And then he gasped dozens of them!

Abruptly the scurrying sounds stopped. Barry stood motionless, hardly daring to breathe. The red, glittering eyes seemed to regard him with an almost inhuman intelligence. Cautiously he took a step backwards.

And with a hideous, screeching cry that echoed off the tunnel walls, the rats rushed towards him!

The tall, white-haired man in the purple smoking jacket strode through the hospital corridor like a visiting dignitary. In his wake hurried a short, pretty blonde girl with an exasperated expression on her face.

'Where's the fire, Doctor?' she gasped.

The white-haired man cast a puzzled glance over his shoulder. 'Fire?'

'What I mean is, can't you slow down? Why the big rush?'

The Doctor sighed. 'My dear Jo, you really must cultivate a sense of urgency. According to the Brigadier-'

'Pa.s.s please, sir.'

The order, cutting off the Doctor mid-flow, had been issued by an armed squaddie whose beret bore a circular badge identifying him as belonging to UNIT. His muscular form blocked the double doors that the Doctor had been approaching.

With a withering look the Doctor said, 'Don't be absurd, man. Surely you recognise me?'

Before the squaddie could reply, the blonde girl, Jo Grant, produced two UNIT pa.s.ses.

'We're here to see the patient,' she said. 'Didn't the Brigadier tell you we were coming?'

'Captain Yates did, miss,' replied the squaddie. 'But the Brigadier gave orders not to allow anyone through without authorisation. No exceptions, he said.'

The Doctor harrumphed.

Once their pa.s.ses had been verified, the Doctor and Jo pa.s.sed through the double doors into an area that seemed a world away from the bustle of the rest of the hospital. The sense of hush was almost expectant. A bespectacled, harried-looking man in a white coat rose from a nearby desk and scurried forward, hand outstretched.

'Dr Raith,' he said. 'You must be the scientists from UNIT.'

The Doctor was already peering over Raith's shoulder. It was Jo who took his hand. 'I'm Josephine Grant. This is the Doctor.'

Instead of saying h.e.l.lo, the Doctor asked, 'Where's the patient?'

Responding to his authoritative tone, Raith turned smartly. 'This way.'

As they marched along the corridor, Jo fell into step beside him. 'What's wrong with the patient, exactly?'

Raith gave her an anxious look. 'It's probably best if you see for yourself.'

'He was a sewer worker, wasn't he? We heard he was attacked in the tunnels, that his injuries were... unusual.'

Raith laughed without humour. 'Unusual is putting it mildly. Here we are.'

The sign on the door read: ISOLATION AUTHORISED PERSONNEL ONLY. The two UNIT soldiers standing sentinel checked their pa.s.ses again, then Raith produced latex gloves and surgical masks in sealed plastic bags from the pockets of his white coat.

'If you could wear these? For your own protection.'

'I really don't think that's...' the Doctor began, then noticed Jo frowning at him. 'Oh, very well.'

When they were masked and gloved, Raith led them into the isolation chamber. A bed surrounded by white drapes dominated the featureless room. Aside from gentle bleeps and clicks from unseen machines beyond the drapes, all was quiet. Raith crossed the room and drew one of the drapes aside, ushering Jo and the Doctor forward. As soon as Jo set eyes on the patient she gasped.

'Good grief,' murmured the Doctor.

The young man was lying on a crisp white sheet, naked from the waist up. His stomach and chest were etched with a fine tracery of metallic circuitry, which appeared either to have embedded itself into his flesh or to be growing out of it. To Jo the circuitry resembled a tree, with branches fanning out like dark veins around the man's ribs, across and down his arms, and up his neck. Tiny white lights, set at points where the fibres split into smaller tributaries, winked and flashed, as though alive.

'What's happening to him?' Jo murmured.

The Doctor's face was grim. 'I'm very much afraid, Jo, that the poor chap is turning into a machine.'

Jo wrinkled her nose. 'Bit whiffy, isn't it?'

Their guide, a stocky Irishman called Joe McGowan, grinned. 'You get used to it after a while, miss. Give it five years and you'll not notice a thing.'

With McGowan and Jo were the Doctor, Brigadier Lethbridge-Stewart and three UNIT squaddies. The soldiers were armed with rifles, the Brigadier with his trusty service revolver, which he clutched in his leather-gloved hand. McGowan, the Doctor and Jo wore hard hats with lamps attached. The overlapping beams played across the curved, dripping walls of the sewer tunnels and gleamed on the water flowing along the central channel.

