Clara.

'A curse,' said Ulpia. 'You may run too, if you like. To be bitten by one is to invite death.'

'They look like mosquitoes,' said the Doctor.

'They're silver,' Clara pointed out.

'Robot mosquitoes?' the Doctor suggested as the insects entered the square.

He took out his sonic screwdriver and ran towards the swarm. As he activated the sonic there was a sudden change in the swarm's buzzing, and it became more high-pitched and tinny. They paused in their flight and then scattered, fleeing from the Doctor's sonic signal.

Ulpia stared at him in surprise. 'You're a doctor of magic.' A statement, not a question like magic was something ordinary and everyday.

'Hardly just insect repellent,' said the Doctor off-handedly. 'How long's this been going on?'

'They've been attacking for five days, spreading disease amongst my soldiers, killing dozens. I cannot risk contact with any of my other legions.' Her eyes moved to the darkening horizon, and her voice grew harder. 'My husband defeated the Alamanni, the Vandals, the Gallic Empire and countless others. He restored Rome to greatness, and yet I am paralysed by mere insects. I don't know how to save my people.' She turned her attention to the Doctor. 'Can you help me?'

Clara glanced at the Doctor. 'Of course we can,' she said.

'We can try,' said the Doctor. 'Where are the patients?'

Before they'd even set foot inside the medical tent, the stench of rotting meat a.s.saulted Clara. Her eyes stung and her throat burned as she and the Doctor stepped inside. She covered her mouth with her hand and took shallow breaths, but it was still overpowering. 'I'm not sure I can stay in here.'

'You think it's any better where you're from?' he asked, approaching the nearest patient. He looked him over with an impersonal precision, checking his eyes and his pulse before moving on.

'Yes!' said Clara.

The Doctor looked at her as though she'd just grown a second head. 'You still cut people open and poke around inside. You dose patients with radiation hoping that it does more damage to the cancer than the cancer does to the patient. You haven't the first clue how to tackle hundreds of diseases.' He moved to the next bed, offered a few calming words to the wounded soldier as he examined the dark patches of mottled skin on his face and neck.

Clara glared at him. 'Antibiotics, anaesthesia, X-rays.'

'They've got anaesthetic here,' said the Doctor, as he moved on to an unconscious patient and quickly examined the injuries beneath his bandages. 'These symptoms are very odd; very different.'

'Insulin,' said Clara. 'Vaccinations, knowing what the heart actually does. We've made progress!'

'All right, fine, there may have been one or two slight improvements,' conceded the Doctor, 'but on the grand scale of things-'

'On the grand scale of things, I'm right, and you'd rather be a patient in twenty-first-century Britain than here.'

'That would depend on what was wrong with me,' said the Doctor.

'You know, Doctor, it's healthy to admit when you're wrong.'

He pretended not to hear, caught up in his examination of the sick young man in front of them.

'Doctor?'

'Would you let me concentrate? And if you are going to throw up, please do it outside.' He moved on to another patient, and walked straight into a Roman clothed in a long tunic and fresh bloodstains.

The Roman looked into the Doctor's eyes and frowned. 'You're new,' he said.

'Yes,' agreed the Doctor. 'Just recruited by her Royal Highness to help with your little medical problem.'

The Roman's frown deepened. 'I'll a.s.sume you're referring to the Empress. I'm Vettius Falco, commander of the camp physicians. How do you think you can help?'

'Treatment hasn't been going very well so far, has it?' said the Doctor. 'Hardly. The best news we have is that it isn't a contagion. Only those bitten directly by the mosquitoes have been infected, but there've been dramatic differences in their symptoms. No herbal remedies have aided them, and when infected limbs have been removed, the infection has resurfaced in other parts of the body.'

'So what have you been doing for them?'

'Praying,' said Vettius.

'Don't you dare,' said Clara to the Doctor as he opened his mouth to respond.

He raised his eyebrows. 'You've no idea what I was going to say.'

'I can guess, and it wasn't anything helpful.'

The Doctor turned back to the row of patients he'd just examined. 'They've all been bitten by these mosquitoes, but are exhibiting reactions diverse enough to suggest they've not all been infected by the same illness. The mosquitoes themselves are artificial, and that attack didn't look random. It all feels to me like someone is using your army as laboratory specimens.' He paused, then spoke in a low voice: 'Clara, I hope you're paying attention and have noticed the noise outside.'

