With a scowl of annoyance the guard unwillingly checked Gelvert's shelter number, detailed another sentry to watch over them, and stumped off into the rain.

He was back inside three minutes at a run and rapidly began punching b.u.t.tons on the control board.

Additional lights came on all around the camp perimeter and a siren started to wail mournfully. Spotlights on the fence pylons flared into life and began quartering the waste ground.

'Looks as though Gelvert has done it,' Ian said to the Doctor. 'How did you guess he would go so quickly?'

'It was a strong possibility. I don't think he would risk such a blatant theft unless he was ready to leave. My ring was no doubt intended to top up his funds for use on the outside.

When the attempt failed he suspected we might alert the authorities and so left immediately. From his behaviour earlier I suspect he has been planning the escape for some time.

Ahh... I presume this is the camp commandant.'

A large red-faced man entered the guardhouse still adjusting his uniform. 'What's going on here?' he demanded.

'A complaint from these prisoners led us to investigate unit R8, sir,' the guard reported unhappily. 'All its occupants are missing. We believe they have escaped.'

'Turn out the camp for roll call.'

In minutes the camp was roused and every internee was paraded and checked off, shivering and bleary-eyed in the rain. Apart from Gelvert fourteen men and two women from four different huts were unaccounted for.

'Dismiss them,' the commandant ordered wearily when the roll was complete. 'Then start checking the fence and review the visual log.'

'What about the two who raised the alarm, sir?' the guard asked, indicating Ian and the Doctor.

'What? Oh no, it's best if you keep them here for the moment. I may want to question them later. Meanwhile I'd better alert the Watch.'

He walked heavily through to the inner office.

'He sounded fed up with the whole business,' Ian observed, as they sat on a bench in the anteroom. 'I would have expected him to sound less resigned and more angry.'

'Considering the lax standards of this camp I suspect this is not the first such escape he has been obliged to report,' said the Doctor.

A few minutes later a car painted military drab rolled in through the gates. A smart young officer in a dark green uniform emerged from it and entered the guardhouse. He glanced curiously at Ian and the Doctor before being shown through to the commandant.

As he rested his head on the adjoining wall, Ian found he could just hear what was being said within. He nudged the Doctor and they both listened intently.

'This is the fourth escape in three months,' the newcomer was saying. 'Haven't you been able to plug the gaps in your security yet?'

'I've done my best, Captain,' the commandant replied stiffly. 'But they don't send me the extra men or equipment I need. It's a matter of priorities, apparently, and right now NC2s are at the bottom of the list. If you think you can do better, you can have my job any time you want and welcome to it!'

'There's no need to take that tone with me, Breen. I'm just doing my job. When NC2s escape they become the City Watch's responsibility.'

'Yes, and how many do you catch and bring back? A handful, less than ten per cent. That doesn't sound so very efficient. When you start recovering half of them maybe you can start telling me how to run my camp.'

'What are you talking about, ten per cent?'

Breen smiled grimly.

'I keep records, you know, and I can count.'

'Well, I can't explain the numbers. I'm only standing in for Captain Terrel... he's off sick... and his squad reports directly to Commander Pardek. But I'll do what I can to get you the extra resources you need.'

'Thank you, Lant. I'd appreciate that.'

'Meanwhile we have your NC2s to find. Do you know how they got out yet?'

'None of the fence alarms were triggered. At the moment your guess is as good as mine. Frankly, we're still puzzling over how the last group managed it. We even had the compound scanned for tunnels, though it's bedrock only a metre down. There was nothing, of course.'

'I had no idea things were so bad,' Lant said.

Breen gave a hollow laugh. 'It's not been widely advertised. They don't want the populace alarmed. You know what most people think of NC2s. The few we do get back won't say how they got out, and we can hardly beat it out of them. What can we threaten them with? Probably they hope to use the same method again.'

'I'm beginning to understand your problem.'

'Well, try to impress it on the council. This sort of thing is going to happen more often when the NC2s accept that we really can't take them with us and they realise they've nothing to lose. Well have to increase security soon or else they'll riot.

If only they'd sent me the...'