It was the Doctor who had insisted on retracing Barry Jenkins's steps and Jo who had insisted on accompanying him. The Brigadier had raised a token protest but knew that Jo had faced far greater perils in her travels with the Doctor. Most recently the two of them had returned from a planet full of man-eating plants, invisible aliens and Daleks! A London sewer tunnel was small potatoes by comparison.

McGowan, leading the way, suddenly halted. 'That's odd.'

The Doctor appeared at his shoulder. 'What is?'

'That wall.' McGowan pointed. 'It shouldn't be here.'

'Let me see.' The Doctor took the map, his eyes skimming across it. Half-turning to Jo and the Brigadier, he said, 'He's quite right, you know.'

'Renovations?' suggested the Brigadier. 'Channelling the flow, that sort of thing?'

McGowan gave him a pitying look. 'There speaks a man who knows nothing about what's under his own two feet.'

'I'll have you know-' began the Brigadier, but the Doctor waved him to silence.

'Shh.' He had moved across to the wall and was standing with his ear pressed against it.

'What is it?' hissed Jo.

The Doctor produced a stethoscope from a jacket pocket and used it to listen to different sections of the wall, umming and ahhing as he did so.

Finally, in a dry voice, the Brigadier said, 'In your own time, Doctor.'

The Doctor arched an eyebrow. Stuffing the stethoscope back into his pocket, he said, 'Do you know what this is?'

'Surprise us,' said Jo.

'It's a box.'

'A box?' repeated the Brigadier.

The Doctor nodded. 'A stone box. The question is what's inside?'

The Brigadier stepped forward and rapped on the wall with the b.u.t.t of his revolver. 'Could get a couple of my chaps to blow a hole in it for you.'

The Doctor winced. 'Or we could employ more subtle methods.' With a grin that simultaneously quadrupled his wrinkles and made him appear boyish, he produced a hammer and chisel so archaic they looked as though they could very well have once belonged to Mary Anning and began to tap the wall.

'Take a long time to-' the Brigadier began and then the squealing started.

It came from the opening on their right, a shrill, terrifying cacophony, like a hundred panes of gla.s.s shattering at once. As the UNIT troops ran forward, the Doctor and the Brigadier whirled towards the sound, the Brigadier raising his revolver and pointing it into the tunnel.

McGowan clutched Jo's arms and looked at her wild-eyed. 'What in the Lord's name is that? It's like a million banshees all wailing at once.'

Jo was scared too, but McGowan's panic made her feel almost calm. 'It's animals,' she said. 'I think.'

'Look!' McGowan screamed.

Jo twisted her head. Through the crush of bodies in front of her, she glimpsed multiple red points of light swirling through the blackness towards them. She was baffled at first, and then, as the lights drew closer, she recognised what they were.

Red eyes. And now the light from the Doctor's lamp was picking out the first wave of the creatures surging towards them.

They looked like rats. Small, sleek, scampering on four legs, tails snaking behind them.

But these rats were not black or brown. They were not covered in fur.

They were silver. They gleamed.

They were robot rats! Made of metal!

The three UNIT troops began to blaze away, the din of their rifles deafening in the confined s.p.a.ce. Bullets ricocheted off the rats, raising sparks. A few of the creatures were ripped apart by gunfire, but the majority kept coming a squealing, scuttling wave of metal.

'Retreat!' roared the Brigadier, letting off more shots.

McGowan was already halfway up the tunnel, running as fast as he could. Jo, the Doctor and the Brigadier followed, the UNIT troops bringing up the rear.

Suddenly one of the soldiers screamed. Jo whirled, to see him go down, his body immediately engulfed by a twitching, writhing ma.s.s. The Brigadier sprang forward, but the Doctor shouted, 'No! Let me!'

Before the Brigadier could protest the Doctor pointed his sonic screwdriver. The sound that came from it was so piercing that Jo clapped her hands over her ears. Dimly she was aware of the Brigadier and the soldiers covering their ears too. The effect on the rats was spectacular.

They began to jerk and spin, to go haywire. A few sparked and became instantly motionless, but most, once they had recovered from their momentary disorientation, turned and fled back into the darkness from which they had come.

'Well done, Doctor!' yelled the Brigadier. 'Handy little device, that weapon of yours!'

The Doctor turned off his sonic screwdriver with a frown. Almost primly he said, 'The sonic screwdriver isn't a weapon, Brigadier. My intention was to confuse the creatures, not destroy them.'