The buzzing noise was faint but steady and getting closer. 'They're coming back,' she said.

'I guess I'll have to be more convincing this time.' The Doctor took out his sonic screwdriver. Clara looked out of the open tent flap to see the approaching swarm heading straight towards her. The Doctor pulled Vettius down behind a table of gleaming surgical tools.

'Do they know we're here?' asked Clara, ducking back inside and joining the other two behind cover.

'I don't think it's us they're after,' the Doctor told her.

Vettius' eyes widened with realisation. 'The patients.' He tried to stand but the Doctor pulled him back down.

'There's nothing you can do. No point in making yourself a target.'

'Isn't there?' snapped Clara. 'We're not the ones who're defenceless.'

'I have a duty,' said Vettius.

'Stay. Here. Both of you.'

As the shimmering swarm swept into the tent, the Doctor stood and activated his sonic screwdriver, sweeping it round in a wide arc. The swarm reacted with a shudder and sudden swerve before dozens veered away in random directions. A handful fell to the ground, but the rest were unimpeded as they swept down on the first patient.

'Can't you just get rid of them again?'

'I could, but then they'd just come back. I'm trying to deactivate a few. We need to get a closer look at them.'

Clara rolled her eyes, ducked up to grab a basket from the table, and tipped all the tools out.

She edged round the swarm, closer to the bed they were focused on, upturned basket in hand. She moved closer, focusing on several mosquitoes close to the ground.

In one swift, smooth movement, she leapt forward and slammed the basket down over them.

The Doctor adjusted his sonic and tried again. The swarm seemed confused, as though they'd lost their sense of direction, before they flew out of the tent and up into the air, climbing higher and higher until they'd vanished from sight.

'Well,' said the Doctor, 'I don't think we'll be seeing those ones again.' He crouched down next to Clara's basket. 'They don't sound happy.'

'What are they?' asked Clara. 'Some sort of alien robot mosquitoes are invading the planet?' She gave the basket a speculative look. 'Should we squash them?'

'Let's try for a more subtle approach. I should be able to deplete their power, then we can take a closer look.'

Vettius hurried over to the patient who'd been the swarm's target. 'He's dead.'

The Doctor stared over the corpse as he thought. 'Terminating the specimen after they collect the results.'

'That's horrible,' said Clara.

The Doctor nodded. 'That's why we're going to put a stop to it.'

He waved his sonic screwdriver over the basket and, when the trapped insects ceased their buzzing, he gingerly flipped the basket over. Three of the mosquitoes lay beneath it, inert.

Clara crouched down and picked up one of the fallen insects. It was exquisite. Beautiful. Less than two centimetres long, but she could make out the delicately constructed legs, the antennae, the diamond sheen of the compound metal eyes. 'Are these safe now?'

'Safer,' the Doctor told her. 'I'd avoid the mouth if I were you; it may still bite.'

He carefully plucked another of the fallen mosquitoes from the floor. 'All right, you wee beastie, let's take a good look at you.'

Ulpia Severina paced the room, her hands clasped behind her back as she considered what the Doctor had told her.

'Then this is an attack?'

'Possibly. Or someone's made a terrible mistake. Either way, those mosquitoes don't belong here. You've nothing like the technology needed and the component alloys don't even come from this planet.'

'Someone sent them here?' asked Clara. 'Why?'

'I don't know, but if we can find where they're coming from, we might be able to stop them. They need an energy field to recharge their power, so their range is limited.'

'Energy field?' asked Ulpia.

'They need to rest before they come and attack you again,' said Clara.

'And when you find their resting place, you intend to destroy it, alone?' said Ulpia.

'I think it'll be rather a delicate operation,' the Doctor told her. 'A lot of soldiers trampling around the place would only attract unwanted attention, and might prompt another attack.'

Ulpia was silent for a few moments as she held the Doctor's gaze. 'Why are you helping me?' she asked.

He gave her a slight smile. 'I like helping people. My generous nature.'

'I want your word that you will do everything in your power to destroy these creatures.'