'Time to make our move,' the Doctor said to Ian quickly.

He got up, opened the door of the inner office and was halfway through it before the surprised guards could respond.

'Perhaps I can be of a.s.sistance,' he said to Lant and Breen amiably.

'Who are these men?' Lam asked, regarding them with deep intelligent eyes.

'They reported the escape. Go back outside, you two. I'll speak to you later.'

'But we overheard you speculating on the method by which Gelvert and his a.s.sociates may have escaped,' the Doctor continued, 'and I thought you might benefit from our advice. But apparently I was wrong. Come on, Chesterton. It seems we are not needed.'

And he made to withdraw.

'Wait!' Commandant Breen snapped. 'Do you know how it was done?'

'I do not know from first-hand knowledge, if that is what you are implying, sir,' the Doctor replied with dignity, 'but I can deduce the most likely method used, bearing in mind the prevailing conditions at the time of the escape. Unless it is proven that the escapees simply walked out of the front gate past your somnambulistic guards, I can suggest the most likely section of fence to examine and save you much wasted time.'

He's got their full attention now, Ian thought. I only hope he knows what he's talking about.

'All right,' Captain Lant said, 'how do you think it was done?'

'First I must confirm that your security system works as I suspect. You have visual monitors mounted on the towers, infrared sensors beyond the perimeter and the fence itself is no doubt alarmed? Come, sir, do not be reticent. Your security has already been breached. You can lose nothing by admitting the facts.'

Breen hesitated for a moment, then nodded.

'Then I can tell you what most probably occurred. Taking advantage of the downpour which obscured the cameras, Gelvert and his accomplices would have made their way to a point equidistant from the towers on the longest stretch of fence facing the waste ground. There they either cut through the fence themselves, after connecting bypa.s.s leads around the section, or else pa.s.sed through a hole that had already been prepared for them...'

'What!' Breen exclaimed. 'You cannot be suggesting that a citizen helped them?'

'Very probably. To evade your heat sensors they would have needed thermal scattering blankets. Where would they get them from? Perhaps you should ask the trader, what was his name... yes, Lesitor. We noticed Gelvert talking to him at some length earlier in the day. Anyway, once clear of the camp they could have reached the edge of the city in five minutes. You really should level the intervening ground properly, it provides far too much cover.' Before Breen could respond, the Doctor glanced out of the window. 'Ah, I see the rain is easing off. Shall we examine the fence?'

It was just as the Doctor had said.

A corner of mesh panel, adjoining a stanchion post between two watchtowers, had been cut through and repaired several times with conductive metallic adhesive of a matching colour. The bulges formed by the adhesive on the mesh were so slight that they would have pa.s.sed all but the closest inspection. A couple of lengths of wire with spring-clip ends remained in place across the cut section.

'They were too hurried this time to finish hiding their tracks,' said the Doctor. 'No doubt due to our prompt raising of the alarm.'

As Breen and Lant conferred, Ian said quietly: 'I'm not sure I like the way you've implicated that trader, Doctor. After all, he was only helping what amount to refugees and political prisoners to escape.'

'Lesitor was no doubt exploiting those he aided for his own ends, not acting out of some n.o.ble sense of altruism,' the Doctor replied brusquely. 'My ring was probably intended as payment to him. He was playing on the hopes and fears of vulnerable people. But was it simply for monetary reward? I suspect there is more going on here than meets the eye. These are not isolated incidents, there is method behind them. How else can you explain...'

He broke off as Lant came up to them.

'I want to know why you are doing this,' he said simply.

'If you hadn't pointed this escape route out, you might have used it yourselves.'

'Ah, but we have no intention of attempting anything so crude,' the Doctor said with a twinkle in his eye. 'When we leave here it will be quite open and above board.'

Lant frowned. 'Well, Breen's grateful enough, I grant you.

No doubt you'll have an easy time here. But if you think this buys your way on to the Ship, I'm sorry but you're mistaken.'

'That was never our plan either. This little exercise has simply served to attract the attention of somebody in authority, such as yourself,' the Doctor explained with disarming candour.

