No,' the Doctor a.s.sured him quickly, 'but that's the key, isn't it? This plane is invisible.'

'That thing's got a shroud?' Chris snorted. Munin was anything but invisible. Theoretically it was possible to bend light waves around an object, but the technology required was almost impossible to maintain, even in Chris's time. It required knowledge of force fields, gravitronics, computer management systems and lasers. It was incredibly energy intensive. If Hartung could build force fields and lasers, why bother with making his plane invisible? He could make it invulnerable instead - that would be a weapon worth having.

The Doctor was shaking his head.

'Have a closer look,' the German invited. The Doctor stepped forward, reaching up as high as he could and brushing his hand along the nosecone. Chris followed his example. The surface was irregular, uneven. Tiny little pyramids of black rubber covered the plane. The surface had then been coated in some rough paint.

'Munin is covered in a revolutionary new carbon foam,'

Steinmann announced.

'It acts as a Jaumann absorber,' the Doctor remarked, poking it experimentally.

Chris struggled to remember what the jargon meant.

'You've lined the walls of your hangar with it, too,' he noted, not wanting to be left out.

'Very observant. As well as absorbing light and radar energy, the material makes almost perfect soundproofing.'

'Hartung has built the first radar-invisible plane. A stealth bomber.'

Chris looked again. It certainly had primitive stealth characteristics. The plane had no sharp edges: the wing surfaces and tips were rounded, the tail was almost pear-shaped. The plane's outer surface was free of any scoops or ridges. The air intakes were as small as they could be and there were no protruding antennae. There was virtually nothing to bounce a radar beam off: no breaks in the skin, no right angles, no wing fences. He hadn't seen the back of the plane yet, but he imagined that the heat emissions were controlled in some way.

Not just radar-invisible, Doktor. Look closer.' Steinmann indicated the side of the plane.

The Doctor peered up. 'Yehudi lights!'

'I beg your pardon?' Steinmann said. Chris was puzzled too, and the Doctor was only too happy to explain.

'That's what the Americans call them. They would normally be used to light up engine intakes and other places where shadows build up, but here they are arranged all over the surface of the plane. The exact colour of a camouflaged plane isn't that important, believe it or not, it's all to do with its shape and the way light falls on it. The human mind can be tricked into thinking that a flying object isn't a plane if it is lit in an unusual way and makes the silhouette look odd.

Remarkable.'

Chris wasn't impressed. 'It's not terribly advanced, is it?'

The Doctor had that faraway look again. 'Ten years ago aeroplanes were built out of balsa wood and canvas. Any plane in production now could outperform them.'

Chris could easily have tracked Munin using a gravity displacement sensor, or a ma.s.s detector. Hartung, of course, wouldn't even suspect that such equipment existed. In twenty years' time, any reasonably competent operator could have spotted the plane using radar, but Munin was certainly stealthy enough to slip past the primitive radar they had in 1941. In a war where everyone was using state-of-the-art technology, being only a couple of years more advanced than your enemy made all the difference. Some Adjudicator instinct buzzed a warning.

'How long does it take to design and build a plane like this?' he found himself asking.

Steinmann was only too happy to tell him. 'First, we work out what sort of planes we need. We work out the exact operational requirements: how many pilots it will have, how fast it will fly, the range of the plane, its rate of climb, its service ceiling, its bomb load, its armament -'

'We get the message,' the Doctor said distractedly. He had wandered over to the exposed flank of the plane and was toying with his abacus. Despite the Doctor's indifference, Chris had a hunch that he was on to something and was interested in what Steinmann had to say.

'It usually takes six months to draw up the exact specification for a large aircraft like this. We send out the requirements to the various aviation companies for compet.i.tive tender. The companies work out whether they can build the plane, how long it would take if they did, and they tell us what the unit cost would be. Nine or ten months later, the Luftwaffe decide which company has put in the best bid.'

