'To stop you looking too closely at what's going on, perhaps,' the Doctor said. 'And to act as a front man, to head off nosy people like me. Unless, of course, you already know what's going on and this is all a bluff.'

Jeff was shaking his head. 'We all know what's going on.

It's a website, that's all. So it takes a bit more power to run than we thought. That's no surprise really. It's not the end of the world.'

The Doctor looked him in the eye. 'I hope you're right about that.'

There was a moment of silence. Then the Doctor clapped his hands together, coming to a decision.

'This website is the key to everything. So let's have a look at it. Fire it up, please, Jeff. I'm going to take the Brainy Test.'

'OK, so it's a bit weird,' Jeff had to admit later.

It was dark outside, and everyone else seemed to have gone home.

The Doctor had taken the test. At first it was all simple and what he had been expecting a few sums, some missing words to fill in, some spot-the-next-shape-in-the-sequence puzzles, and so on.

After a while, though, the tests changed. It was as if the website had got to know the Doctor was clever enough to solve harder problems.

It was not a sudden change. The Doctor gradually found that he was getting more and more questions he wouldn't expect in a normal test. There were really quite tricky questions about DNA and the way it was made. There were boxes where he had to fill in sums to work out how evolution took place. There were problems to solve that the Doctor knew no human being could have devised or understood.

But the people who had eaten the Brainy Crisps would be able to do them. This was not a test. This was real, new research on a ma.s.sive scale.

'How many people use this website?' the Doctor asked.

'At any time there are over half a million users on the network,' Jeff said.

'That's a lot of computer power.'

'You know how much,' Henry said. 'We told you earlier.'

'I didn't mean the machines, I meant the people doing the tests.' The Doctor sat back and thought about his next move. 'So, who devised this test? Who set it up?'

'The web pages were already built,' Jeff said. 'We were given the questions, and we just had to put them on the server so people could get to them on the internet.'

The Doctor clicked on the web browser. He got back into the firm's systems and found a way into the secure Accounts area.

'How did you do that?' Jeff asked, impressed.

'It's a knack,' the Doctor said modestly. 'I was thinking there must be a payment record. Whoever built the website got paid, or had their time billed to it, so there must be a note of that.'

He found the right folder for the website work, but it refused to open.

'Problem?' Henry asked.

'Looks like higher management has its own network within the main firm's systems.'

'That's right. Even we aren't allowed access to that.'

'Can you get into it, Doctor?' Jeff asked eagerly.

The Doctor shook his head. 'It's protected by a digital deadlock seal. There's no way in, even for me. Well, not unless I destroy the whole system, and that seems a bit extreme. Who would have access?'

'To the whole system?' Henry said. 'Only Sir Manning Cross.'

The Doctor leaped to his feet. 'Then we need to use his computer. I'll be able to bypa.s.s the seal from there. Then we can get into his personal network and data.'

'Isn't that a bit naughty?' Henry asked.

'Oh yes. Let's do it. He'll be long gone by now, I hope.'

The Doctor turned to Jeff. 'Can I ask you to stay here and keep an eye on the systems? I need to know if anyone else detects us getting inside the network. Keep an eye out for any unusual activity can you do that?'

'Sure thing, Doctor.'

It was the end of the day, and the offices were dark and empty. Henry led the Doctor to Sir Manning Cross's office on the seventh floor. It took only a moment for the Doctor to open the door, and they slipped inside.

'Are you all right if I go and set some things going in my office?' Henry asked. 'We run some of the accounts programs at night. That way we don't disrupt the systems while people are working.'

The Doctor was happy to be left to hack into Sir Manning's computer. He sat alone in the near-darkness, his face lit by the glow from the screen. The rest of the office was a jungle of shadows. Soon the Doctor was lost in his work, breaking the digital deadlock seal and hacking into the management systems.

The office the Doctor had been given was filled with shadows. Jeff's face was lit up by the glow from the screen as he worked.

A draught shifted the papers on the desk. A deeper, darker shadow fell across him, and Jeff looked up.

'Oh,' he said, relieved, 'It's just you. I thought you'd gone.

Did you forget something?'

The shadow changed shape as the creature that cast it started to transform into its true body. Jeff stood up, mouth open in fear. He beat at the figure looming over him, trying to fend it off. The chair toppled over behind him. Jeff caught hold of something and pulled. It came away, but he had no time to wonder what was in his hand.

Huge, leathery wings beat the air, scattering papers. A high-pitched shriek drowned out Jeff's cry of terror. Sharp, alien claws slashed down.

Jeff's lifeless body slumped to the floor. The beating wings stopped and the shadow shrank back to its original size. A hand that once again looked human closed the door.

Chapter Six.

The Doctor was getting bored. He had been through so many files that the screen was starting to blur before his eyes.

Despite his hopes, he had actually found very little of interest.

'Any luck?' Henry asked, stepping back into the room.

The Doctor leaned back and stretched. He peered at Henry's dim shape in the gloom. 'Not much, I'm afraid. Most of it is the sort of rubbish management reports we got at the meeting today.'

'That's why I stopped going. That and the way they always seemed to find fault with what I was doing. They said it was too slow, or too costly, or just plain wrong. I know I'm not really very good at my job, but they put me in it. They could at least help.'

'There was one thing,' the Doctor confessed. He shut down the computer and turned off the screen. 'There's a company meeting tomorrow morning.'