'You have my attention,' Lant said. 'Go on.'

'We only ask three simple favours. First, would you please check on the condition of my granddaughter, Susan Foreman.

She was taken to your City Hospital earlier today after being badly injured. I have not yet had any news of her.'

'I can do that. Next?'

'A friend of ours may be trapped in the damaged building where your people first found us... I do not know the address but I'm sure it is on record. Please monitor the progress of the work for any news of her. Also there will be a blue box, rather taller than a man, in the remains. It will be undamaged and you will not be able to open it by any means at your disposal.

You may wish to bring its existence to the attention of the mayor, and to remind him that the explanation we gave when we first met was the truth.'

'What explanation?'

'Ahh, if I told you that now, without evidence, you would not believe me.'

'Really,' said Lant dubiously. 'And the third favour?'

'If you find the man Gelvert, he may have a key on him tied to a length of black ribbon. It was the item he stole from me. If you find the key, I will show you what is inside the box.'

Lant gave them a very searching look, then nodded. 'All right, I'll see what I can do. Meanwhile, I suggest you get back to your tent.'

The Doctor rubbed his hands together with satisfaction as they walked back through the darkened camp. 'Most promising, eh, Chesterton? Captain Lant's curiosity has been piqued sufficiently for him to cooperate. We shall soon be out of here, never fear.'

'Yes, Doctor,' Ian said grudgingly. 'Very neatly done. But what is going on here?'

The Doctor frowned. 'There is something very wrong with this city and this camp.' He gazed into the distance, musing half to himself. 'Escapes on this scale, primitive security systems... No, no. If it wasn't absurd, I would almost say somebody was giving these people a chance to escape.'

'But that's ridiculous!'

'As you say, my dear boy, ridiculous. But ask yourself; what happens to the nine out of ten NC2s that are not recaptured?'

Chapter Seven.

End of the Tunnel Laboriously Barbara scratched another B> mark on the side wall of the tunnel. She was feeling desperately tired and increasingly dispirited.

As far as she could estimate, her watch having stopped, she had been travelling for six hours. It had been hard, slow going. The ledge pathway was narrow and slippery and twice she had fallen into the culvert stream, on the second occasion cracking her knee badly so that she now walked with a limp.

In several places she had to work her way round piles of rubble from collapsed walls. Each time she glanced nervously up at the cracks radiating across the arched concrete roof over her head, hoping she would not bring it down on top of her.

So far she had found two ladders set in the sides of the tunnel, rising through shafts to what might have been manholes. But the heavy metal covers would not budge and there had been no response when she pounded on them and shouted for help. She had not expected when she set out that the only exits from the drains might be sealed.

Now she was wondering how far she could continue.

Thirst would soon force her to drink some of the drain water and pangs of hunger were also making themselves felt.

Meanwhile the chill of her damp clothes was steadily sapping her strength.

Then she became aware of a new sound beyond a bend ahead of her, rising above the soft lap and splash of the stream.

It was an echoing rush of breaking water and was growing steadily louder. Could it be the outfall of the drainage system?

She almost ran forward, slipping and sliding dangerously, longing for the first sight of daylight.

She rounded the corner... and her heart sank.

Three feeder tunnels, similar to the one she had travelled along, met in a chamber half-full of dark swirling water. On the far side was the low arch of the main outlet into which they emptied, but its mouth was closed off by a heavy grille of close-set bars that extended from the roof down into the water.

Perhaps the stream did eventually lead to the outside, but there was no way she could follow it any further.

Barbara sank down on to the narrow ledge that skirted the edge of the chamber and buried her head in her hands. Either she would have to return to her starting point, or else explore the side tunnels. How many miles might they run? Would all their access shafts be closed off as well?

As she brooded she switched off her torch to save its battery. But after a few minutes the absolute darkness became oppressive. She began to hear words hidden within the rush and gurgle of the water and thought Ian was calling to her. She almost shouted back in response before she realised it was all in her mind.

She switched the torch on again.

The beam shone by chance at a low angle along the line of the ledge, and picked out imprints in the thin crust of damp silt that had been deposited over the years. She peered closer.