I couldn’t afford to make a mistake; people would die if I did.

The dense grove of trees was clear in the image, but the large dark shape near it was not.

What concerned me was that the railway was less than half a mile from the smudge, and the strategically significant port at Calais a half mile in the other direction.

It could be the ideal location for a munitions factory.

Or was it simply a consequence of the photographic development process, a shadow from a cloud, dust on the camera or the negative?

For my whole life, maps had made everything possible. I knew this to my very core, aware that the war would be won or lost based on the quality of maps. I stared at the aerial photograph in my hands. Every missing bridge, new road or altered building was important.

The sound of the teleprinters in the next room interrupted my thoughts.

I put the photo down on the battered wooden table in front of me and picked up the map I was trying to update.

Aside from the well-recorded build-up around the port and a few artillery installations along the coast there had been no changes observed for some time.

But that didn’t mean things weren’t happening; it simply meant they hadn’t been seen.

In the files, I searched for an older photograph for comparison but the last images of this area of Calais were over six months old.

So this smudge could be something or it could be nothing.

I needed to decide. At least there appeared to be no new armaments unlike the last photograph I’d looked at taken near Brest. I straightened and rolled my shoulders back.

For five hours I’d been studying the latest photos and comparing them to the previous ones.

Any variations I discovered, I marked in preparation for the Combined Forces meeting.

Decisions could be made on even the smallest changes I could observe.

My colleague, Lieutenant Harling, looked up from the map he was working on and held up a pack of cigarettes.

His solemn face broke into a smile, making his features almost handsome.

But it was his charm that made him exceedingly attractive.

Half the women in the building were a bit in love with him.

I was not; I had learned early on to avoid that sort of distraction, although I had fallen in love once.

Now I kept all my affection for a fictional detective.

It ensured there were no complications or distractions that way.

‘No, thanks.’ I took a deep breath and looked at the slim gold watch on my wrist. ‘If I start at noon, what will I use to keep me alert at eight tonight?’

‘A gin?’ he suggested with a grin.

‘Hmm.’ I laughed, thinking of the many drinks we had shared since I accepted the secondment here seven months ago from the Inter Services Topographical Department based at Manchester College, Oxford.

Our team of two occupied one small room amidst the command and operations centre for the Navy.

We coordinated our efforts with the large teams in Oxford and Cambridge where they made geographical handbooks for the services.

Organising the photographs in front of me into a neat pile, I clasped onto the one near Calais that was troubling me and held it out to him. ‘What do you make of that large mass in the bottom left, Harling?’

He squinted then turned the picture in every direction. ‘Your guess is better than mine.’

‘The last aerial photograph of this area we have was taken months ago and it showed nothing.’ I circled the blob with my finger.

He rose and we both walked to the large map covering the back wall of our underground room. A trickle of air from the vent in the ceiling moved the loose corner of the chart but did nothing to shift the lingering cigarette smoke that hung in the air.

‘Could be nothing. Still . . . ’ I tapped the spot. It needed marking despite my uncertainty.

‘Miss Tremayne?’ a breathless woman called from the door.

I swung around and smiled. It was a typist from the pool.

‘You’re wanted up in The Zoo.’ She drew a breath.

All the typists and the secret ladies as they were known called Room 39 The Zoo.

It held the Naval Intelligence team who were an odd assortment of men including an art historian, a writer, a journalist, a barrister and the former head of Thomas Cook’s West End office.

They all reported to Vice-Admiral Godfrey.

Like a modern Noah’s Ark, Room 39 contained a collection of somewhat exotic creatures for naval intelligence, including my boss.

‘And you’re to bring your maps for your meeting,’ she said.

In haste I marked the unknown object on the map of Calais then rolled it up with the other ones we’d been updating.

‘Knock ’em dead,’ Harling said, perching on the desk lighting a cigarette.

I made my way down the corridor, stopping in the powder room to check my appearance.

With my boss I never quite knew what to expect.

I smoothed my hair into place and freshened my red lipstick.

Now I was ready to face whatever it was that required my attention.

It had to be important to call me away from my preparation for the meeting at five.

It wouldn’t be an excursion. They were always well planned, and it normally involved soothing ruffled feathers and unpicking the muddles various officers and ministers found themselves in when they looked at a map rather than reading it.

Climbing the stairs, I left behind the artificial daylight lamps and stuffy air conditioning and entered the corridors of the Admiralty.

Passing the uniformed guards who knew me well, I walked down the long dull corridor.

Only the mosaic tiles on the floors lifted the atmosphere.

Their swirling wave patterns were almost whimsical and always struck me as at odds with the solemn activity that took place in the building.

I strode on with the sound of my heels echoing in the bleakness on my way to the beating heart of Naval Intelligence.

Reaching a large transept the size of a station ticket booking hall, I noted the chaos of paperwork in transit, boxes, trays, and tea-making equipment. I waved to the women who kept everything working. Without them we didn’t have a chance of winning this war and yet here they were almost invisible.

I opened the door to Room 39 and was immediately struck again by the three long west-facing windows.

They looked out on Horse Guards and over to the garden of number 10 Downing Street, the Foreign Office, and St James’s Park Lake.

The parade ground was filled with barrage balloon equipment.

The scene was so familiar, I could map it from memory.

The first man to make eye contact was Lieutenant Commander Shawcross, who liaised with me on all things for the topographical handbooks created in Oxford and Cambridge.

Lieutenant Commander Montagu nodded a welcome.

I was a frequent sight in this hub of activity.

The other occupants of The Zoo were too engrossed in their work to acknowledge my presence and the green baize door to Vice-Admiral Godfrey’s office was closed.

My boss, Commander Fleming, was the vice-admiral’s personal assistant and the department’s fixer, knowing everyone who was useful.

Currently he was at his desk with a solemn expression on his face.

At thirty-three, he was as smooth as they come, with not a hair out of place and a cigarette always in hand.

His uniform was immaculately tailored to flatter.

He, like Harling, caused the female staff’s hearts to flutter.

We had met when Fleming visited Oxford with Harling and they’d sat in on one of my briefings.

Two days later I was summoned to London and by the end of the week I was working for the intelligence service.

Gone were the hours spent teaching map reading to new officers and making geographical information booklets among the dreaming spires.

Commander Fleming looked up, stood then walked around the desk and pulled out a chair.

‘Do sit, Merry,’ he said then lit a cigarette. ‘The meeting with the Combined Forces has been brought forward.’ Fleming tapped on the pile on the desk. ‘And new intelligence photographs have come in.’

‘Aerial or oblique?’ I asked. Each of these provided vital information with the oblique ones identifying natural and man-made defences on the beaches and the aerial providing detailed information on roads, bridges and railways.

‘Both.’ Fleming perched on the corner of the desk.

‘Do I have time to review them before the meeting?’ I asked, reaching for the uppermost image.

He glanced at the clock which sat above the huge marble fireplace that dominated one wall.

‘No,’ he said.

‘Do you have any idea what they want?’ I was almost certain he didn’t, or that, if he did, he wouldn’t say.

Everything was on a need-to-know basis and I was becoming accustomed to not being given access to all the information I needed to be fully useful.

This frustrated me daily, but I couldn’t change the system as much as I might like to.

‘Are you ready?’ He stubbed his cigarette out and stood.

‘Of course.’ I clutched the maps I’d brought with me and followed him down the long corridor and up the stairs, suppressing my laughter as he vetted my attire.

He struggled to understand my confirmed single status.

I was a mystery to most men and the commander was no exception.

They couldn’t accept that a twenty-seven-year-old woman with a pretty face and a good figure could have a brain and would choose to give marriage a miss.