There was a new sound, a clack clack coming from the inspection platform. Chris saw the Doctor up there, opening up his briefcase. The Doctor retrieved large sheaves of paper from it: blueprints. He struggled with them for a moment, trying to unfold them. Steinmann was still speaking. 'Then comes the hard part: the prototype. It took over two and a half years to build Hugin and her sister Munin, here. A whole host of problems had to be solved, from perfecting the revolutionary new jet engine to working out the exact positioning of the controls.' The n.a.z.i's voice receded as he stepped over to the Doctor. The little man had his arms wide, trying to stop the blueprints escaping from his grasp. He had even managed to tie a knot in one of the sheets. Steinmann helped to straighten out the papers. The Doctor examined them intently for a moment. Steinmann frowned, and tried to turn the plans the right way up. In the resulting struggle, a couple of the blueprints escaped, drifting down from the platform to the oily hangar floor. Steinmann gave up; instead he turned his attention back to Chris, and grasped the safety railing like a vicar at a pulpit. The sermon continued. coming from the inspection platform. Chris saw the Doctor up there, opening up his briefcase. The Doctor retrieved large sheaves of paper from it: blueprints. He struggled with them for a moment, trying to unfold them. Steinmann was still speaking. 'Then comes the hard part: the prototype. It took over two and a half years to build Hugin and her sister Munin, here. A whole host of problems had to be solved, from perfecting the revolutionary new jet engine to working out the exact positioning of the controls.' The n.a.z.i's voice receded as he stepped over to the Doctor. The little man had his arms wide, trying to stop the blueprints escaping from his grasp. He had even managed to tie a knot in one of the sheets. Steinmann helped to straighten out the papers. The Doctor examined them intently for a moment. Steinmann frowned, and tried to turn the plans the right way up. In the resulting struggle, a couple of the blueprints escaped, drifting down from the platform to the oily hangar floor. Steinmann gave up; instead he turned his attention back to Chris, and grasped the safety railing like a vicar at a pulpit. The sermon continued.

'Then we begin the air trials: our top pilots test the plane for its handling, performance and suitability. The trials of this magnificent plane were completed at the end of February. Air trials take about sixteen months. Then, normally, production begins on a squadron of aircraft and they are tested in action.

We see how easy it will be to construct and operate the planes. Typically, this takes another two years, perhaps two and a half. Then, the Luftwaffe decide how many planes they actually need. Another year.'

Chris had been doing some mental arithmetic.

Steinmann was now so far away that he felt he had to shout.

'That's about eight years in total. The British have by far the best radar technology in the world. They are - what? - two or three years ahead of the Germans. But not even the British network is that good. The Chain Home network is still being set up. You can't possibly have started building this radar-invisible plane eight years ago - there wasn't any radar back then!'

'In wartime, the need is more urgent so we cut corners.

We don't ask the aircraft companies what they want to build, we tell them. We build two prototypes: a simple measure that halves the trial period. This particular aircraft is a special case, it will -'

'What's that doing there?' the Doctor grumbled, interrupting. Steinmann just smiled, and bent over to see what had caught the Doctor's eye. 'Heat sink,' he whispered.

The Doctor looked up at the n.a.z.i officer for a moment, bemused, then checked his plans. Apparently satisfied, he returned to his investigations.

Steinmann continued. 'Munin is a special case, and it will never be ma.s.s-produced. At a unit cost of three million Reichsmarks, we couldn't afford to. Not that we need many. I have calculated that five of these planes would be enough for the British invasion. They could wipe out every radar station, every airstrip and every major bridge in the south of England before the British even knew we were in the air.'

'It still doesn't add up,' objected Chris, who had joined the odd pair on the inspection platform. The Doctor had all but disappeared into the workings of the aircraft now. Chris continued. 'Hartung's a genius. A hardworking genius. You've thrown resources at him, given him everything he wants. But even working flat out on an unlimited budget you couldn't build this plane fast enough.'

'Construction took just under four and a half years. It started in November 1936. It was completed on 28 February this year. I don't understand your objection. We have managed to design and build new fighters in under three years.' Steinmann shifted uncomfortably.

'No, listen. The first ever radar test was only a couple of years ago at Daventry. That was -' Chris struggled for the date.

'- February the twenty-sixth 1935,' supplied the Doctor, who had emerged from the depths, his clothes spotless. He made a show of wiping his hands on his handkerchief.

'There you go. Over a year before Hartung began work,'

Steinmann a.s.sured Chris. Cwej was not impressed.

'That wasn't when the British had radar, that was the first time that a radar echo was ever detected. It wasn't until the summer that anyone proposed a coastal network of radar stations. Britain's got its own boffins, and they worked flat out, but the first station wasn't completed until the summer of 1937. By then, even if you hadn't cut corners, you must have started to build the prototype. How can you possibly have built counter-measures before the device you are trying to counter has been built? It's like building anti-tank guns before the invention of the tank! The German government wasn't going to throw millions of marks away on a pie-in-the-sky scheme like this rubber plane in 1936. They had quite enough to spend their money on.'

'Hartung convinced them,' Steinmann said simply.

Chris was sceptical. 'How?'

'He claimed that he had been told. A vision of the future, of n.a.z.i destiny.' He hissed the last words, like an invocation.

Chris glanced over at the Doctor to check his reaction, but the little man was still deep in study.

'The Doktor has been uncharacteristically silent,'

Steinmann observed coldly.

The Doctor looked up from the blueprints at the mention of his name. 'Where is Hugin?'

