They passed through a massive stone archway and found a flat stretch of ground beside a ruined tower.

'Bit of a bumpy wicket,' said the Doctor. 'Still, it'll do.' He set up the stumps at one end, carefully laying the bails across the top. Then he paced down the wicket and set more up at the other end. He offered Tegan the bat. 'Want to open the batting for Australia, Tegan?'

'And face the bowling on a pitch like that? Far too dangerous. That ball's hard, you know.'

The Doctor turned to Turlough. 'You or me, then.'

Turlough fished in his pocket and produced a coin. 'Let's toss for it, shall we?'

'Still got your two-headed Trion ten-credit piece, I see,' said the Doctor amiably. 'We'll use my English half-crown. Heads or tails?'

Turlough chose heads, the Doctor tossed the coin and Turlough won. He took up his position at the wicket and the Doctor walked back behind the bowler's stump.'Ready Turlough?'

Turlough tapped the crease with his bat and nodded determinedly. 'Do your worst, Doctor!'

The Doctor started his run up and suddenly checked, staring aghast over Turlough's shoulder

'What's up?' called Turlough, turning to see what the Doctor was staring at with such horrified astonishment. Some distance away stood a motionless man-shaped silver figure. It seemed to be watching them.

'Keep still,' roared the Doctor. 'Keep absolutely still!'

Ignoring the advice, Turlough leaped for the shelter of the ruined tower. The silver figure swung its arm and a silver javelin flashed past Turlough's body, missing him by inches. He dived into cover and then peered out.

The Doctor was standing motionless, his arm still drawn back. Tegan began running towards him.

'No!' yelled the Doctor. 'Don't move!'

The silver figure swung round towards Tegan. As its arm flashed down, the Doctor bowled the fastest ball of his life. The cricket ball thudded into the silver figure's chest.

Although not enough to harm it, the impact was enough to spoil its aim. The second javelin flashed past Tegan as she completed her run and dashed into the tower beside Turlough.

The Doctor joined them with a flying leap. 'Keep down!' he shouted. 'And don't move!'

A hail of silver javelins flashed towards them, whizzing over their heads or striking sparks from the sheltering stone. The Doctor looked round. They were in a low, circular stone chamber at the base of the tower. Thick walls protected them on all sides. The only gap in their defences was the broken door through which they'd entered.

'Quick,' said the Doctor. 'Grab some of this masonry and barricade the doorway.' They dragged the chunks of stone that littered the floor across to the doorway and built a hasty barrier.

'All right, that will do,' said the Doctor. 'Now lie down and lie still! Its sensors detect any movement.'

'What is that thing, Doctor?' whispered Tegan.

'A Raston Warrior Robot - the most perfect killing machine ever devised.'

'Can we make it back to theTARDIS?' asked Turlough.

'Back through the archway and across the grass? I doubt it. The Raston Robot moves like lightning. I'm afraid it's got us pinned down.'

*

The robot was standing motionless on a nearby hillock, quite close to them, surveying the surrounding countryside. Its field of view included the TARDIS, just visible through the stone archway, the ruined tower in which they were hiding and all of the route between the two.

Tegan and Turlough studied the robot with fascination. It was extraordinary only in its simplicity. It was manshaped, not particularly large, with a body surface of smooth silvery metal. The head was blank, a featureless metal oval. As far as they could see it was unarmed.

'Where does it keep those javelin things it was chucking at us?' asked Tegan.

'Maybe it's run out!' suggested Turlough hopefully.

The Doctor shook his head. 'It extrudes the javelins from its body. Its weapons are all built-in.'

'What is that thing anyway?' asked Turlough. 'What did you call it?''A Raston Warrior Robot,' said the Doctor.

'Well, what's it doing here?' whispered Tegan indignantly. 'It wasn't here before.'

'I think someone must have sent it after us. What you might call a particularly nasty practical joke!'

Turlough studied the silver figure in fascination. 'Sent it from where, Doctor?'

'From the Death Zone. Sarah and I encountered it on the way to the Dark Tower.'

