As they passed through the building, two men fell in behind them: Inspector Thompson from Scotland Yard, and Churchill’s private bodyguard. They emerged on the parade ground, passed a team operating a barrage balloon, and went through a gate in the barbed-wire fence to the street. London was blacked out, but a crescent moon gave enough light for them to find their way.

They walked side by side a few yards along Horse Guards Parade to Number One, Storey’s Gate. A bomb had damaged the rear of Number Ten, Downing Street, the traditional residence of the Prime Minister, so Churchill was living at the nearby annex over the Cabinet War Rooms. The entrance was protected by a bombproof wall. The barrel of a machine gun poked through a hole in the wall.

Digby said, “Good night, sir.”

“It can’t go on,” said Churchill. “At this rate, Bomber Command will be finished by Christmas. I need to know who or what Freya is.”

“I’ll find out.”

“Do so with the utmost dispatch.”

“Yes, sir.”

“Good night,” said the Prime Minister, and he went inside.

PART ONE

1

On the last day of May 1941, a strange vehicle was seen on the streets of Morlunde, a city on the west coast of Denmark.

It was a Danish-made Nimbus motorcycle with a sidecar. That in itself was an unusual sight, because there was no petrol for anyone except doctors and the police and, of course, the German troops occupying the country. But this Nimbus had been modified. The four-cylinder petrol engine had been replaced by a steam engine taken from a scrapped river launch. The seat had been removed from the sidecar to make room for a boiler, firebox, and chimney stack. The substitute engine was low in power, and the bike had a top speed of about twenty-two miles per hour. Instead of the customary roar of a motorcycle exhaust, there was only the gentle hiss of steam. The eerie quiet and the slow pace gave the vehicle a stately air.