The black ship began to turn, slowly at first. It steadied and zipped out of the cloud, its roaring rear thrusters trailing thick superheated fumes.

Spiggot crushed another plastic cup. Unfortunately, he had forgotten to drink all of the coffee that had been inside it and the scalding liquid splashed over his sweater.

'Do try and be more careful,' the Doctor advised, handing him a napkin. 'We'll get nowhere with you being so jumpy.'

They had returned to Spiggot's cabin to wait for Romana, as arranged. Spiggot had spent anxious minutes going over the facts of the case and failing to see any pattern. 'Trouble is, Doctor,' he said, mopping up the mess, 'there's no reason why this person should kill Zy, is there?'

'Should there be?' The Doctor had returned to the study of the file on Xais. He tapped the page he was reading. 'It says here that one of Xais's great weaknesses was her irrational hatred of all humans. She was extremely intelligent, yes. But sometimes she lost control and just killed the nearest human for the sake of it.'

Spiggot lit another of his cigarettes. 'But we're not looking for Xais, Doctor. Xais is dead. We know that.'

The Doctor vaulted from his chair, seemingly consumed by some revelation. 'Do we, Spiggot? Do we really? We know that she was put to the particle reverser. But do we know that she's dead? Do we even know what dead means? "How wonderful is Death, Death and his brother Sleep." ' He coughed and pointed to Spiggot's cigarette. 'Please put that out, it stops me from thinking.'

Spiggot ignored his request. 'What do you mean, do we know she's dead. Of course we do.'

'But how do you kill someone, eh? Eviscerate them, crush them, reverse their particles. But do the dead always stay dead, mm?'

Spiggot found himself almost disturbed by the Doctor's intense gaze and ominous words. But common sense prevailed. 'I thought we'd decided that there was no ghost.'

The door of the cabin opened and Romana walked in.

Without any attempt at greeting, she asked urgently, 'Doctor, what do you know about helicon?'

'It's a soft mineral,' he replied. 'And not a very useful one, either. Low conductivity, low strength. Low interest. Don't they mix it with Ball's ore to line pipes?'

'Nothing else?'

Spiggot spoke up. 'It's worthless. Any schoolkid knows that.'

Romana looked between them. 'It might not be.'

The Doctor tilted his head, a sign Spiggot recognized as a portent of trouble. 'Romana, I'm glad to see you taking an interest in planetary geomorphology, but please try to keep your mind focused on one thing at a time.' He turned to Spiggot. 'This is her trouble, you see. No concentration.'

Romana went on, 'Doctor, Stokes made a mask of Xais using helicon. It was taken from the gallery this evening. The only item that was removed.' She lowered her voice. 'Do you think the mask might be connected to the killing?'

His reaction was immediate. He raced for the door of the cabin, his long legs taking him from one side of the room to the other with a couple of loping strides. 'Well, come on, then!'

Romana raced after him. 'Where are we going?'

'To find Pyerpoint. Do try to concentrate!'

Spiggot tapped Romana on the shoulder. 'Would you mind telling me what's going on?'

'I'm afraid I would,' Romana shouted back as she raced after the ta.s.selled ends of the Doctor's scarf, which were disappearing around the corner of the corridor. 'There isn't time.'

Shom watched anxiously as K9 continued his silent communion with the security computer. The technicians, now feeling more than slightly redundant, were huddled in a bemused group on the far side of the room. The big screen of the control centre still displayed the revolving cylinder that served as graphic representation of the system. K9's investigations had revealed the full extent of the corruption of the computer and the cylinder was now cracked and chipped in several places.

Shom knelt to address K9. 'How was this done?' he asked incredulously. 'The entire system's been degraded.'

K9 was too busy to reply. 'Please do not ask questions until a.n.a.lysis is complete.'

'It cannot answer you,' Pyerpoint told Shom, 'because this is another of Spiggot's distractions. Such interference is impossible. This,' he pointed to the screen, 'is supposed to fool us. They've programmed the machine to falsify its findings.'