'In the other hangar.'

The Doctor was examining the plans again. 'In one piece?'

'Why, of course.' But his Adam's apple rose and fell ever so slightly, and his tone was just a little too defensive.

'He's lying,' concluded Chris.

'I know he is. Didn't your mother ever tell you that lying was naughty, Oberst Oskar?' Steinmann's face was a picture of indignation. The Doctor rounded on him. 'Hugin blew up just before midnight on March the first above a small cove in St Jaonnet on Guernsey. If you remember, Herr Wolff met me there the morning afterwards. I recovered a small piece.'

He flicked his wrist and a twisted sc.r.a.p of metal appeared in his hand. The Doctor held it up to a point on the fuselage.

'Component F-989A,' he concluded, pointing to the corresponding place on the blueprints.

'Very good. Would you care to explain why the plane exploded?'

The Doctor hesitated. 'I'm not sure about that bit.'

'No, neither are we. I thought you might know.' There were footsteps behind them. Chris turned and saw a pair of Luftwaffe pilots stepping towards them. They wore full flight suits.

'What's going on?' Chris asked. The Doctor suddenly looked worried.

Steinmann smiled. 'These officers are about to launch an attack on Southampton in Munin. The town will be razed to the ground. The British government have been informed, but will not be able to stop the attack.'

The pilots began mounting the platform. Chris looked over to the Doctor. 'I thought you were pledged to peace, Herr Steinmann,' the little man said softly. 'You saw what happened in Granville. Yet now, later the very same day, you are willing to do exactly the same.'

'We have offered peace to the British. If they choose not to accept our offer, then we shall destroy them, and have peace that way. This is a demonstration of our power.'

Chris watched helplessly as the pilots began closing up the inspection hatch. His mind was racing. 'Doctor. I've worked it out. How Hartung built it.'

The Doctor was standing back, the blueprints clenched in his hand. 'Really? Do you still think it's a UFO?'

No, it's terrestrial, but Hartung was obviously forewarned about radar. Someone's gone back in time and told him.' The Doctor didn't answer; instead he buried his nose in the plans.

Steinmann had a gleam in his eye.

' Der Tausendjahrige Reich Der Tausendjahrige Reich!' he declared, clenching his fist.

' Gesundheit Gesundheit,' offered the Doctor.

'The tiniest piece of information,' Steinmann whispered.

He grasped the Doctor's upper arms, almost lifting him into the air. 'When is the time barrier broken?' he demanded.

Chris moved forward to protect the Doctor, but Steinmann had already let go of the little man. The Doctor was readjusting his collar, getting his breath back.

'How long is a piece of string?' the Doctor answered finally.

Steinmann removed his revolver from its holster, flicked off the safety catch, and levelled it at the Doctor's forehead.

Chris glanced across at the pilots. They were not large men and they weren't armed, but he and the Doctor were outnumbered. Chris stepped back, not wanting to provoke anyone.

'There's nothing you can do, Doktor. n.a.z.i scientists from the future have invented a time machine. They sent Emil Hartung a message. Vital technical information about radar: how it worked, when the British would have it, how to counter it. Not too much, but enough.'

'Is that what Hartung told you?' said the Doctor, genuinely curious.

'Hartung never confided the exact source of his information. It doesn't matter, now. You are watching history in the making, the moment of victory that Hartung promised.

For over four years we have been planning this moment, and now it is complete. Sieg Heil Sieg Heil!'

The pilots had completed their tasks. They joined in the salute.

'You know your mission?' Steinmann asked, not taking his eye, or aim, off the Doctor.

'Yes, Oberst Steinmann.'

'Go to it.' The pilots saluted again.

'It's not over yet. The Doctor will think of something,'

Chris declared. The first pilot was mounting the steps.

Steinmann shook his head.

'The Doktor? Christopher, the Doktor is dead.' He squeezed the trigger.

Click. Click. Click. Click. Click. Click.

The Doctor smiled thinly. 'I took the precaution of removing the bullets about an hour and a half ago, while we were having coffee.'

Steinmann glanced down at the gun. It was the lapse in concentration that Chris had been waiting for. He batted the revolver from Steinmann's hand, then rounded on the two pilots. He easily swatted one over the side of the platform, grabbing the other one before he could escape. A blow to the back of the neck rendered him unconscious. Steinmann stepped back.

'What was that about inevitable n.a.z.i victory?' asked the Doctor innocently.

'You are surrounded. There's no way that you'll get out of here.'

Chris glanced back at the hangar door. Steinmann was right: they had been lucky the last time they escaped from this base. They might not be as lucky this time.

'There's no way out,' Chris concluded.

'Never overlook the obvious,' the Doctor smiled.