'Shine the torch at my feet,' commanded the Doctor.

'There you see?'

Ian shone the powerful beam ahead along the ledge and the cavern wall.

'Quite a chasm, is it not?' the Doctor said.

'There's not much to hold onto either, Doctor,' said Ian.

'We'll have to press ourselves against the rock.'

The old man shook his head morosely. 'If I press myself against it any harder, my dear Chesterton, I shall do myself an injury. Now do get a move on! We cannot afford to stand here admiring the view. We have got to find Barbara, you know.'

Ian threw the Doctor a warning glance and cautiously continued edging his way along the perilous shelf. 'I only hope this leads somewhere useful,' he murmured to himself.

They worked their way slowly sideways for several metres and then reached a section where the ledge was barely wider than the length of their shoes. Not only was it extremely brittle, but in places it sloped away at an alarming angle from the rock wall. If it got any worse they would have no choice but to retreat, but where to? They had followed the only viable route out of the chamber where the TARDIS had materialised and it had brought them onto this ledge. They had not found any alternative way down to the cavern floor.

Pressed flat against the wall, they were just negotiating a particularly nasty sloping section when the t.i.tanic bellowing noise suddenly erupted again. Ian stopped dead and the Doctor, only centimetres away, collided with him for a second time, almost knocking him down into the abyss. Ian's fumbling fingers nearly dropped the torch, but at the last moment he managed to trap it between his knees. At the same instant, the Doctor lost his balance and started toppling forwards. With superb reflex action Ian grabbed his sleeve and dragged him back against the rock face. Wringing with cold sweat, they stood rigid against the wall listening to the dying echo of the awesome roar.

'You must be more careful,' the Doctor scolded. 'You almost dropped the torch.'

'What the devil was that?' Ian whispered.

'Well, it certainly wasn't me me!' snapped the Doctor, angry with himself for almost causing a disaster. 'Stop showing off and shine the torch down there.'

Clenching his teeth in frustration, Ian extricated the torch from between his knees and directed its broad, brilliant beam over the precipice.

What he and the Doctor saw in the bright pool of light made their flesh creep. The sandy floor of the cavern appeared to have come alive and to have formed itself into a huge beast of terrifying size and menace. Its vast head was the size of a small room and it tossed savagely from side to side as if trying to tear the stale air apart.

The enormous jaws were armed not with teeth but with curving scimitar gums as sharp as blades. On each side of the head was a giant luminous red eye whose dilated pupil enabled the beast to see quite easily in its dark habitat.

Around the thick neck there was a kind of ruff of bony spines alternating with weblike plates. The creature's ma.s.sive body was plated and hinged like that of an armadillo or a rhinoceros, and its dry h.o.r.n.y skin, pitted and grooved, was the colour of the sand itself. The monster's thick legs were so short that its belly dragged perpetually along the ground and its long tail thrashed the sand like a whip.

The Doctor and Ian stood transfixed on the ledge above, watching the behemoth as it caught their scent and reared up on its hind legs. It uttered another deafening raucous bellow and its hot foul breath made them turn aside in disgust, their gorges rising.

'What's that nightmare thing?' Ian whispered, trying to press himself into the rock out of harm's way.

The Doctor shook his head grimly. 'I have no idea, my boy. My only concern is that it is down there and we are up here...' The Doctor emitted a squawk of alarm as a portion of the ledge gave way beneath him.

Ian grabbed his companion's sleeve and managed to drag him to safety a little further along the ledge. The monster's baleful eyes glowed like red-hot rings just a few metres below them and its huge purple tongue lashed greedily out of its cavernous mouth.

'Thank you,' muttered the Doctor grudgingly. 'But we really cannot dawdle along gawping at the local fauna, Chesterton. This is not a zoo. Come on!'

Ian Chesterton could quite happily have pitched his infuriating companion into the monster's gaping jaws, but he controlled his irritation with heroic forbearance and watched as the creature slumped back on all fours and dragged itself off along the cavern floor in the direction they themselves were taking. 'Doctor, that thing's got eyes, so presumably it must have come in from the outside,' he declared, easing cautiously along the perilous shelf again.

'Good. Very intelligent observation my boy,' the Doctor said affably, following close behind him. 'Sort of reasoning I might have employed myself...'

Ian grinned smugly to himself as he edged, like a crab, along the ledge.

'However, I happen to know better,' the Doctor added mischievously. 'You should also have noticed that the beast possesses luminescent irises and can therefore provide its own light source. Ergo Ergo, it does not necessarily inhabit the open air.'

Ian bit his tongue and fumed in silence, trying to concentrate on his hazardous task.

'However,' the Doctor agreed after a pause, 'it is possible that the beast may lead us out of the caves.'