'Even so,' the Brigadier said. 'Seems to have done the trick.'

As the two soldiers pulled their downed colleague back to his feet, the Doctor wandered across to pick up one of the inert rats. Giving it a cursory examination, he dropped it almost absentmindedly into the pocket of his purple jacket.

Gingerly rubbing one ear, the Brigadier asked, 'So what were those things, Doctor?'

'A deterrent,' the Doctor said, and peered thoughtfully back up the tunnel. 'Whatever's behind that wall doesn't seem terribly keen on house calls.'

Although the man in the isolation chamber did nothing but lie there, Dr Raith was terrified of him. He wished the fellow had not been brought to the hospital; that the responsibility for his care belonged to someone else. It was not that he was afraid the man might die. What really frightened him was not knowing what his patient would eventually become. That odd chap from UNIT had said Jenkins was turning into a machine but how could that be? And yet the results of their various examinations seemed to corroborate the Doctor's diagnosis.

Raith was sitting at his desk, staring glumly at the man's X-ray results, when the peace of the Isolation Unit was shattered by several thumps and a crash. His head snapped up. He had no doubt the sounds were coming from the isolation chamber which housed their patient. He could only suppose the man had woken up and was now flailing about, distressed and confused. Wishing he were anywhere but here, he jumped up and ran towards the sounds. He arrived to find the two UNIT guards facing the door, training their rifles on it.

One of them turned to him nervously. 'He don't sound too happy in there, does he, Doc?'

Raith was about to reply when the door of the isolation chamber burst open.

For a moment n.o.body moved. Raith goggled at his patient in horror. In the thirty minutes since his last examination, the man's condition had worsened considerably. The complex network of circuitry now covered his entire upper body, including his face, which stared blankly out through eyes filmed over with a silvery sheen. Hesitantly Raith stepped forward.

'Mr Jenkins, why don't you...'

His voice tailed off. Jenkins' mouth had dropped open, as though on a hinge, and Raith could see something moving in there.

Suddenly a ma.s.s of silver worms, each about the size of a finger, poured from Jenkins's mouth. Moving with lightning speed, they flowed towards the two soldiers and began to slither up their bodies.

As the men screamed and dropped their rifles, Raith turned and fled. But he wasn't fast enough to outrun the worms.

Within seconds they were on him.

Although the Brigadier would never have admitted it aloud, he desperately needed the Doctor's advice. Something had gone badly awry at the hospital and the Brigadier had no idea how to deal with it. According to Sergeant Benton, the place was overrun with silver worms, which were infecting people, turning them into machines like that poor chap in the Isolation Unit. There was apparently no stopping the things. They were multiplying and spreading at an alarming rate. The Brigadier had ordered Benton to quarantine the hospital and seal every exit but it was only a matter of time before the worms escaped into the wider world.

As he strode towards the UNIT laboratory, he heard the familiar trumpeting bellow of the Doctor's TARDIS.

'No!' he shouted, breaking into a run. 'Don't you dare, Doctor! I absolutely forbid it!'

But as he burst through the doors he saw the familiar blue box fading away. The Brigadier couldn't believe it. Where the blazes had the Doctor gone now?

Standing at the six-sided console the Doctor said, 'We've arrived, Jo.'

Jo looked dubious. 'But where? You know what the TARDIS is like. We might just as easily be in Timbuktu.'

The Doctor looked pained. 'The old girl homed in on energy signals emanating from the chamber. We're right on target.'

Jo raised her eyebrows. 'Well, there's only one way to find out, isn't there?'

They emerged from the TARDIS into a dank-smelling stone chamber strewn with debris. Jo switched on the torch she was holding and shone it around. The debris, which she had thought was rubble, was actually bits of machinery. There were cogs, circuit boards, engine parts and all manner of other paraphernalia.

'It's a sc.r.a.pyard,' she said, torchlight playing over a nearby heap of metallic components, which gleamed silvery grey.

The Doctor was about to reply when, from behind the heap of components, they heard the sc.r.a.pe of movement.

They froze. Slowly the Doctor raised a warning hand, indicating that Jo should stay still. He edged towards the metal hummock and peered over the top. As he did so something rose up from behind it with a clicking and a whirring and a creak of metal.

It was a figure. Huge and dark and shadowy. The Doctor jumped back as Jo jerked the torch upwards.