'At least let's make the kind of attempt that will look good on the report,' I said. 'Let's take this old lady over to Müggelsee and put her in the truck.'

'You kept your man well wrapped up,' said Werner. Twenty years at least he's been operating in this town, and I'd never seen him until today.'

'Deep cover,' I said, imitating the voice of Frank Harrington at his most ponderous.

Werner smiled. He enjoyed any joke against Frank.

Werner got in the driver's side and took the wheel. He started up and turned the car south for Berlinerstrasse and the city centre. 'For Müggelsee the autobahn will be quicker, Werner,' I said.

'That would take us out of the East Sector and into the Zone,' said Werner. 'I don't like crossing the city boundaries.'

'I came that way to get here. It's quicker.'

'This is Himmelfahrt – Ascension Day. A lot of people will be taking the day off to swim and sun. It's not an official holiday, but there's a lot of absenteeism. That's the only kind of "ism" that's really popular here. There will be cops on the roads that lead out of town. They'll be taking names and arresting drunks and generally trying to discourage people from having a holiday whenever they feel like goofing off.'

'You talked me out of it, Werner.'

Mrs Munte leaned forward between the seats. 'Did you say we're going to Müggelsee? That will be crowded. It's popular at this time of year.'

'Me and Bernie used to swim out there when we were kids,' said Werner. 'The Grosser Müggelsee is always the first to warm up in summer and the first to freeze for ice skating. It's shallow water. But you're right, gnädige Frau, it will be crowded out there today. I could kick myself for forgetting about the holiday.'

'My husband will be there?'

I answered her: 'Your husband is there already. We'll join him and you'll be across the border by nightfall.'

It was not long before we saw the first revellers. There were a dozen or more men in a brewer's dray. Such horse-drawn vehicles, with pneumatic tyres, are still common in Eastern Europe. But this one was garlanded with bunches of leaves and flowers and coloured paper. And the fine dapple-grey horses were specially groomed with brightly beribboned manes. The men in the dray wore funny hats – many of them black toppers – and short-sleeved shirts. Some wore the favourite status symbol of Eastern Europe: blue jeans. And inevitably there were Western T-shirts, one blazoned 'I love Daytona Beach, Florida ' and another 'Der Tag geht…Johnnie Walker kommt'. The horses were going very slowly and the men were singing very loudly between swigging beer and shouting to people in the street and catcalling after girls. They gave a loud cheer as our car went past them.

There were more such parties as we got to Köpenick. Groups of men stood under the trees at the edge of the road, smoking and drinking in silence with a dedication that is unmistakably German. Other men were laughing and singing; some slept soundly, neatly arranged like logs, while others were being violently ill.

Werner stopped the car well down the Müggelheimer Damm. There were no other vehicles in sight. Plantations of tall fir trees darkened the road. This extensive forest continued to the lakes on each side of the road and far beyond. There was no sign of Werner's big articulated truck, but he'd spotted its driver standing at the roadside. He was near one of the turnoffs, narrow tracks that led to the edge of the Müggelsee.

'What is it?' Werner asked him anxiously.

'Everything is in order,' said the man. He was a big beefy rednecked man, wearing bib-and-brace overalls and a red and white woollen hat of the sort worn by British football supporters. 'I had the truck here, as we arranged, but a crowd of these lunatics…' He indicated some small groups of men standing in a car park across the road. 'They began climbing all over it. I had to move it.' He had the strongest Berlin accent I'd ever heard. He sounded like one of the old-style comedians, who can still be heard telling Berliner jokes in unlicensed cabarets in the back streets of Charlottenburg.

'Where are you now?' said Werner.

'I pulled off the road into one of these firebreaks,' said the driver. 'The earth's not so firm – all that bloody rain last week. I'm heavy, you know. Get stuck and we're in trouble.'

'This is the other one,' said Werner, moving his head to indicate Mrs Munte in the back seat.

'She doesn't look too heavy,' said the driver. 'What do you weigh, Fraulein? About fifty kilos?' He grinned at her. Mrs Munte, who obviously weighed twice that, didn't answer. 'Don't be shy,' said the driver.

'And the man?' said Werner.

'Ah,' said the driver, 'the Herr Professor.' He was the sort of German who called any elderly well-dressed fellow-countryman 'Professor'. 'I sent him up to that lakeside restaurant to get a cup of coffee. I told him someone would come for him when we are ready.'

While he was saying that, I saw the black Volvo and the minibus coming down the road from the direction of Müggelheim. They would have made good time on the autobahn, flashing their lights to get priority in the traffic or using their siren to clear the fast lane.

'Get the professor,' said Werner to me. 'I'll drive the old lady down to where the truck is parked, and come back to meet you here.'

As I hurried along the woodland path towards the lake, I could hear a curious noise. It was the regular roaring sound that waves make as they are sucked back through the pebbles on a long stony beach. It got louder as I approached the open-air restaurant, but that did not prepare me for the scene I found there.

The indoor restaurant was closed on weekdays, but there were hundreds of men milling around the lakeside Biergarten in inebriated confusion. They were mostly young workers dressed in bright shuts and denim pants, but some wore pyjamas and some had Arab headdress, and many of them had brought the black top hat that is traditional for Himmelfahrt. I could see no women, just men. There were long lines of them waiting at a serving hatch marked 'Getränke' and an equally long line at a hatch marked 'Kaffee', where only beer, in half-litre plastic cups, was being served. Tables were crammed with dozens and dozens of empty plastic cups stacked together, and there were more empties scattered in the flower beds and lined up along the low dividing walls.

'Heiliger bim-bam!'' said a drunk behind me, as surprised as I was at the sight.

The roars of sound were coming from the throats of the men as they watched a rubber ball being kicked high into the air. It went up over their heads and cut an arc in the blue sky before coming down to meet yet another skilfully placed boot that sent it back up again.

It took me a few minutes to spot Munte. By some miracle he'd found a chair and was sitting at a table at the edge of the lake where it was a little less crowded. He seemed to be the only person drinking coffee. I sat down on the low wall next to him. There were no other chairs in sight; prudent staff had no doubt removed them from the danger zone. 'Time to go,' I said. 'Your wife is here. Everything is okay.'

'I got it for you,' he said.

'Thanks,' I said. 'I knew you would.'

'Half the clerks in my department have taken the day off too. I had no trouble walking into the chiefs office, finding the file and helping myself.'

'I'm told you had a visit from the police.'

The office had a visit from the police,' he corrected me. 'I left before they found me.'

'They came out to Buchholz,' I said.

'I was trying to think of some way of warning you when a man came up to me in the street and brought me here.' He reached into his pocket and produced a brown envelope. He put it on the table. I left it there for a moment. 'Aren't you going to open it and look inside?' he asked.