The doors of the control centre opened and Margo walked in. She was dressed in a clean white tunic. Her movements were as they had always been, calm and efficient. It was as if the disturbances of the previous days had not occurred. 'Sir.'

She addressed Pyerpoint.

He hurried to her side and took her arm. 'What are you doing here?' he asked. 'I advised you to relax in your cabin.'

She stared back at him. Shom noted an unusually insolent tone in her reply. 'I wished to apologize for my behaviour, sir.

And to convey my willingness to submit to your authority in person.'

Pyerpoint relaxed his grip on her arm. 'There is no need for that. Please. Return to your cabin.'

K9 spoke up. 'Warning. Danger. This station is in danger.'

Pyerpoint turned abruptly. 'What is it talking about now?'

He waved Shom forward. 'Disconnect it.'

'Warning. Danger!' K9 continued. 'The security net has been blinded to signals on band 456601 of the hyper three carrier beam since February twenty-eighth. My sensors indicate that a coded signal on that frequency is now being transmitted from this location.' He whirred and clicked, his delivery speeding up. 'Furthermore, interference extends to override of the station's transmat system. Records of transmat communications on February 28th have been falsified.'

'It's talking nonsense,' Pyerpoint insisted. 'Disconnect it.'

Shom moved to comply, but then the door crashed open and the Doctor stormed in, followed by an anxious Romana and a breathless Spiggot. 'I'm afraid he isn't, Pyerpoint! K9 never talks nonsense, well hardly ever.'

The High Archon flushed. 'I've had quite enough of you and your colleagues, Doctor. I must ask you to leave immediately. I shall be making a complaint to the police authority over your con'

The Doctor reached forward and shook him by his scrawny shoulders. 'Listen, listen! Your computer has been overridden by an expert. Xais!'

Pyerpoint knocked his hands away. 'Doctor, Xais is dead. I saw her die!'

Spiggot leapt forward. 'It sounds crazy, I know. But we think she's found a way to bring herself back to life.'

'Your behaviour is starting to bore me, Spiggot,' Pyerpoint said. 'Get off this station or I'll have you thrown off.'

So, the Normals were perhaps not all so stupid. These investigators had uncovered her presence. But it was of no consequence. They would soon be dead.

She looked around the control centre. She had to complete the task she had come here for and this argument would serve as a good cover. The control panel of the security computer was unattended. She went to it and reached a hand out to a particular panel of switches. Her fingers fluttered over the keyboard in a blur. She nodded, satisfied, as a row of green lights flicked to red and then died. And the Normals believed this was a sophisticated system!

Unnoticed by those engrossed in the argument, a wisp of smoke started to curl from the collar of the ridiculous mobile computer. Xais was pleased. It was a conceited, ugly little thing.

She walked from the control centre unnoticed, and made for the nearest lift. Safely inside, she took the silver mask from her tunic and clamped it over her face, then pressed the control that would take her down to level four.

'Helicon is a quite worthless mineral,' Pyerpoint insisted. 'As any schoolchild could tell you. It is used sometimes as part of an alloy, to line pipes, I believe.'

'It may have all kinds of properties we can't comprehend,'

the Doctor went on. 'Listen, you must evacuate this station and call for help from Five.'

'Certainly not.' Pyerpoint straightened himself. 'I do not intend to evacuate on the strength of one very straightforward murder and a technical fault. Now, you are going to leave. Do I have to use force?'

'Pyerpoint,' the Doctor stormed. 'This station is in terrible danger!'

The argument was interrupted by a high-pitched mechanical groan that issued from K9's voicebox. A steady stream of smoke came from the robot dog. His casing glowed red.

'K9!' cried Romana. She hurried to his side.

'Imperative disconnect, Mistress!' he was able to gasp as he was overpowered. 'Power supply to systems overloaded.

Imperative disconnect!'

Romana attempted to obey, but the heat surrounding K9 was too great and she s.n.a.t.c.hed her hand back. A roar started to come from the console to which he was connected.