'You weren't with Sarah,' began Tegan, and then broke off.

'I was in several places at once, remember.'

'So how did - you and Sarah deal with it?'

The Doctor cast his mind back, struggling to recall the memories of his other self. When a number of his incarnations came together and acted independently, their memories were shared. But to each Doctor, the memories of the others were shadowy, a little dreamlike. 'We didn't. We were pinned down, just as we are now, when a squad of Cybermen turned up.'

'And the Cybermen scuppered the Raston Robot?'

'On the contrary, it was the Cybermen who got scuppered. We managed to slip past during the battle.'

'Are you telling me that thing fought off an entire squad of Cybermen?' demanded Turlough incredulously.

'It didn't just fight them off, Turlough. It massacred them.' The Doctor closed his eyes, recalling the slaughter. Cybermen staggering back and collapsing, their chest-units exploding as they were pierced by the Robot's javelins. Cybermen with arms and legs and heads sliced off by the sword that suddenly grew from the robot's hand. 'Believe me, they didn't stand a chance,' he concluded.

'Then neither do we,' said Turlough grimly. 'We've got no food, no water... We can't stay hiding here forever. But if we move that thing will kill us.'

'Never say die,' said the Doctor. 'We can't out-fight it, but perhaps we can out-think it.'

'Where does it come from - originally, I mean?' asked Tegan. 'Who made it?'

'Nobody knows. According to legend, it was created by a race that was old when the Time Lords were young. A race that devoted all its great powers to war and the creation of super-weapons. They vanished without trace -probably destroyed themselves. Unfortunately, they left a few of their weapons behind - like this one!'

'It must have some weak point,' she said. 'It's hard to think of one. It feeds on the atomic radiation in the atmosphere, so it never runs down. It can convert energy into matter. And it moves like lightning. Watch!' Cautiously the Doctor reached out for a chunk of broken masonry. Leaping up he threw it, not at the robot but at a nearby pile of rubble, and dropped down again. As the rock clattered on the nibble, the robot instantly fired a javelin at the sound. It stood poised for a moment, then blurred and vanished, reappearing immediately on top of the pile of rubble.

It scanned the area for a moment until it was satisfied there was no danger. Then it vanished again, reappearing on top of its original hillock. There was a moment of horrified silence as the Doctor's companions absorbed what they'd just seen.

'Look, it must know we're here,' said Tegan quietly. 'Why doesn't it just flash over and turn us into pin-cushions?'

'Because we're no threat to it,' said the Doctor. 'It was designed as a guard robot and it's got fixed behaviour patterns. It chooses a patch of territory and defends it -and it interprets all movement as being hostile.'

'Marvellous,' said Turlough bitterly. 'If we attack it'll kill us, if we run away it'll kill us - but we're perfectly at liberty to sit here and die in our own time.'

'That's about it.' The Doctor was thinking hard. 'What we need is a distraction.'

'Like a squad of Cybermen?' suggested Tegan.

'Anything,' said the Doctor. 'Anything at all.'

*

The Doctor stepped out of the TARDIS surveying the picturesque scene around him. He had barely registered the silver figure posed on its hillock when a young man in a cricket blazer jumped up from behind a wall and shouted, 'Go back!'

Their eyes met and time froze. The remaining memories of his fourth incarnation and the memories to date of his fifth flooded into the Doctor's mind. The gaps were filling fast now - he was almost himself again. As always, the process was over in moments. He then strode towards the Fifth Doctor, who was emerging from the base of a ruined tower and running towards him.

'How nice to meet you at last!'

'You've chosen a very inconvenient moment - as usual,' said the Fifth Doctor. 'That thing up there is a Raston Warrior Robot and it's got us trapped. As soon as this time bubble breaks you'll be trapped too. I'd advise you to get back in your TARDIS.'

'And leave you all in the lurch? Never!' said the Doctor. Disregarding the Fifth Doctor's protests, he went over to the ruined tower, where Tegan and Turlough crouched like statues behind the barricade. 'I don't suppose we could get them over to the TARDIS?'

