It was not easy to get a cab but eventually an old white VW beetle stopped for me. The car's interior was battered and dirty, but the driver's position was equipped like the flight deck of a Boeing jet. The dashboard was veneered in walnut and there was an array of small spanners and screwdrivers and a pen-shaped flashlight as well as a large coloured medallion of the shrine of the Virgin of Guadalupe. In contrast to the derelict bodywork of the little car, the young driver was dressed in a freshly starched white shirt with a dark-grey tie and looked more like a stockbroker than a cab driver. But Mexico is like that.

The traffic moved slowly through the heavy rain but it didn't make less noise. There were two-stroke motorcycles and cars with broken mufflers and giant trucks – some so carefully painted up that every bolt-head, rivet and wheel-nut was picked out in different colours. Here on the city's outskirts, the wide boulevard was lined with a chaos of broken walls, goats grazing on waste ground, adobe huts, rubbish tips, crudely painted shop-fronts in primary colours and corrugated-iron fences defaced with political slogans and ribaldry. Despite the rain, drunks sprawled full-length on the pavement and the barbecue fires hissed and flared at the taco counters.

By the time we got near to Werner Volkmann's apartment, the rainstorm was flooding the gutters and making great lakes through which the traffic splashed, and in which it sometimes stalled. There was a constant racket of car horns and engines being over-revved by nervous drivers. The cab moved slowly, and I watched drenched and dirty kids offering dry, clean lottery tickets that were protected inside clear plastic bags. And plenty of well-dressed shoppers had chauffeurs who could hold an umbrella in one hand and open the door of a limousine with the other. I couldn't imagine Zena Volkmann anywhere but here in the Zona Rosa. Within the area contained by the Insurgentes, Sevilla and Chapultepec there are the big international hotels, smart restaurants, the shops with branches in Paris and New York. And in the crowded cafes that spill out on to the pavement are to be heard every new rumour, joke and scandal that this outrageous town provides in abundance.

Zena Volkmann could live anywhere, of course. But she preferred to live in comfort. She'd learned to respect wealth, and the wealthy, in a way that only a poverty-stricken childhood teaches. She was a survivor who'd climbed up the ladder without benefit of any education beyond reading and writing and painting her face, plus a natural ability to count. Perhaps I did her an injustice but sometimes I had the feeling that she would do anything if the price was high enough, for she still had that fundamental insecurity that one bout of poverty can inflict for a lifetime, and no amount of money remedy.

She made no secret of her feelings. Even amid the contrasts of Mexico she showed no great interest in the plight of the hungry. And like so many poor people she had only contempt for socialism in any of its various forms, for it is only the rich and guilty who can afford the subtle delights of egalitarian philosophies.

Zena Volkmann was only twenty-two years old but she'd lived with her grandparents for much of her childhood. From them she'd inherited a nostalgia for a Germany of long ago. It was a Protestant Germany of aristocrats and Handküsse, silvery Zeppelins and student duels. It was a kultiviertes Germany of music, industry, science and literature; an imperial Germany ruled from the great cosmopolitan city of Berlin by efficient, incorruptible Prussians. It was a Germany she'd never seen; a Germany that had never existed.

The elaborate afternoon Kaffee-Trinken that she'd prepared was a manifestation of her nostalgia. The delicate chinaware into which she poured the coffee, and the solid-silver forks with which we ate the fruit tart, and the tiny damask napkins with which we dabbed our lips were all parts of a ceremony that was typically German. It was a scene to be found in the prosperous suburbs of any one of a hundred West German towns.

Zena's brown silk afternoon dress, with embroidered collar and hem below the knee, made her look like a dedicated hausfrau. Her long dark hair was in two plaits and rolled to make the old-fashioned 'earphone' hairstyle virtually unknown outside Germany. And Werner, sitting there like an amiable gorilla, had gone to the extent of putting on his tan-coloured tropical suit and a striped tie too. I was only too aware that my old rain-wet open-necked shirt was not exactly de rigueur, as I balanced the coffee-cup on the knee of my mud-splashed nylon pants.

While Zena had been in the kitchen I'd told Werner about my trip to Biedermann's house, about the Russians I'd seen there and Biedermann's confession to me. Werner took his time to answer. He turned to look out of the window. On a side-table the broken fragments of a cup and saucer had been arranged in a large ashtray. Werner moved the ashtray to the trolley that held the TV. From this sixth-floor apartment there was a view across the city. The sky was low and dark now, and the rain was beating down in great shimmering sheets, the way it does only in such tropical storms. He still hadn't answered by the time Zena returned from the kitchen.

'Biedermann always was a loner,' said Werner. 'He has two brothers, but Paul makes all the business decisions. Did you know that?'

It was small talk, but now Zena was with us and I was undecided about how much to say in front of her. 'Are both his brothers in the business?'

Werner said, 'Old Biedermann gave equal shares to all five of them – two girls and three boys. But the others leave all the decisions to Paul.'

'And why not?' said Zena, cutting for me a slice of fruit tart. 'He knows how to make money. The other four have nothing to do but spend it.'

'You never liked him, did you, Bernie?' said Werner. 'You never liked Paul.'

'I hardly knew him,' I said. 'He went off to some fancy school. I remember his father. His father used to let me steer the trucks round their yard while he operated the accelerator and brakes. I was only a tiny child. I really liked the old man.'

'It was a filthy old yard,' said Werner. He was telling Zena rather than me. Or perhaps he was retelling it to himself. 'Full of junk and rubbish. What a wonderland it was for us children who played there. We had such fun.' He took a piece of tart from Zena. His slice was small; she was trying to slim him down. 'Paul was a scholar. The old man was proud of him but they didn't have much in common when Paul came back with all those college degrees and qualifications. Old Mr Biedermann had had no proper education. He left school when he was fourteen.'

'He was a real Berliner,' I said. 'He ran the transport business like a despot. He knew the names of all his workers. He swore at them when he was angry and got drunk with them when there was something to celebrate. They invited him to their marriages and their christenings and he never missed a funeral. When the union organized a weekend outing each year they always invited him along. No one would have wanted to go without the old man.'

'You're talking about the road transport business,' said Werner. 'But that was only a tiny part of their set-up.'

'It was the business the old man started, and the only part of the Biedermann empire he ever really liked.' A timer began to ping somewhere in the kitchen but Zena didn't move. Eventually it stopped. I guessed the Indian woman was there but banished to the back room.

'It was losing money,' said Werner.

'So, when Paul Biedermann came back from his American business management course, the first thing he did was to sell the transport company and pension his father off.'

'You sound very bitter, Bernie. That couldn't be why you hate Paul so much, could it?'