Bletchley Park Museum was a large, Victorian Gothic mansion situated in parkland in the county of Buckinghamshire. During the Second World War, it had been the base for the Government Code and Cypher School- a branch of the British intelligence services dedicated to breaking the encrypted communications of various branches of the Nazi armed forces.

Until the 1970s, Bletchley had been a government secret. Only selected people who had worked in the intelligence services or within the park itself had any idea of its existence. And nobody knew the full story.

Bletchley Park had been home to the best minds across a number of disciplines throughout the war. Alan Turing, father of modern computing, had been based at the site for the duration of the war and his efforts alone had made a significant impact on the success of the allies. It truly was a fascinating place.

As the car she had hired turned to drive towards the looming main house, Maura couldn't help but imagine what her role would have been had she been alive during the war. It was such a significant part of history, especially for women, and she could understand exactly why Rachel Maloney had dedicated years of her life to studying exactly what had gone on in this magnificent, but secretive place.

She had spent a large portion of the previous evening thumbing through Rachel's thesis: her notes, her scribbles, in the hope of finding something significant. When nothing jumped out at her, she decided it would be best to commit as much as she could to memory. Now that she was here, approaching the gravel driveway and round lake Rachel had described in her writing, her words were given life and meaning and Maura felt confident that a little time in this place would be all she needed to make sense of Rachel's work.

"Hello, I called earlier. We're here to see the research manager? He said he would meet with us this morning.." As Harriet spoke, Maura noted how much her Bostonian accent had softened from her years in the UK.

"Okay. If you can give me a moment, I'll call Professor Mountford for you. His office is just through that door, if you'd like to wait in there."

It felt strange to be ushered into an office in the middle of a museum, but Maura surmised that Professor Mountford would simply rather have use of a functional office that had always been an office, rather than take a room that had a different function and destroy any historical significance they might find in there.

The door creaked open. Professor Mountford, who looked to be in his early seventies, strode confidently into the room. "Are you ladies here for me?" He asked, beady eyes surveying them both as he stretched out his hand.

With a firm handshake for each of them, Professor Nicholas Mountford introduced himself. "Now, what can I do for you?"

Harriet took a breath. "Professor, I am a friend of Rachel Maloney. Your name was in her doctorate file."

"Ah, Rachel. Such an enthusiastic character. It's so nice to see the young taking an interest in their history."

"Professor Mountford, Rachel is dead."

The old man gripped the leather desk with both hands, his knuckles white and swollen from arthritis.

"Dead? Oh good heavens. That is a shame. When she didn't appear for her presentation at the festival, I asked my colleague to call. She didn't get an answer, but I didn't persevere. I just supposed she was ill. I do hope she didn't suffer."

Maura shook her head, looking into the man's eyes searchingly. "I'm afraid it was murder, Professor."

Nicholas Mountford raised a hand to his heart. "Well. Poor Rachel. What a damn shame. I… I thank you for coming all this way to tell me. I will truly miss her. Is there anything else you would like to know whilst you're here?"

"Rachel left…" Maura nudged her friend and shook her head, unsure if she could trust this man with such important information.

"Professor, what did you know about Rachel's work?"

"Rachel's focus was mainly on the women employed at Bletchley. When she visited, it would be to roam through old testimonials, old letters. She knew more about the women who worked here than anyone else I know. She would meet regularly with a group, the Bletchley Park Historical Preservation Society, and had met a handful of the women who did work here. Most, unfortunately, are no longer with us, but she had the joy of hearing first-hand how these women changed the world for the better. Not many more people will get that privilege, I'm afraid."

Maura smiled at the man's enthusiasm. "It sounds like wonderful work."

"Preserving the stories of history's forgotten heroes? There is nothing better, my dear."

Trying her best to recall the chapter names of Rachel's work, there was nothing of significance Maura thought she could ask.

"Do you mind if we have a look around? I would really like to know more about this place."

Professor Mountford nodded, and gestured to the door. "Absolutely. As friends of Rachel's I would say go through free of charge, but a small donation is always useful to us here. I'm afraid I have a meeting to attend with the events team- I shall have to let them know about this sad news- so I won't be able to accompany you. There is a guided tour setting off from the main doors at ten-past. It is truly fascinating stuff. I should know- I wrote the script."

Rising from his seat, Nicholas Mountford strode confidently back towards the door. "I am sorry our meeting has been under such terrible circumstances." After shaking the hands of both women, he was gone.

"What now?" Harriet asked.

"We find an enigma machine".

Strolling quietly behind a group of tourists, Maura half-listened to the guide's explanations as she looked around for any sign of a working enigma machine. As they approached the huts of the old naval codebreakers, she spotted a display case.

"Harriet!" She whispered, pointing. "Enigma."

The large, black machine looked like an old typewriter, but with more cogs and no space for paper. "See, the machines were wired in pairs with different settings. Depending on the rotor positions, each key gave out a different letter. But always in pairs. It was one of the biggest breakthroughs early on. Fascinating".

"There's one problem" Harriet said, studying the machine in the glass box before them. "We don't know the rotor positions. We… without them we have no idea what we're looking for. It doesn't make any sense."

"I agree."

"So, we're back to square one?"

"Unless Rachel managed to remember a series of exact rotor positions and make note of them before she…" Maura stopped herself before she upset her friend.

"Yes, square one."

The tour was disrupted by the shrill tone of Maura's telephone.

Glancing down at the name scrolling along the top of the screen, she let out a deep sigh. "I should take this" she explained, her hand gently resting on Harriet's forearm.

"Is it work?"

Maura shook her head.

"It's Jane."