'In a state of temporal stasis? Never!'

'Then I'd better join you.' The air blurred and shimmered and both Doctors ducked behind the barricade as time resumed its normal flow. A javelin streaked above their heads, striking the tower wall. Everyone froze, and the robot resumed its motionless vigil.

To Tegan and Turlough, it was as though the Doctor had simply appeared from nowhere. They looked at him unbelievingly. Forestalling the hail of questions the Fifth Doctor said hurriedly, 'Tegan, Turlough, this is an old - no, a very new - friend of mine. He's called the Doctor too, as it happens and he's come to help us.'

'How?' asked Turlough bluntly.

'I'm sure we can think of something,' said the Doctor. 'After all, two heads are better than one.'

'Even when they're the same head?' said the Fifth Doctor sceptically. Tegan and Turlough watched as the two Doctors sat staring intently at each other, their minds in telepathic communion.

'Well, I suppose it might work,' said the Doctor dubiously. 'Let's try it then.'

'Buridan's Ass?' said the Doctor.

The Fifth Doctor nodded. 'Buridan's Ass!'

'Risky?' 'Undoubtedly. But it's our only chance.' To the horror of their companions, the two Doctors rose and began to walk towards the Raston Warrior Robot.

'Equidistant now, mind,' called the Doctor warningly. 'Absolutely! Any divergence and it'll go for the nearer.' Tegan realised that they were pacing themselves so that they stayed the same distance from each other, and also from the robot. As they got nearer to the robot, they moved nearer to each other, their paths maintaining, what was it called - Tegan racked her brains to recall long-ago geometry lessons - maintaining the shape of a perfect isosceles triangle.

Astonishingly, the robot did not attack either of them. It swung from one to the other as if about to fire, but never did. As the Doctors approached nearer and nearer, the robot froze into immobility. Then, clasping its hands to its head in a curiously human gesture, it toppled over, rolling down the hillock to land at the Doctors' feet. They looked down at it.

'Well, it worked;' said the Doctor.

'It certainly did,' said the Fifth Doctor. He turned towards the tower. 'You can come out now!'

Tegan and Turlough emerged, stretching cramped limbs. They looked down at the prone robot.

'What did you do to it?' asked Tegan.

'I think we gave it a nervous breakdown,' said the Fifth Doctor. 'I know how it feels!'

'Who's this Buridan you were on about?' demanded Turlough.

'Ah, a bit of an old sophist,' said the Doctor, fondly, 'Buridan was a French philosopher in the 14th century who once carried out a rather cruel experiment with a hungry donkey.'

'He placed it exactly between two mangers filled with hay,' continued the Fifth Doctor. "The poor beast starved to death because it couldn't work out which one to go for.'

'Something similar happened with the robot,' said the Doctor. 'When it's about to attack it locks on to the electrical impulses from its enemy's brain. Now it happens that my brain patterns and those of my friend here are remarkably similar. To the robot it seemed as if it was registering the same target in two places at once. The confusion caused an overload.'

'Temporary or permanent?' asked Turlough.

'I'm not sure,' said the Fifth Doctor thoughtfully.

"Then let's not stay around to find out,' said Tegan practically.

'Seconded,' said Turlough.

The Doctors looked at each other.

'They're quite right,' said the Fifth Doctor. 'I hate to cut this reunion short, but we'd better be on our way. You first!'

The Doctor shook hands with Turlough and took Tegan's hand and kissed it. 'A very great pleasure to meet you again!'

Tegan looked hard at him. 'You two aren't just very alike are you?' she said. "That wouldn't have fooled the robot. You're both the same!'She looked at the Fifth Doctor. 'Is he that one of your past selves we never met? The one who got trapped in a time loop?'

'Certainly not!' said the Doctor.'Excellent fellow, though. We met quite recently.'

'Then he's one of your future selves,' said Tegan. 'How does it feel, seeing your future self for the first time?'

The Fifth Doctor smiled. 'To tell you the truth, Tegan, it's a little spooky!'

