Benny stepped forward, looking out of place in her movie-star outfit.

'I'm Professor Bernice Summerfield, these are my friends Christopher Cwej and Roslyn Forrester. Lieutenant, there's been a mistake - these people may look strange, but they are on your side.'

Reed stepped forward, and couldn't keep his eyes off Benny. 'I know. I work with Cwej and I'm engaged to Roz.'

Chris and Benny looked at each other.

'No, really, he is,' Roz insisted, look at the ring.'

The British soldiers had let go of Chris, and Reed grasped his hand and welcomed him back. A second later and Reed was briefing Cwej and Forrester, leading them a little way away from Benny. The archaeologist tiptoed after them. 'We've arrested von Wer. He flew in on this.'

'But that's how the Doct-' Roz jabbed her heel down hard on Chris's foot.

'The Von Wer from the photograph?' she asked.

'The very same. It looks like you were wrong for once.

We caught him underneath this plane, carrying a set of plans, a million Reichsmarks and a signed confession. Kendrick's taken him to SID HQ.'

Benny had already guessed. Roz caught her eye and nodded.

'Anything else?' Forrester asked.

Reed nodded grimly. 'They're going to bomb Guernsey.'

'No!' Benny shouted.

Reed whirled to face her. 'I don't like it myself, Miss Summers, but the Cabinet don't have any choice.'

Chris grasped his shoulder. 'They are trying to bomb the airstrip where this was built?'

'Yes.'

'It's not in Guernsey. We need to find Kendrick.' Reed stared at him for a moment, then the two men ran off together towards Whitehall.

Benny faced Roz. 'And we have to find the Doctor.'

'He's at SID HQ.'

'Where's that?'

Roz grinned. 'It's where those two are jogging to. Just under a mile away.'

Benny groaned. 'Couldn't we grab a taxi?'

Roz nodded. I've got a staff car waiting.'

Reed and Cwej caught up with Kendrick as he was about to step into his office. The admiral was beaming, and he ushered them both into the room. It was the first time that Chris had been here. The office was small and the oak-panelling and blackout curtain over the window made it even more claustrophobic. As with every other room in the building, maps and reports littered every surface. A scale model of a battleship took pride of place on the desk.

'It's good to see you back, Lieutenant.' Kendrick shook Chris's hand. The two lieutenants were still out of breath from their run. Chris couldn't waste any more time, though.

'Sir, I have heard from Lieutenant Reed that you intend to bomb Guernsey tonight. Hartung isn't based there; the Luftwaffe have a camouflaged airstrip just outside Granville.'

'We bombed the -'

'Sir, this is a different airstrip, one that you don't know about.'

Reed had found a map, and he brought it over. 'Show us, Cwej,' the lieutenant suggested.

Chris ran his finger over the map. The Gerard farm was marked on it. It was easy enough to trace the escape route he and the Doctor had used - back from the farm, through the wood, along the stream. On this map, an old one from before the war, there was still a farm on the site of the camouflaged airstrip. The country lane leading from Granville to this farm was marked. Chris estimated the position and drew a circle around it, carefully avoiding the Gerards' land.

'Send the bombers there, three miles or so to the south.

There are no ground defences - they don't need them if no one knows that they are there. Everything is underground, though.'

Kendrick tapped the map. 'We've not even had a whiff of this from our agents in France, Cwej.'

Chris was insistent. 'I broke into the base on two occasions, sir. The n.a.z.is had been testing Hugin and Munin there for months. You've received reports about mysterious flying objects. You know that Hartung is in the area.'

'For G.o.d's sake, Admiral, call Bomber Command before the raid on Guernsey,' urged Reed.

Kendrick picked up the phone.

'Open up the door,' Forrester ordered.

The young private did as she asked. Roz followed Benny into the cell. The Doctor was sitting at the end of his bed, in the lotus position. He was stirring a mug of tea.

'Look at that,' said the Doctor crossly, holding up the spoon. 'What use is only one spoon? Oh, Benny! You found my umbrella.' He took it from her, hugging it joyfully.

'Hartung is dead,' said Benny softly. 'I saw his body in the morgue. He died on March the second at St Jaonnet.'

'When Hugin exploded,' the Doctor muttered under his breath. 'But...'

It was the first time that Roz could ever remember the Doctor looking surprised. It disconcerted her.

'So this has all been for nothing, hasn't it?' Benny suggested.

The Doctor put down the mug. He was deep in thought.

'Look at me, Doctor,' Benny insisted. She pulled down her sungla.s.ses. Her bruising was still horrific, and both Roz and the Doctor winced. 'Hartung managed to blow himself up without any help from you.'

'I thought you were safe,' the Doctor said weakly.

'Hang on, though, Bernice,' Roz said. 'Without the Doctor and Chris, the Germans would still have Munin and the plans to build more.'

