But the American high command thought the attack might come at Hawaii, and the army feared an invasion of the west coast of the United States. Even the team at Pearl Harbor had a nagging suspicion it could be Johnston Island, an airstrip a thousand miles south of Midway.

They had to be one hundred per cent certain.

Chuck had a notion how it might be done, but he hesitated to say anything. The cryptanalysts were so clever, and he was not. He had never done well in school. In third grade a classmate had called him Chucky the Chump. He had cried, and that had guaranteed that the nickname would stick. He still thought of himself as Chucky the Chump.

At lunchtime he and Eddie got sandwiches and coffee from the commissary and sat on the dockside, looking across the harbour. It was returning to normal. Most of the oil had gone, and some of the wrecks had been raised.

While they were eating, a wounded aircraft carrier appeared around Hospital Point and steamed slowly into harbour, trailing an oil slick that stretched all the way out to sea. Chuck identified the vessel as the Yorktown. Her hull was blackened with soot and she had a huge hole in the flight deck, presumably caused by a Japanese bomb in the Battle of the Coral Sea. Sirens and hooters sounded a congratulatory fanfare as she approached the Navy Yard, and tugs assembled to nudge her through the open gates of No. 1 Dry Dock.

‘She needs three months’ work, I hear,’ Eddie said. He was based in the same building as Chuck, but in the naval intelligence office upstairs, so he got to hear more gossip. ‘But she’s putting to sea again in three days.’

‘How are they going to manage that?’

‘They’ve started already. The master shipfitter flew to meet her – he’s on board already, with a team. And look at the dry dock.’

Chuck saw that the vacant dock was already swarming with men and equipment: he could not count the number of welding machines waiting at the quayside.

‘All the same,’ Eddie said, ‘they’ll just be patching her up. They’ll repair the deck and make her seaworthy, and everything else will have to wait.’

Something about the name of the ship bugged Chuck. He could not shake the nagging feeling. What did Yorktown mean? The siege of Yorktown was the last big battle of the War of Independence. Did that have some significance?

Captain Vandermeier walked by. ‘Get back to work, you two girlieboys,’ he said.

Eddie said under his breath: ‘One of these days I’m going to punch him out.’

‘After the war, Eddie,’ said Chuck.

When he returned to the basement and saw Bob Strong at his desk, Chuck realized he had solved Strong’s problem.

Looking over the cryptanalyst’s shoulder again, he saw the same sheet of paper with the same six Japanese syllables:

YO—LO—KU—TA—WA—NA

He tactfully tried to make it sound as if Strong himself had solved it. ‘But you have got it, Lieutenant!’ he said.