'I must go,' said the Doctor, shaking hands with himself. 'Goodbye!'

As he turned to go a harsh voice boomed, 'Stop! Do not move!'

They turned and saw a small group of figures grouped on top of the nearby hillock. Squat and dome headed, they all wore space armour. The heavy-duty military blasters they carried were trained on the Doctors and their companions.

Tegan glared at them in irritation.'Now what? Who the hell are this lot? What are they doing here?'

The booming voice answered her question.'This territory has now been annexed. You are all prisoners of the glorious Sontaran Empire!'

*

As the group began moving towards the blue box, Vrag noticed the silver form prone in the grass nearby. 'What is that?'

'Be careful,' said the yellow-furred Doctor. 'It's -'

The brown-furred Doctor cut across him. 'It's a servo-robot - vital to the operation of theTARDIS.'

'Why did he warn me to be careful?'

'It's an extremely complex piece of robotics,' said the brown-furred Doctor loftily. 'Far in advance of anything on your planet.'

'Why doesn't it move?'

'It's in a dormant phase. You can easily rouse it.' As he spoke he stared challengingly, not at Vrag but at the other Doctor.

Vrag sensed some kind of emotional undercurrent between the two of them, but he was too impatient to bother himself about it. Instead he turned to the nearest trooper. 'Get that thing moving.'

The trooper moved over to the robot and booted it hard in the side. The robot rose in one smooth motion. It extruded a sword from its right arm and sliced off the Sontaran trooper's head, which rolled away across the grass.

The headless Sontaran stood for a moment, the stump of its obscenely thick neck gushing a stinking fluid, and then fell to its knees. The robot surveyed its astonished enemies for a moment, blurred and apparently vanished. It reappeared on the far side of the group. Its arm swept down and a metal javelin pierced the probic vent of another trooper, killing him outright...

*

'Back to the TARDIS!' shouted the Fifth Doctor, starting to run. The Doctor, Tegan and Turlough followed him .

Without hesitation. The Fifth Doctor opened the door and ushered Tegan and Turlough inside, then turned to his other self.

'Are you coming?' The Doctor said, 'Just a moment. There's no hurry, not now'

He was watching the battle between the Raston Warrior Robot and the commando squad of Sontarans. It was not unlike the battle with the Cybermen his other self had witnessed in the Death Zone. But the present reality was far more ghastly than that memory. Perhaps it was because the Sontarans, for all their squat, troll-like shape, were somehow more human. They were creatures of flesh and blood, and they were being butchered.

The Raston Warrior Robot was everywhere at once. Sometimes it flashed from point to point on the perimeter of the group, bringing down trooper after trooper with its deadly projectiles. At other times it was suddenly in their midst, lopping off arms and legs, and shearing heads from bodies with its sword blade. The Sontaran troopers fired their blasters wildly, but the Robot was never in one place long enough to form any sort of target. In the swirling confusion of the battle, they succeeded only in killing each other. It was over at last - or almost. Only Vrag was left on his feet. Five javelins had pierced his body, and his shoulder was bleeding from a sword slash, but he refused to die. The Raston Warrior Robot appeared beside his swaying form. It drew its sword arm back for the final stroke - and Vrag's great leathery hands sprang out and seized it by the throat.

The robot struggled wildly, but Vrag's hands tightened in a death-grip. With a final paroxysm of dying strength, Vrag wrenched the head from the robot's body and hurled it from him. The headless robot stood swaying for a moment and then toppled over. A moment later Vrag crashed to the blood-soaked ground beside his enemy.

The Doctor ran over, skirting the mangled bodies of the dead Sontaran troopers. Vrag opened his eyes and saw the Doctor kneeling beside him.

Feebly he gasped, 'I thank you, Doctor.'

The Doctor looked down at him. 'Thank me? Why?'

'You trapped me - but you gave me a true warrior's death. I killed that thing, did I not?'

The Doctor looked at the headless Raston Warrior Robot. 'Oh, yes. If it's any consolation, you won.'