'That does it. We go ahead.'

'That's my my girl!' girl!'

Ruth gave him an exasperated look and went over to the controls.

Jo Grant looked furiously at the Doctor who was still hard at work on his complex piece of circuitry. He was fitting it into a carrying case which was shaped rather like a table tennis bat. The rounded end held dials and a little rotating aerial.

'You know, Doctor,' said Jo conversationally, 'you're quite the most annoying person I've ever met. I've asked you at least a million times. What is is that thing?' that thing?'

The Doctor looked absently at her. 'Extraordinary. I could have sworn I'd told you...

It's a time sensor, Jo.'

'I see.'

'Do you? What does it do then?'

'Well, it . . . it's a . . . Obviously it detects disturbances in the Time Field.'

The Doctor gave her an admiring look. 'Very good. You're learning, Jo. Yes, this is just what you need if you happen to be looking for a TARDIS.'

'It's a TARDIS sniffer-outer!'

'Precisely; Or any other time-machine for that matter. So, if the Master does turn up.

'Bingo?

'As you so rightly say, Jo - Bingo!'

Stuart was laboriously climbing into an all-enveloping protective suit which made him look like a rather comic astronaut. 'I feel like the back a pantomime horse.'

'Very suitable for a keen young man like you,' said Ruth briskly.

'Come again?'

'Starting at the bottom?

Stuart groaned. 'Anyway, it's all a waste of time. Why should there be any radiation danger at the receiver? We're only going to use about ten degrees.'

'Are you willing to take the risk?'

Stuart thought for a moment. 'No!'

'Then stop beefing and get on with it!'

Fitting the visored helmet over his head, Stuart went through into the inner section of the laboratory - the receiving area.

Ruth operated controls and the TOMt.i.t noise began, rising steadily in pitch and volume . . .

(Blissfully unaware of all this scientific activity, the Inst.i.tute's regular window cleaner was setting his ladder up against the laboratory window. He peered curiously at the radiation suited figure in the lab, then reached for his wash-leather.) Ruth went to a shelf and took down marble vase. It had curved sides and a domed lid, and looked rather like a giant chess p.a.w.n.

She put the case on a fiat surface beneath a complex looking focussing device, then returned to her control panel.

Stuart's voice came from the intercom. 'Interst.i.tial activity - nil.'

Ruth checked the dial on her console. 'Molecular structure, stable. Increasing power.'

The oscillating whine of TOMt.i.t rose higher. In the inner lab the crystal began to glow.

With the Doctor's time sensor in her hand, Jo stood looking apprehensively at the open door of the TARDIS, which was making a strange wheezing, groaning sound. 'I say, Doctor, you're not going to disappear to Venus or somewhere?'

The Doctor's voice came through the TARDIS door. 'No, of course not. Just keep your eyes on those dials!'

Suddenly the dials began flickering wildly, the aerial spun frantically, and the device gave out a high pitched bleeping sound.

'It's working!' said Jo excitedly.

'Of course it is. Make a note of the readings will you?'

Jo grabbed a note pad and pencil.

Ruth was still calling out the readings. 'Thirty-five . . . forty . . . forty-five . . .'

Stuart's voice came back. 'Check, check, check.'

'Increasing power . . .'

The circular aerial on top of the Doctor's device was revolving wildly. It slowed and stopped as the TARDIS noise died away.

The Doctor came marching out, took the note pad with the readings from Jo's hand and began studying it absorbedly.

'Well done,' said Jo.

'Thank you,' said the Doctor modestly.

'It's a bit out on distance though. Says the TARDIS is only three feet away.'

'Those are Venusian feet,' said the Doctor solemnly.

'I see. They're larger than ours?'

'Oh yes, much larger, Jo. The Venusians always tripping over themselves.'

Suddenly the time sensor came to life again. Jo jumped, 'You must have left something switched in the TARDIS, Doctor.'

'I most certainly did not. Why?'

Jo handed him the sensor. 'Look, it's working again. And the readings are different.'

The Doctor stared indignantly at the sensor. 'That's impossible - unless...'

'Unless what?'

The Doctor said slowly, 'Unless someone's operatring another TARDIS.'

In the inner laboratory Ruth's voice came to Stuart over the intercom. 'Isolate matrix scanner.'

