Overnight, the sightings of the ghost submarine had increased dramatically. No one knew if this was because of White’s tinkering with the navy’s sonar and radar systems or some sloppiness by whoever was messing about in the Channel, but everyone wanted to take advantage of the opportunity they’d been gifted.

‘Has anyone else noticed?’ Knox asked.

White shook his head. ‘Nothing from the French, Dutch, or Belgians. At least nothing we’ve picked up. It feels like the ghost is putting on a show specifically for us. I’ve got every engineer in the place trying to work out why.’

‘Is anyone looking at the other why?’ Knox asked.

White frowned.

‘If there’s a pattern to the appearances it might reveal their mission,’ Knox said. ‘Give me the coordinates of the sightings and maybe I can help predict where they’re going to show up next.’

‘That’s actually a very good idea,’ White replied.

Then he told one of his assistants to clear some space and get Knox a map of the Channel.

Knox settled into the melee, thinking he might just have found what he needed to shake him out of his foggy mood—if the submarine was Russian, piercing whatever invisible armour they’d wrapped it in would be a major coup.

Unfortunately it turned out that this particular chink was a false start.

Knox studied the movements of the ghost, from its very first sighting to the new data he was being handed almost constantly. He tried to discern some logic to what the Russians were doing, but there wasn’t any. He sifted through his limited knowledge of the history of the Channel—and borrowed some of the navy’s—in case the Soviets were trying to send some kind of coded message, but if they were he couldn’t find it.

There were a few points on the map the submarine kept its distance from, but it wasn’t hard to work out why—they were all naval bases. There was one spot on the south coast of England that Knox avoided too: the inverted Y of water that cut the Isle of Wight off from the mainland and stretched up to Southampton. This was where de Gaulle had arrived to see Macmillan after crossing the Channel on the Surcouf, the French navy’s latest T 47-class destroyer seven years ago. It was also where Jack Williams had died, the only member of a dive team sent on Knox’s orders to take a closer look at the new French destroyer who hadn’t returned from the mission.

White’s attempts to come up with a technological solution to tracking the submarine also failed to bear fruit. More echoes were being caught, but they were still just echoes.