'We have so many meetings in this place. It's amazing anyone has time between them to get any real work done.'

The Doctor grinned in the near-darkness. 'But this meeting is at seven in the morning. It's in a posh hotel, and it's not in Sir Manning's diary. There's no agenda, and no list of who is attending. There's just a time and an address in a coded email from Miss Sark.'

'A secret meeting,' Henry said. 'I wonder what it's about.'

'The coded email says it's been called by the firm's main shareholders. I'm hoping it's about what's really going on.'

'Doctor what is is really going on?' Henry sounded annoyed. 'I don't know who you are or what you're doing here. You tell me the computers are running some secret program that's linked to the Brainy_Crisps website. So what? really going on?' Henry sounded annoyed. 'I don't know who you are or what you're doing here. You tell me the computers are running some secret program that's linked to the Brainy_Crisps website. So what?

Why are we sneaking about in the dark, breaking into the boss's office and hacking his computer?'

The Doctor walked slowly round the office. He peered into the corners and opened drawers and cupboards. 'What if I told you the Brainy Crisps are bad for you? What if I told you that eating them makes you clever but slowly burns away your brain? Would you believe me?'

'I might,' Henry said. 'I have wondered what side effects there might be. I mean, if the science exists to make everyone brainy, why can't you get it on the NHS? It can't cost too much because it's as cheap as crisps.'

The Doctor found a pile of papers in a drawer and started leafing through them. 'What if I told you that the science the brainy formula, if you like is secret. It's known only to Sir Manning Cross and Miss Sark and a few others?' He closed the drawer and moved to a door at the side of the room.

The door was locked. It couldn't lead anywhere as it was close to the building's outside wall, and the Doctor guessed it opened into a storeroom. Maybe useful files were locked away inside.

'Well, I suppose that makes sense,' Henry said. 'But why waste it just making crisps?'

The Doctor set to work on the door with his sonic screwdriver. 'What if I told you that Sir Manning and Miss Sark know the secret because they're not human?'

'Not human?' Henry joined the Doctor by the door. 'What do you mean, not human?'

'Aliens,' the Doctor whispered. He slipped the sonic screwdriver back into his jacket pocket.

'Aliens? Selling crisps?'

'A means to an end. They disguise themselves as humans.'

Henry took a deep breath. 'So what do they really look like, these aliens?'

The Doctor pulled the door open. There was just enough light to see what was inside. 'Um,' the Doctor said quietly.

'Well... They look exactly like that, in fact.'

Inside the small room behind the door, hung the dark shape of a Krillitane. The huge, winged creature was hanging upside down from the ceiling. Its wings were folded round its body. The long head looked like it was carved from brittle stone.

'Is it asleep?' Henry asked in a hushed whisper.

The creature opened its eyes. 'No,' it said.

The Doctor slammed the door shut. He whipped out his sonic screwdriver and aimed it at the lock. 'Time we were going.'

Henry didn't move. He was standing frozen with fear.

The Doctor pushed him roughly towards the office door.

'Run!'

Behind them, the door to the storeroom exploded into splinters. The Krillitane smashed its way out, and set off after the Doctor and Henry. It took long lolloping strides. Once in the open-plan area outside Sir Manning's office, it opened its wings.

Henry and the Doctor ran for their lives. They could hear Krillitane wings beating behind them. The creature was half running, half flying. It bounded after them, launching itself forward from panels and walls. Its screeches filled the air.

'Where are we going?' Henry gasped. He was having trouble keeping up with the Doctor. He was also having trouble seeing where they were going.

'Who knows?' the Doctor shouted back. Then: 'Ah!' He suddenly dived sideways down an aisle between desks.

Henry almost fell as he turned to follow. The Krillitane was close on his heels. Ahead of him, the Doctor had grabbed something from a wall mount a fire extinguisher.

The Doctor brought the fire extinguisher up, aiming the hose at the Krillitane. Henry dived to one side as the Doctor pressed the lever. The Krillitane shrieked louder as it saw what was happening. It launched itself through the air at the Doctor. A jet of carbon dioxide gas gushed out over the Krillitane. It caught the creature full in the face, the pressure knocking it backwards.

The Doctor stepped forward, still spraying. As soon as the fire extinguisher ran out, he dropped it. He shouted again at Henry to run.

The Krillitane was thrashing in pain. It struggled to get up.

The Doctor and Henry now had enough time to get away.

The Doctor let Henry lead the way since he knew his way round the offices.

'Fire stairs,' Henry suggested. 'We need to warn Jeff about these alien creatures.'

Soon they were back at the Doctor's office.

'How many of those things are there?' Henry asked as they went inside.

'More than one, clearly,' the Doctor said sadly.

Jeff's body lay stretched out across the floor. The Doctor checked it briefly, but he could tell at once the man was dead.

'Poor Jeff,' he said quietly. 'I am so sorry.'

Henry was standing in the doorway, his face pale in the light cast by the computer screen.

The Doctor stood up and walked slowly over to Henry.

'When you left me in Sir Manning's office,' he said coldly, 'it was only for a few minutes. But you had time to come back here.'

Henry nodded. 'If only I had. I might have been here when...' He looked away. 'I might have been able to save him.' A single tear escaped from one of Henry's eyes and rolled down his cheek. 'Is that what you mean?'

The Doctor watched the tear glisten on Henry's cheek.