DogNut looked at the bright lamps and the glow of monitors attached to functioning computers as if they were some kind of magic. They had a generator at the Tower and one night a week they fired it up so that they could listen to music and watch DVDs in a communal room, but it was the warmth and cosiness of the lights that the kids enjoyed most.

Looking through the windows into this bright laboratory was like looking into another world. The world that had existed before the disease wiped it all out. Except there was an unreal quality to it, since all the scientists wearing the lab coats were children – fourteen, fifteen years old. As if they were involved in a school film project or something, playing at being grown-ups.

They were making a pretty good job of it, however. DogNut had to admit that life at the Tower was little better than medieval. This was something different.

‘So you got another generator in there to power it all?’ he asked.

‘Not in there,’ said Justin. ‘The fumes would kill us. There’s a couple up on the roof.’

He took DogNut inside, and they wandered among the desktops. Kids were peering in microscopes, looking at things in dishes, writing notes, reading books … DogNut’s brain was beginning to ache.

‘How d’you have time to run the museum and all this?’ DogNut asked.

‘Oh, I don’t run the labs,’ said Justin. ‘To tell you the truth, I’m not great at biology – physics is more my thing.’

‘I thought it was all the same. Just science.’

‘No. Ah, here he is …’

DogNut saw a boy approaching across the laboratory floor. He was tall and wearing a grubby white lab coat over black jeans, a tweed jacket and a tatty old sweater. He had a shock of untidy dark hair and would have been quite good-looking if his teeth hadn’t been yellow and blackened, jutting out like horse’s teeth from rotten, receding gums.

‘This is Einstein,’ said Justin.

‘For real?’ said DogNut, grinning.

‘Yeah, obviously for real,’ said the boy sarcastically. ‘I really am Einstein. Justin regenerated me from cells found in a preserved lock of his hair.’

‘So it’s a joke name?’

‘Yes. My real name’s Isaac Newton.’

‘OK. Cool. Hello, Isaac.’

The boy snorted and looked to Justin then back to DogNut.

‘You’ve never heard of Isaac Newton, have you?’

‘Nope. Should I of?’

‘He was only the most famous British scientist of all time, the man who discovered gravity, worked out the laws of the universe.’

‘I didn’t really do a lot of science at school,’ said DogNut.

‘You surprise me. What did you specialize in? Finger painting?’

‘I liked history.’

‘Give the man a banana. History. Not a lot of use in the modern world, but it’s better than nothing, I suppose.’

DogNut stuffed his hands in his pockets. Otherwise he was in danger of hitting the boy, whose superior, sarcastic manner might have been even more devastating if his breath hadn’t stunk.

DogNut made a last effort to be polite.

‘So, if you’re not really Einstein, and you’re not really Isaac Newton, then who are you?’

‘Stephen Hawking.’

‘Listen, dickwad,’ said DogNut, grabbing the boy’s throat. ‘Stop taking the piss or I’ll knock your green teeth out.’

‘Oh, how brave.’

DogNut slapped him and let him go. The boy looked shocked for a second and then laughed in DogNut’s face.

‘I can see that the world of meticulous, patient, scientific enquiry is not for you. I rather think what we do here is going to be lost on you.’

‘Whatever.’

‘But at least you seem to have heard of Stephen Hawking.’

‘He was the dude in the wheelchair with the robot voice.’

‘Bravo. Full marks.’

‘Back off, both of you,’ said Justin. ‘This is getting stupid.’

The boy smirked and offered to shake DogNut’s hand.

‘My name’s Orlando Epstein,’ he said theatrically and not entirely sincerely, ‘but you may call me Einstein. Everyone else does.’

‘Yeah, right.’ DogNut slapped the hand by way of a greeting. ‘My real name’s Danny Trejo, but you may call me DogNut. Everyone else does.’

‘OK, Danny.’

DogNut didn’t let his face give anything away. He may not know much about scientists, but Einstein clearly didn’t know much about hard-faced, ex-con Mexican action-movie stars.

‘This is all very cool and impressive, like,’ he said blandly, keeping his little triumph to himself, ‘but what have you actually found out?’

‘Loads of stuff,’ said Justin, sounding like an excited eight-year-old.

‘Amaze me!’

‘We’ve proved there’s a definite link between ultraviolet light and the progress of the disease,’ said Einstein.

‘Yeah, and what does that mean?’

‘Basically, sunlight makes the disease act faster,’ said Einstein. ‘It blisters their skin, accelerates the process, and we’ve observed the effects of UV light on their blood under a microscope.’

‘Light kills them?’

‘Only ultraviolet light,’ said Justin. ‘Like you get from the sun. Electric light makes no difference.’

‘I always knew the sickos didn’t like the sun. But you saying it actually hurts them?’

‘Yes.’ Justin nodded enthusiastically.

‘And you can prove that, can you?’

‘Yes.’

‘How?’

‘We tied up one of our specimens in the courtyard for a week and watched it die,’ said Einstein.

‘You mean a sicko?’

‘Yes.’

‘You left one out in the sun?’

‘We wanted to observe it. That’s what scientists do. They observe things and create theories based on their observations.’

‘So what happened to him?’

‘Her,’ said Einstein. ‘She burst. Splat! She couldn’t have made more of a mess if she’d swallowed a hand grenade.’

‘It seems the longer they stay out of the sun the worse it is when they’re eventually exposed to it,’ said Justin. ‘Those that go outdoors in the day develop more of a resistance, like getting a suntan. We tried it with another subject, exposed him to low light levels every day, gradually increasing the time he spent in the sun. After three weeks he hadn’t got much worse than the subjects we kept in the dark. Of course the sunlight irritated him.’

‘It sent him crackers,’ said Einstein, and he giggled. ‘We had to put him down in the end.’

‘When they stay in the dark, though,’ said Justin, ‘they stay quite calm, like the ones in the lorry. Take them outside and they lose it.’

‘The brighter the sunlight, the worse it affects them,’ said Einstein.

‘So we got to pay more attention to the weather forecast,’ said DogNut. ‘Cloudy with a chance of zombies.’

Justin laughed. ‘Something like that.’

‘They sound more like vampires than zombies.’

‘They’re neither,’ said Justin, irritated. ‘They’re not any kind of walking dead. But we think the secret to what they really are is in their blood. It’s that we need to look at most carefully.’

‘We need to know if the disease is a virus,’ said Einstein. ‘Or if it’s bacteriological, or if it’s a type of cancer, or autoimmune disease, maybe it’s caused by poisoning of some sort.’

‘Or space dust,’ said DogNut.

‘It’s a possibility,’ said Justin. ‘A disease could have come into the atmosphere off an asteroid, or a meteor, or was maybe brought back by a space mission.’

DogNut had mentioned space dust as a joke, and he decided to push it further, enjoying Justin taking his idea seriously.

‘Could be an alien attack,’ he said.

‘Possible, but unlikely,’ said Justin. ‘The nearest inhabitable planet is many millions of light years away from Earth.’

‘Yeah, well, if they’d set off before breakfast, they could be here before tea time.’

‘That really wasn’t very funny,’ said Einstein.

‘Whatever,’ said DogNut, ignoring Einstein’s insult. ‘I think I get what you’re doing here.’

‘Do you?’ said Justin.

‘Yeah. You’re saving the world.’