Stuart reached for a control with his gloved hand. 'Check.' In front of Stuart there was a square metal platform with a focussing device suspended over it - the exact duplicate of the one before Ruth in outer lab.

Suddenly on that platform there appeared ghostly outline of a vase.

'It's going to work!' shouted Stuart excitedly.

Ruth's calm voice came back. 'Pipe down and concentrate. Stand by. Initiating transfer.'

Stuart began the countdown. 'Ten . . . nine eight . . .'

The crystal glowed brighter.

In the Director's study the Master had installed himself at the Director's desk, calmly drafting a proposal to double his own grant for the Director to sign. The clock of what had once been the old stables began to chime. Suddenly the Master frowned and looked up. The chiming was slow, dragging, slurred, as if the old clock was somehow running down. But the Master knew better. It wasn't the clock that was slowing down - it was time itself.

'The fools!' he snarled, and hurried from the room. he snarled, and hurried from the room.

'Four... three.., two.., one!' chanted Stuart.

In the outer lab the vase became transparent, then faded slowly away . . .

. . . to reappear, solid and real on the receiving plate in front of Stuart.

Rapidly he operated controls. 'Transfer stabilising. Okay Ruth, switch off. We've done it!'

He expected the noise of TOMt.i.t to die away, but it didn't. The oscilliating whine rose higher.

He heard Ruth's voice over the intercom. 'Stuart, come here. There's a positive feedback. She's overloading!'

Pulling off his helmet, Stuart rushed back to the outer lab where he found Ruth busy at her console.

Without looking up she said, 'You'll have to bring the surge down as I reduce the power or she'll blow.'

Stuart ran to the console. 'Right.'

The astonished window cleaner was still perched at the top of his ladder, staring at the glowing crystal as if hypnotised. Suddenly a giant surge of power struck him, like a push from an invisible hand.

He flew backwards off his ladder, and floated floated rather than felt to the ground below. rather than felt to the ground below.

The Master, crossing the courtyard observed the phenomenon without surprise. He hurried towards the door that led to the laboratory.

As he came closer, he leaned forward against the thrust of some invisible resistance, like a man walking against a high wind.

The stable clock was still giving out its low, dragging chime.

In the laboratory itself, the calm centre of this localised temporal storm, things seemed normal enough.

Ruth and Stuart were in the inner lab examining the vase on its metal platform. The crystal was still glowing brightly.

Carefully, Stuart lifted the vase from its platform. 'It looks fine!'

Ruth nodded. 'Be careful. Bring it through here.' She led the way back into the main lab.

Carefully, Stuart stood the vase on a bench. 'I don't believe it. We've really done it!'

'It'll have to be checked for any structural changes,' said Ruth cautiously.

'OH, FOR Pete's sake,' said Stuart explosively, 'it's as good as new, you can see it is.' He grabbed her by the shoulders and began waltzing her round the room to a triumphant chant of 'We've done it, we've done it, we've done it!'

The dance stopped abruptly as they waltzed straight into the Master. He was standing in the doorway, an angry scowl on his face.

The Doctor was studying a map. 'I'd place it in that segment there, Jo. Anything from fifty to a hundred miles from here.'

'Not much to go on.'

'Not unless he switches his TARDIS on again...'

Jo looked hopefully at him. 'Well, you never know. He might.'

'And in that case Jo, if we were a bit nearer, and in Bessie...'

'Right,' said Jo. 'Come on then, Doctor, let's go. You bring the map.'

The Master was in a towering rage. 'You are a fool, Doctor Ingram.'

Ruth felt herself quailing beneath the sheer force of his anger, which made her all the more determined to stand up for herself. 'You have no right to talk to me like that, Professor.'

'Be silent! You might have caused irreparable damage.'

'I was in full control the whole time. If you have no confidence in me -'

The Master cut across her. 'That is quite irrelevant. Mr Hyde, why did you you allow this stupidity?' allow this stupidity?'

'Hang about,' protested Stuart. 'I'm not my sister's keeper, you know. She's the boss.'

He hesitated and then admitted, 'In any case, I was the one who suggested it.'

The Master turned away. 'I might have known. Just like an irresponsible schoolboy.