He recalled a heated discussion as they watched the burning towers – Nadja had said that she couldn’t understand people who committed suicide because they were afraid of death. Was this the same woman who, years later, was supposed to have plummeted into the depths of the ocean, herself now a puff of grey?

It was as unimaginable as two aeroplanes flying into the World Trade Center one after another.

But that happened too…

‘Do we have another view?’ Martin asked. Bonhoeffer puckered his lips apologetically. They were in the living room of the captain’s suite, the curtains closed and the lights dimmed. Half a minute ago Martin had asked him to stop the DVD at timecode 085622BZ, which was 20.56 and 22 seconds nautical time.

‘Your family had cabin 8002, which is almost outside the range of the hull camera, right at the other end.’

The captain sounded chesty, a consequence of the bulging plaster across his nose, which restricted his breathing. Dr Beck had attended to him. Martin didn’t know whether he’d admitted to his fiancée the true cause of his injuries or if he’d told her a white lie. He didn’t much care either.

‘It’s a miracle you can see anything at all,’ Bonhoeffer said, and he was right.

That first puff of grey had been illuminated for no more than a split second by the ship’s lights. Before the body hit the water it had already disintegrated into the darkness.

My son disintegrated!

‘Do you want to watch it to the end?’ the captain asked, waving the remote control in his hand.

Yes. Absolutely. But before that, Martin wanted to know something else. He pointed at the timecode at the bottom of the screen, which was flickering in the freeze-frame.

‘When did Nadja and Timmy last enter their cabin that day?’

Bonhoeffer sighed. ‘Please don’t lay into me again, but back then our access control data was routinely wiped at midnight. That’s our system for recording the use of electronic key cards. Five years ago we were only allowed to store the data for twenty-four hours. Things are different today.’

‘So you don’t know how often they went in and out that day?’

‘All we know is that they skipped dinner.’

‘Okay.’ As Martin opened his mouth it felt as if his heart were beating louder. ‘Then please continue the video.’

To the end.

Bonhoeffer pressed a button on his remote control and the gloomy images resumed moving. The timecode at the bottom of the screen counted up in seconds until it happened again at 085732BZ: the second puff of grey fell.

Wait.

‘Stop there!’ Martin shouted frantically.

The words shot from his mouth before the realisation had quite dawned on him.