It was a powerful boat and as fast as anything that even the river-police might have had. We raced up the Havel towards Spandau, Six holding the white steering-wheel grimly, oblivious to the effect that the slipper’s enormous wake was having on the other waterway craft. It slapped against the hulls of boats moored under trees or beside small jetties, bringing their irate owners out on deck to shake their fists and utter shouts that were lost in the noise of the slipper’s big engine. We went east on to the Spree.

‘I hope to God we’re not too late,’ shouted Six. He had quite recovered his former vigour, and stared resolutely ahead of him, the man of action, with only a slight frown on his face to give a clue to his anxiety.

‘I’m usually an excellent judge of a man’s character,’ he said, as if by way of explanation, ‘but if it’s any consolation to you, Herr Gunther, I’m afraid I gravely underestimated you. I had not expected you to be as doggedly inquisitive. Frankly, I thought you’d do precisely what you were told. But then you’re not the kind of man who takes kindly to be being told what to do, are you?’

‘When you get a cat to catch the mice in your kitchen, you can’t expect it to ignore the rats in the cellar.’

‘I suppose not,’ he said.

We continued east, up-river, past the Tiergarten and Museum Island. By the time we turned south towards Treptower Park and Köpenick, I had asked him what grudge his son-in-law had had against him. To my surprise he showed no reluctance to answer my question; nor did he affect the indignant, rose-tinted viewpoint that had characterized all his previous remarks concerning members of his family, living and dead.

‘As well-acquainted with my personal affairs as you are, Herr Gunther, you probably don’t need to be reminded that Ilse is my second wife. I married my first wife, Lisa, in 1910, and the following year she became pregnant. Unfortunately things went badly and our child was still-born. Not only that, but there was no possibility of her having another child. In the same hospital was an unmarried girl who had given birth to a healthy child at about the same time. She had no way of looking after it, so my wife and I persuaded her to let us adopt her daughter. That was Grete. We never told her she was adopted while my wife was alive. But after she died, Grete discovered the truth, and set about trying to trace her real mother.