'January is the equivalent,' K9 rambled, his delivery speeding up. 'Harriet married a soldier planetary density varies if Otto has six marbles, who is Vla.s.silivich...'

The Doctor pulled Romana back and cried, 'Down!' He grabbed Spiggot with his other hand and flung them both to the floor. Pyerpoint, Shom, and the technicians, warned by instinct of what was to happen, covered their eyes and dropped to the ground.

The console bellowed, groaned, and erupted in a shower of dazzling green sparks. Molten blobs of what had been circuit boards and processor links spattered the occupants of the room. A second explosion tore through adjacent consoles.

Palls of choking black smoke were released. The lights flickered, went out and were replaced by emergency systems that glowed feebly.

Typically, the Doctor was the first to react. Covering his mouth and nose with his scarf, he stumbled through the smoke to where he remembered the fire apparatus was located. He hefted a fire extinguisher and tossed it to Romana. She caught it and started to beat back the flames with jets of foam. The Doctor joined her and they had the blaze under control in under a minute.

The odorous smoke started to clear slowly and K9 was revealed. His personal defences had protected him from much external damage, although his sh.e.l.l was blackened and scarred, but his head was angled upward and his eyescreen was unlit. Romana stepped forward carefully and wiped the foam from his casing.

'His integrators have gone,' the Doctor diagnosed. 'But he's designed to shut down if something like this happens. I wonder why he didn't?'

Romana patted the nose of the deactivated dog. 'He was trying to warn us. Don't worry, K9, we'll soon have you patched up.' To her astonishment K9's eyescreen flashed briefly.

'Thank goodness,' said the Doctor. 'For a moment there, K9, I thought your goose had been cooked.'

K9's head moved slightly and he said in a small voice, 'Not understood, Master. This unit does not contain Earth water-fowl.' His voice slurred and he became inactive again.

Spiggot had picked himself up from the floor. Coughing and choking, he joined them at the burnt-out wrecks of the computer consoles. 'Flaming h.e.l.l,' he said.

'Quite,' the Doctor observed.

Spiggot looked along the lines of smouldering data stores.

'Looks like they've lost it all. Did your K9 do all of that?'

'It wasn't K9,' Romana said. 'The power to the computers was stepped up. The failsafes must have been overridden.'

'And only a genius could have done it,' she and the Doctor said together.

'A genius like Xais,' the Doctor completed. He stopped abruptly and put a hand to his head. 'Of course! That woman who was here, er...'

Romana supplied the name. 'Margo?'

'Margo. She must have set the power to overload. It would explain her breakdown,' he rationalized. 'If '

'If she's possessed by the mask,' Romana went on.

She picked up a cooling chunk of debris from the nearest console. 'And she's left us defenceless.'

In the grim silence that followed these words, she heard a clamour build up outside as the citizens of the Rock began to realize that something was wrong.

Pyerpoint staggered up to them. His scorched eyebrows and sooty cheeks might have amused Romana in different circ.u.mstances. It seemed that, at last, the truth was beginning to reach him.

'Doctor,' he said urgendy, 'K9 said that a signal was being sent from here, on a frequency that the security net had been blinded from picking up. If that's true, somebody out there knows our position. And without control of our laser cannon, we can do nothing to repel them.'

Spiggot flung his arms wide in frustration. 'Oh, that's just great, man! So we're sat here like a barmcake waiting for the toaster!'

Pyerpoint turned to Shom. 'Get after Margo. Find her!

She's gone insane.'

'Right away, sir.' Shom turned to leave.

'But you must not kill her, Shom,' Pyerpoint said firmly.

'She must not die!'

Shom hurried away.

Romana righted one of the plastic chairs that had been knocked down by the explosion and sat dispiritedly. 'If she's sent for accomplices, they could arrive at any moment.'

The Doctor waved an arm airily. 'Not necessarily. She may have done all this just to shut K9 up. I shouldn't worry if I were you, Romana.'

The asteroid lurched to one side and they were all thrown to the floor again.

Romana lifted her head and looked across at the Doctor.

'Of course,' he said, 'I could be wrong about that.'