When the door was opened, by a slight Mexican boy who wore a straw hat, brim curled cowboy-style, I noticed there was a sheet of steel layered into the woodwork of the door. Another boy stood behind him, studying us warily. He recognized Tiptree and nodded.

'This is the bank,' announced Tiptree. It was a large room that overlooked the square, but the blinds had been pulled down. The room, with its ornate Victorian wallpaper and brass light-brackets, had the atmosphere of some Wild West saloon a century ago. Three almost identical men sat at three almost identical old tables. The men were dressed in white short-sleeved shirts with black trousers and black ties and black well-polished shoes: the uniform used throughout the world by men who wish to be entrusted with money. Each man was equipped with half a dozen ledgers, a small cash-box, a scribbling pad and a Japanese calculator. Through a half-open door I could see another room where girls were typing on the wide-platen typewriters that are required for account sheets.

'It's a money-change office,' I said.

'Three partners; brothers. They used to run a loan company… One that was always ready to change money too. But, when the government nationalized all the banks, larger horizons opened.'

'Is it a legal bank?' I asked.

'Strictly speaking it's not legal and it's not a bank,' said Tiptree. 'But it's right for what we want. I've spent a lot of time in Mexico, Samson. I know how things work here.'

I looked at the old man sitting inside the door with a shotgun across his knees. The teenage boys who'd let us in looked like blood relations. Perhaps it was a family business.

Tiptree greeted Zena. She was sitting on a wooden bench and nodded politely to both of us. Despite the heat, she was dressed in a linen suit with Paris labels, and her make-up and the low-heeled shoes made her look like someone who'd prepared for a journey. There was no sign of Werner.

'Is this where the money is supposed to be?' I asked.

Tiptree smiled at the doubt he heard in my voice. 'Don't be misguided by appearances. A quarter of a million dollars is a bagatelle to these people, Samson. They could have ten million, in any of the world's major currencies, laid out across the floor within an hour.'

'You've got it all worked out,' I said.

'You're the muscle; I'm the brains,' said Tiptree, without expending too much energy to persuade me it was a joke.

Tiptree exchanged polite, British-style greetings with one of the partners and formally introduced me. The senior partner was called Pepe, a soft-spoken man with white hair, a pock-marked face and a pocket full of pens. Tiptree told him that Zena was the one to whom the money was to be paid. I looked at Zena and she smiled.

When they were ready to count the money, Zena went to the table to watch the man piling the hundred-dollar bills on the table. I went to watch too. They were used notes; 250 of them in each thick bundle. They were held together by heavy-duty red rubber bands into which torn scraps of paper had been inserted with '$25,000' scrawled on each of them. There were ten bundles.

Perhaps in some other bank, in some other town, the money might then have been passed across the table. But this was Mexico and these were men well accustomed to the mistrust that peasants show for bankers. It all had to be counted a second time note by note. Despite Pepe's fumbling, it took only a few minutes.

When he'd finished counting, Pepe opened a cupboard to get a cardboard box for the money. There were many other boxes, of all shapes and sizes, stacked in the cupboard. On the side of this box it said 'Flat fillets of anchovies 50 tins – 2 oz.' I wonder who first discovered that fifty tins of anchovies fit into exactly the same space as a quarter of a million dollars. Or vice versa.

Perhaps I should have given more attention to Pepe's nervous manner and to the clumsiness he showed in handling the notes but I was too concerned with the prospect of Zena departing with the money before Stinnes arrived. I looked at my watch and I looked at the clock on the wall. Stinnes was late. Something had gone wrong. All my professional intuition said leave, and leave right away. But I stayed.

While Pepe was putting strapping-tape on the box, Zena went to the window. She was holding back the edge of the blind to see down into the square when Pepe told me and Tiptree to put our hands on our heads.

'I'm sorry,' said Pepe, whose drawn white face, the stubble of tomorrow's beard already patterning his chin, bore a frown of desolate unhappiness. 'I'm doing only what I must do.'

Tiptree, despite his excellent Spanish, did not understand Pepe's soft instruction.

'Put your hands on your head,' I said. 'Do as he says.' Even then I think Tiptree would not have understood except that he saw me put my hands on my head. 'Someone got here ahead of us.'

'Your friends?' said Tiptree, looking round the room.

'How I wish they were,' I said. But I had no time for Tiptree's stupid suspicions. I was trying to decide what role the old man with the shotgun was playing in this business, and whether the two boys with him were armed.

Now Zena also had her hands on her head. She'd been pulled away from the window in case her shadow on the blind was seen by someone in the street. 'What's happening?' said Zena.

It was then that a burly, dark-suited man came from the next room. Beside him there was a Mexican boy with a machine-pistol. I didn't like machine-pistols. Especially cheap machine-pistols like this one. Hoping to survive a false move against a man with a machine-pistol was like shouting abuse at a man with a garden hose and hoping not to get wet. I looked at it carefully. It was a Model 25, a Czech design that dated from the time before they changed over to Soviet calibres. An old, cheap gun, but the boy liked waving it around, and he kept the metal stock folded forward to make this easier to do.

I recognized the dark-suited man from the night I'd spent at Bieder-mann's house. It was Stinnes's companion, the man who called himself Pavel Moskvin; the 'fink' – a tough-looking fifty-year-old with a cropped head and the build of a debt-collector. 'You,' he said to me in his abominable German. 'You make sure your friends know that no one will harm them if they do as they are told.'

'What's it all about?' I said.

He looked at me but didn't answer. 'Tell them,' he said.

Zena and Tiptree had heard for themselves. Tiptree said, 'Is this your doing, Samson?'

'Don't be stupid,' I said. 'It's a KGB stake-out. They are waiting for Stinnes. They might leave us out of it if we behave.'

'What will they do?' said Tiptree. 'Are they going to kill him?'

I shrugged. We could only wait and see. The door-buzzer sounded, and Moskvin nodded to tell Pepe to open the spy-hole.

Pepe looked through and after a brief muttering through the hatch said it was a woman who wanted to change some US one-dollar bills into Mexican money. 'Do you recognize her?' Moskvin asked Pepe.

'We have a lot of people asking for change: waiters, hotel workers, shop workers. I don't know. I can't see much through the hatch.'

'Tell her to return tomorrow. Say you've run out of money.' Moskvin's Spanish was even worse than his German. To get a job in the Soviet foreign service with so little aptitude for languages, a man would have to be a very loyal Party supporter.

Pepe sent the woman away and then we all settled down to wait. It was a nerve-racking business. Moskvin had prepared it well. It was the right place. He had all the evidence he needed to nail Stinnes, and this way he'd have the dollars too. There was nothing the KGB liked better than rubbing our noses in it. I cursed Tiptree for changing the rendezvous. It wouldn't have been so easy for Moskvin out there in the dark crowded square.