'Benny's right,' the Doctor said solemnly. 'Hartung was unique. Fifteen years ahead of his time - he was the only person in the world who could have worked out how to build Munin from...' The Doctor paused, before saying, 'Without his genius, the Germans won't be able to carry on with his work.

A brilliant mind...' He trailed off sadly.

This n.a.z.i was a friend of yours, was he?' Roz joked.

The Doctor studiously turned his attention back to his mug of tea.

Benny's eyes narrowed. 'He was. It was you, wasn't it?'

Roz frowned. 'What are you talking about - the Doctor giving state secrets to the n.a.z.is? He walked up to a top n.a.z.i scientist and told him all about Chain Home, and how to reduce his plane's RCS? Somehow I doubt it.'

Benny was unrepentant. 'I'm right, aren't I? You sneaky little git. There isn't any supervillain, or alien incursion.

There's no giant rubber hamster from before the dawn of time. It's just you.'

'It was a mistake, a miscalculation, that's all.'

'Care to tell us about it?' Roz said, suddenly suspicious.

'I met him in Cairo back in January 1936, at a race meeting. He was clearly a very intelligent man, and glad of such company, for once, so I -'

'- told him all about radar and how to build stealth planes.'

'No.' The Doctor was clearly hurt. 'You humans are quite capable of inventing weapons all by your -'

'You're hardly in a position to lecture us, are you?'

'No.' The Doctor looked down at his feet like a naughty schoolboy.

'So what happened?'

Quietly, the Doctor told them about Cairo, about meeting Hartung and the afternoon at the race-track. Then he told them about what had happened that evening...

Mel was wearing a striking creation in sequins and pink organza that she'd found in the TARDIS wardrobe room. She was sitting at a small table at the edge of the ballroom of the Grand Imperial Hotel with the Doctor and Emil. The racing driver was the centre of attention, and so Mel had received more than her fair share of curious looks from the other guests. Although the surroundings were sumptuous, with half a dozen crystal chandeliers hanging from a richly painted ceiling and a full orchestra playing, all three sat staring out of the window, watching owls gently swoop past.

Whooo, ululated the owl, the pitch of its call dropping gradually.

'Fascinating creatures, owls. They've been around for twelve million years,' the Doctor remarked.

'One of the ancient Egyptian hieroglyphs is an owl,' Emil replied.

Illogical though it might be, Mel was afraid of the creatures. 'They give me the creeps,' she admitted.

Emil and the Doctor shared a knowing look.

An owl dived past the window, its feet splayed.

'It's going in for the kill. Do you know they can see well in the dark?' Emil asked.

'Do they have sonar, like bats?' Mel wondered. 'You know, they send out a high-pitched squeak and can build up a picture from the echoes. Like radar.'

The Doctor shook his head. 'No, owls have a combination of sharp eyesight and acute hearing. Owls are binocular, with quite a narrow field of vision, and they sometimes have difficulty seeing things that aren't moving.

However, an owl is quite capable of tracking voles across a field just by the sound they make when they are chewing gra.s.s. As far as I know, only two species of bird use biosonar, the swiftlet and the oilbird. Both breed in very dark caves. It's a fascinating subject: those birds produce twelve clicking sounds a second in the one kilohertz and sixteen kilohertz range and -'

'What's a "kilohertz", please?' Emil asked.

The hertz is the SI unit for cycles per second,' Mel supplied. She didn't tell him that the word was only coined in the 1960s.

'The really clever thing, of course,' the Doctor said, smiling to himself, 'is that the mouse can't see or hear the owl coming. The poor little thing doesn't defend itself, because it doesn't know it is being attacked until it is far, far too late.'

'Come on Emil, let's dance.'

As Emil took Mel by the arm, an owl, the same they had seen before, swooped up into the night with a mouse impaled in its talons.

'Hang on,' said Benny. 'You didn't tell the Germans how to build stealth planes, all you did was talk about owls?'

'Yes, but that conversation sparked off Hartung's imagination, joined up few of the dots for him. It made a few links explicit, encouraged him to research into echo-location, so he found out about the radar experiments. That conversation was the first link in the chain that led to the building of Hugin and Munin. Careless talk costs lives.'

'But...' Roz tried to think about all the things she had told Reed. She remembered the stungun concealed in her jacket and the ma.s.s detector in her holdall. It probably wouldn't be diplomatic to mention all this to the Doctor.

'When I learnt what Hartung had built, I hoped against hope that he had got the information from somewhere else.

But he hadn't; it was all my fault - without me, none of this would have happened.'

'It's still a bit cryptic, isn't it?' said Roz sullenly.

'When an apple fell on his head, Newton discovered gravity. That's how genius works: making a.s.sociations, drawing inferences, putting the pieces together. I... came across his diary in the Soviet Union. The entry for the eve of the race in Cairo made it clear.'