Ian shone the torch down into the well of darkness. The beast had disappeared round a huge b.u.t.tress of rock, though they could still hear its thunderous movements and its stentorian breathing. Ian directed the torch along the ledge again. 'It seems to get wider in a minute,' he whispered, anxious not to attract the beast's attention. 'But it slopes a lot more by the pillar and there are hardly any decent hand-holds anywhere.'

'What is that just ahead?' exclaimed the Doctor excitedly, pointing to something glinting in the rock face near the wider part of the ledge.

Ian aimed the torch. 'Looks like a couple of old fashioned doorknockers.' He squinted at the two heavy metallic rings fixed at shoulder height. 'You know, the sort with rings hanging out of lions' mouths. Somebody's obviously been this way before and thoughtfully provided something to hold on to.'

As they drew closer they discovered that the ledge almost disappeared altogether just before the wider section on top of the b.u.t.tress. Ian found that his toes were overhanging the crumbling edge as he reached forward to grasp the nearer ring.

'Careful, Chesterton!' warned the Doctor. 'Let me have the torch.'

Ian pa.s.sed the torch back to the Doctor. Then he grasped the first ring with both hands and swung himself forward, his feet barely finding any grip on the tiny strip of ledge. He was about to reach for the second ring with one hand when the creature below uttered another bloodcurdling bellow.

This time the noise was even more unbearable, with an edge to it like the sound of fingernails sc.r.a.ping on galvanised steel. Startled, Ian lunged at the second ring and overbalanced. As he swung himself forward onto the wider part of the ledge he felt the second ring shift ominously under his weight.

'Watch this one, Doctor. It's loose!' he warned, landing safely on the top of the b.u.t.tress.

'Loose?' echoed the Doctor, gripping the first ring and preparing to swing himself along to the second one.

'Yes, I'm afraid I dislodged it,' Ian apologised. 'But it's a lot easier over here.'

All at once there was a loud click from deep inside the rock behind the rings followed by the m.u.f.fled whine of some kind of machinery. The Doctor shone the torch on the loosened ring and peered at the pivot which attached it to the rock. There was a viscous silvery trail running down the wall. 'Lubricant!' he exclaimed. 'The ring has some kind of oil on it, which suggests...'

'And what's that noise?' Ian interrupted. 'I don't like the sound of it.'

'Neither do I , Chesterton. Quick, come back here. It may be some kind of trap.'

Suddenly Ian's heart fluttered and faltered, and a horrible p.r.i.c.kling sensation ran up and down his spine. On each side of him, two vertical rows of steel blades had sprung out of narrow slits in the rock wall and locked into position at right angles to the ledge. The blades protruded about thirty centimetres beyond the edge and were pointed at the ends. He was completely trapped on top of the b.u.t.tress.

'Doctor... I'm stuck!' he gasped, his face a vivid white in the torchlight.

The Doctor tucked the torch under his chin and poised himself with both hands on the first ring. 'Really, Chesterton, why can't you leave things alone?' he muttered in a strangled sort of voice.

Puffing with effort, the Doctor hauled himself across and dug his toes into a tiny cleft in the narrowest part of the ledge to help take some of his weight and enable him to have a hand free to try and reverse the mechanism.

Hanging from the first ring with one hand, he reached across with the other and attempted to force the second ring back into its socket. But it was jammed solid and would not budge a millimetre.

Below them, the prowling monster let out another gargantuan bellow even shriller and more grating than the last, and its lashing tail sent a salvo of stinging sand flying up into their faces.

'I have a horrible feeling that it's feeding time,' Ian muttered ruefully.

As he spoke the Doctor gave the ring an extra wrench.

There was immediately another series of clicks inside the rock and to their horror the section of wall between the two rows of blades slowly began to move outwards, narrowing the top of the b.u.t.tress where Ian was trapped with every pa.s.sing second.

Ian's mouth dropped open and his eyes popped incredulously. 'Doctor, it's pushing me... It's pushing me towards the edge...!' he cried, desperately searching the moving slab for a hand-hold.

Below, the creature sat back on its hindquarters and reared its colossal head again, now uttering short staccato roars of apparent relish and antic.i.p.ation.

The Doctor yelled to Ian to hang on while he tugged and twisted and pushed the oily ring in a vain struggle to reverse the machinery. Meanwhile the slab of rock trundled inexorably outwards between the blades, and in a few seconds Ian would be compelled to hang over the precipice by his fingertips.

'Doctor, please do something!' Ian begged, his voice cracking with panic.

'You couldn't climb over the bars onto the other side?'

the struggling Doctor suggested doubtfully.

'Doctor, they're razor sharp!'

The Doctor peered more closely. 'Dear me, so they are.

