He had allowed two considerations to change his mind. First of all, his hosts were serious people. A City guild, the Worshipful Company of Goldsmiths no less, had invited him to be the guest speaker at their annual dinner and they had made it clear that the subject could be of his own choosing, though obviously related to his work. And in return for thirty minutes of his time, he would be rewarded with an excellent dinner, first-class wine and a sizeable payment to the Metropolitan and City Police Orphans Fund, one of his favourite charities.

He splashed a little cologne on his cheeks, then turned off the light and went into the bedroom, where his dinner jacket was waiting for him on the back of a chair. Miss Cain had prepared his speech for him. It was lying on the bed, twelve pristine white pages held together with a paperclip. The title – CRIME AND PUNISHMENT – was in capital letters at the top. Pünd slipped on his jacket, carefully folded the pages, placed them inside his pocket and went into the next room.

He had only recently moved into the flat on the seventh floor of Tanner Court, the elegant mansion block in Farringdon, and he wasn’t quite used to it. The furniture was antique and German. He had brought much of it with him when he moved to England after the war. But everything else was still foreign to him. The rooms with their double-height ceilings seemed much larger than they had any right to be. The carpets and curtains were brand new and he remembered choosing them, aghast at both how expensive they were and how easily he could now afford them. The kitchen was so sparkling clean that he felt nervous about using it – not that he ever cooked for himself. For lunch he would have a salad. In the evening he usually ate out.

He glanced at what had once been his father’s pendulum box clock, hanging in the corner. Made by Erhard Junghans in the nineteenth century and so almost a hundred years old, it had never lost a minute’s time. He didn’t need to leave yet. He poured himself a small sherry and took a black Sobranie cigarette out of an ebony box that had given to him by a grateful client. In fact, the entire apartment had only come his way thanks to a recent case. He lit the cigarette and sat down, trying to relax in his surroundings and remembering the strange affair of the Ludendorff Diamond, which had been, in many respects, his greatest success yet.

On the face of it, the theft had been impossible, a magic trick that had baffled the police, the British public and, most significantly, the frustrated owner, who had lost not just the diamond but several other pieces of jewellery plus cash and certified share certificates to the value of almost a hundred thousand pounds.

His name was Charles Pargeter, a multimillionaire with homes in New York and Knightsbridge who had made his fortune in the oil industry. His wife, Elaine, was a well-known society hostess, a patron of the arts, a member of several boards, a woman of great beauty. The robbery had taken place just before Christmas the year before.

In fact, the Pargeters had been on their way back from a party when they found that their house had been broken into. It was clearly the work of professionals. The alarm system had been disconnected and a window on the ground floor forced. The house had not been completely empty. It was a Saturday and two of the servants – the cook and the maid – had been given the weekend off. The butler had stayed behind but he was almost seventy years old and had slept through the entire business. The Pargeters had returned home with John Berkeley, a business associate and friend, and he had noticed the broken window before they even got in.

To begin with, Charles Pargeter had not been worried. He was a careful man and had installed a safe on the second floor of the house. It was not just any safe. It was the best that money could buy: a solid steel, fireproof box that weighed over two hundred pounds, manufactured by Sentry in America and bolted into the floor. Inside the combination lock, which had been strengthened to prevent any possibility of its being forced, there were no fewer than seven wheels, requiring, therefore, a seven-part sequence to get it open. Only three people knew the combination: Pargeter, his wife, and Henry Chase, their lawyer. There was a second lock, key-operated, and only one key had ever been manufactured. Charles Pargeter kept it close to him all the time. The safe stood against the far wall of a narrow, dark walk-in closet. The thieves would have had to have inside information even to know that it was there.

The three of them – Charles Pargeter, Elaine Pargeter and John Berkeley – had entered the darkened house together and at first they believed that they had arrived in time and that nothing had been disturbed. But when Charles turned the lights on in the bedroom, the terrible truth hit him full in the face. The safe door was wide open. All the contents had been removed.

Elaine Pargeter had called the police while Berkeley led his friend downstairs and poured him a large whisky. They were careful not to touch anything. The police – in the persons of Detective Inspector Gilbert, accompanied by Detective Sergeant Dickinson – arrived very soon after, asked various questions and examined the empty safe. Both the safe and the broken window were searched for fingerprints but none were found.

Pünd remembered reading about the theft in the newspapers. The whole country had been gripped by what had happened – for two reasons. The first was that the safe really was impregnable. The American manufacturers had flown to England immediately and after a careful examination they had announced that their product could not be to blame. The lock could not be forced and had not been forced: whoever opened it must have known the combination, even though this narrowed the crime down to just two suspects: Charles Pargeter and his wife. Their lawyer, Henry Chase, the only other person trusted with the combination, had been abroad on the night in question. Of course, he could have passed on the combination to an accomplice but that still left the problem of the key. Pargeter kept it on his main key ring and it was never out of his sight. He had had it with him at the party and he had handed it to DI Gilbert, who had confirmed that it was the right key and definitely fitted the lock. Could someone have taken it and made a copy? Again, the manufacturers insisted this was impossible. The key was like no other, with a unique, patented design. They had given a press conference in which they had come perilously close to accusing Pargeter and his wife of insurance fraud. But that, too, was highly unlikely. Pargeter had no money troubles. On the contrary, his business was booming. He was one of the richest people in the world.

But it was the Ludendorff Diamond that had really captured the public’s imagination. There are many precious jewels that seem to exist in a world of fantasy and folklore and this one was no exception. A flawless ‘pear double rose-cut diamond’, it had 33 carats and 140 facets. It had been found in Golconda, the same region of India that had produced the Koh-i-Noor. It had belonged to a Russian aristocrat, Prince Andrei Ludendorff, who had been killed in a duel, but not by his opponent. His gun had jammed and exploded in his hand, sending a fragment of metal into his eye. It was said that the diamond had been buried with him, but that his not entirely mournful widow had sent a pair of grave robbers to retrieve it. Pargeter had bought it privately in New York for an undisclosed price, although the figure of two million American dollars had been mentioned. It might well have been more.

And now it was gone. Pargeter had also lost cash and shares. His wife had kept several pearl and diamond necklaces, rings and a tiara in the safe. Even their passports and birth certificates had been taken. But all this seemed inconsequential compared to the Ludendorff Diamond. There was, Pünd noticed, a certain amount of sympathy for the criminals who had pulled off this spectacular coup without violence. At the same time, there was very little sympathy for Pargeter, who was seen almost as the instigator of the crime rather than its victim, as if his extreme wealth had made it only reasonable that he should be targeted.

Pargeter was not, in fact, an unpleasant man. When he had arrived at Pünd’s office in the Old Marylebone Road, he had come across as quiet and self-effacing. He had the look of a Harvard professor, with thick, silver hair and glasses and immaculately dressed in a double-breasted suit and tie. Pünd remembered every word of what he had said.

‘Mr Pünd,’ he had begun, standing with his hands behind his back, ‘my people tell me that you are the best detective in the world, and having looked into your history, I believe you are the only man who can return the Ludendorff Diamond to me.’ He spoke with an American accent, carefully judging his words before he allowed them to pass his lips. ‘I want to explain to you why I have come here today. First, as I’m sure you are aware, the police have been unable to come up with any possible explanation for what appears, on the face of it, to be an impossible crime. I have repeatedly told them – and I will assure you of the same thing – that only three people knew the combination to that lock and I would trust the other two with my life.’

‘You have never told it to anyone else?’ Pünd interrupted.

‘No.’

‘And it was never written down? Perhaps as an aide-memoire?’

‘No.’