From somewhere over the other side of town I heard a police siren start up. I leaned out of the window and then threw the radio as far as I could towards the office. The windows were now dark again, except for the faint flicker of fire.

I buttoned my coat, put on my cap, and looked quickly around the room to make sure there was nothing there left to compromise us. Then I went downstairs to watch the police and fire service arrive.

The firemen arrived immediately after the first police car. And then an ambulance. The noise of their heavy diesel engines throbbed loudly. Batteries of headlights burned through the continuing drizzle of rain and reflected upon tiny bits of broken glass that were strewn all over the roadway and sparkled like ice. There were black pieces of charred paper and broken bits of wood and things that I didn't care to inspect too closely. The fire engine's ladder moved slowly until it was positioned against the office windows, where a red glow was still to be seen. A fireman climbed it. There was a terrible smell of burning and enough smoke for the firemen to be using breathing gear.

The whole street was brightened as everyone drew back their curtains to watch the activity. By now the front door of the offices had been opened. The ambulance men pushed through the little crowd that had formed and went inside to look around. They didn't take a stretcher with them. They guessed they wouldn't be needing one.

It was three o'clock Sunday morning by the time I'd collected the car and driven back to Bret Rensselaer's place in Berkshire. Bret was fully dressed when he came to answer the door to me – he was quick to tell me that he'd never gone to bed – but he'd changed his clothes; he was now in a roll-neck cashmere sweater and matching blue poplin pants. He'd been waiting for the phone call that would tell him everything had gone smoothly.

But when the phone call came, it told him that an explosion had killed two men in an office in Cambridge. The story was on the wire services. It was too late for the Sunday papers, but the national dailies would probably carry it on Monday. If a TV crew had got pictures, it might be on the evening bulletin.

'We need a break,' said Bret. He'd put a drink in my hand and then devoted a lot of time to getting a second log burning in the fireplace. I crouched over it. I was cold.

'Yes, we need a rise in the price of beer or a bus drivers' strike to grab the headlines,' I said. 'But don't worry; a small explosion in the back streets of Cambridge isn't exactly front-page stuff, Bret.'

Bret pulled a little wheeled trolley over to the fire. On it there was a bottle of single-malt whisky that he'd brought out of the cupboard for me and a full jug of iced water. He sat on the fender seat and warmed his hands. The curtains were closed now, but I could hear the rain still beating on the glass, as it had been not many hours before when I'd sat here with Ted Riley, listening to Bret explaining how easy it was all going to be. 'A booby trap,' said Bret. 'What bastards!'

'Let's not jump to conclusions,' I said. I sat on the other side of the fender. I don't like perching on fender seats; it was like trying to get warm on a barbecue – you cooked one side and froze the other. 'Maybe it wasn't intended to kill.'

'You said it was a booby trap,' said Bret.

'It was a slip of the tongue.'

'So what was it?'

'I don't know. It might have been no more than a device to destroy the secret papers. But a heavy-steel filing cabinet makes it into a bomb.'

'They put a lot of explosive into it. Why not use an incendiary device?' asked Bret.

'We had an explosion like it in Berlin back in the old days. They'd only used a small charge, but the cabinet had some special fireproofing liner. When it went, it blew the side of the building out. It was worse than this one.'

Why is he bugging me about all these details? I thought. Who cares about how big the explosive charge was? Ted Riley was dead.

There's no chance that…'

'No chance at all. Two dead. You said the wire services had the story.'

'They get it wrong sometimes,' said Bret. 'Will they be identified?'

'I didn't go in and look around,' I said.

'Sure, sure,' said Bret. 'Thank Christ it wasn't you.'

'Riley's an old-timer. He emptied his pockets and his clothes had no laundry marks. He made me check it with him. The other man I don't know about.'

'The locksmith came from Duisburg. It was a German make. He was the expert on that sort of safe.'

'They'd changed the inside of the lock,' I said.

'I know,' said Bret. He drank some of his tonic water.

'How could you know unless you had a monitor on the radio?'

Bret smiled. 'I had someone monitoring the radio. There's no secret about that.'

'Then why ask me the questions?'

'The old man is going to ask me a lot of questions and I want to know the answers. And I don't want to read the transcript to him; he can do that for himself. I need to hear what you've got to say.'

'It's simple enough,' I said. 'Stinnes told the interrogator that there was some good stuff in that office. You sent Ted Riley in to get it. The filing cabinet was wired to destroy the evidence – bang. What difficult questions can the D-G ask, except why?'

'I don't blame you for feeling bitter,' said Bret. 'Ted Riley was a friend of your father, wasn't he?'

'Ted Riley was good at his job, Bret. He had the instinct for it. But the poor sod spent his life checking identity cards and making sure the burglar alarms were in working order. Just for one little lapse.'

'He wasn't material for London Central, if that's what you are suggesting.'

'Wasn't he? Who do you have to know to be material for London Central?' I said. 'Jesus, Bret, Ted Riley had more intelligence skills in his little finger than…'

'Than I have in my whole body? Or was it going to be Dicky? Or maybe the D-G?'

'Can I have another drink?'

'You won't bring Ted Riley back to life by pouring that stuff down your throat,' said Bret. But he reached for the bottle of Glenlivet and uncapped it before handing it to me. I poured a big one for myself. I didn't offer Bret any; he was quite content with his tonic water.

'I had a talk with Ted Riley last night,' I said. I stopped. The red lights came on in my skull. Everything warned me to be cautious.

'That must have been interesting,' said Bret, keeping his voice just level enough for me not to get up and bust him in the nose.

'Ted told me that Stinnes is tuned to Moscow every morning at eight-thirty. Ted thought he was getting his instructions from them. Maybe one of the instructions they gave him was to tell us about the Cambridge cell and get Ted Riley blown into little pieces.'

'Why are you telling me what Riley thought? Riley was just a security man. I don't need the opinions of security men when the interrogator is doing so well.'

'So why didn't you send the goddamned interrogator to do the break-in last night?'

Bret held up a hand. 'Ah, now I'm reading you loud and clear. You're trying to link the two events. Riley – despite the interrogator's satisfaction – sees through Stinnes and his misinformation scheme. So Riley has to be removed by a Kremlin-planned bomb. Is that what you're trying to sell me?'

'Something along those lines,' I said.

Bret sighed. 'You were the one who's been hyping Stinnes as if he was the greatest thing since sliced bread. Now your friend is killed and everything goes into reverse. Stinnes is the villain. And since Stinnes is virtually under house arrest, Moscow has to be the heavy. You really try my patience at times, Bernard.'

'It fits,' I said.

'So do a million other explanations. First you tell me the bomb was just to destroy the paperwork. Now you want it to be a trap to kill Riley. Make up your mind.'