How very inconvenient for you. Well, it's no good trying to climb over them.'

Ian jerked his head towards the rings. 'Can't you do anything with those?' he pleaded, as he felt his heels reach the edge of the ledge.

His fingers found a small crevice in the slab and he managed to work them into the hand-hold just as his feet were shoved off the ledge into thin air. 'Doctor, I can't hold on much longer...' he gasped, his body sagging and his arms stretching painfully under the weight.

'I am doing my best,' the Doctor a.s.sured him, experimenting with manipulating both rings at the same time while still hanging on to one of them. 'Kindly remember, Chesterton, that it was you you who triggered this fiendish mechanism.' With the torch jammed under his chin, the Doctor was forced to perform the most ape-like contortions in order to shine the beam onto the rings above his head. If Ian had not been trapped in such a perilous predicament, he would have been helpless with laughter. who triggered this fiendish mechanism.' With the torch jammed under his chin, the Doctor was forced to perform the most ape-like contortions in order to shine the beam onto the rings above his head. If Ian had not been trapped in such a perilous predicament, he would have been helpless with laughter.

About fifteen centimetres beyond the edge, the section of wall abruptly stopped moving and Ian was stranded in mid-air above the yawning abyss. Below him the monster continued its hungry bellowing. Unluckily the two bladed barriers stuck out further than the movable slab, so Ian could not even attempt to swing himself round the edge of the slab and back onto the narrow ledge beside the Doctor.

Ian's fingers were growing number every second. He tried to call out but his dry throat would only emit a croak of despair.

'Use my coat!' the Doctor suddenly shouted. Wriggling out of it, he hooked his arm through one of the rings and leaned out as far as he dared to fling his frock coat over the pointed ends of the blades. 'The material's pretty thick. It should protect your hands long enough for you to swing round here onto the ledge.'

Blinking the sweat from his eyes, Ian squinted sceptically at the coat draped over the murderous blades.

He had no sensation left in his hands now but he could feel the monster's hot rancid breath on his legs as it reared in the darkness beneath him. It seemed that he had nothing to lose. 'This'll never work...' he gasped, grabbing at the coat with one hand.

The Doctor grasped the other side of the coat with his free hand and held it firm. 'Now, my boy, swing!' he commanded.

Ian nearly fell. As he tightened his grip on the coat sleeve the cramped fingers of his other hand tore away the brittle crevice in the mobile slab and his body lurched sickeningly against the blades. But the coat material protected him and he ended up hanging with both hands clutching the musty old garment.

'Pull yourself up and round this way!'

Valiantly, Ian hauled himself hand over hand up the Doctor's coat and round the end of the blades. The Doctor seized his arm and Ian jumped for the narrow ledge with a leap worthy of a swashbuckling hero. His flailing toes found the thin ledge and he landed breathless and soaked in sweat next to the panting old man. The Doctor moved back to the first ring, leaving Ian clinging weakly to the troublesome second ring.

'Thanks, Doctor... Thought I'd had it...' Ian whispered, trying to avoid the temptation to look down into the bellowing abyss. When he had recovered a little he peered at the rings and then at the blades and the moving section of wall.

'It looks like something out of Edgar Allan Poe,' he muttered, trembling at the thought of what he had just escaped.

'Poe? Who's he?'

'But what is it for, Doctor?'

'No idea,' the Doctor snapped, removing the torch from under his chin and shining it onto the rings.

There was a long, low rumble from the creature and they heard it dragging itself laboriously away beyond the b.u.t.tress.

'The executioner sounds disappointed,' Ian murmured wryly.

The Doctor grunted, studying the rings through narrowed eyes, his head thrown back and his cheeks sucked in with characteristic concentration. 'Come along, come along. Give me a hand!' he ordered abruptly.

'Barbara could be in grave danger. We have wasted quite enough time as it is.'

'What shall I do?' Ian asked, trying to balance on the thin ledge without putting any strain on the ring.

'Nothing, until I tell you to. Unless I am very much mistaken these rings work in conjunction with one another. It is just a question of working them in the correct sequence,' the Doctor explained mysteriously. He twisted and turned the first ring like a burglar trying to open a combination lock, pressing his ear against the rock and listening for something. 'Half a turn clockwise now!' he cried.

Ian obeyed as best he could without losing his footing.

Nothing seemed to happen.

The Doctor frowned and turned his ring again. 'Two turns anticlockwise now!' he commanded.

Ian accomplished the difficult manoeuvre without slipping.

'And push!'

Ian pushed. The ring eased a little way into its socket.

'Half a turn anticlockwise again!'

Ian obeyed.

'And push!'

Ian pushed again. There was a hollow clang inside the rock followed by a grating whirr.