Just a note – this is assuming that the Doctor never told Jack, nor any of the others, about what he had to do to Donna. Personally, I think it makes sense that he wouldn't have done – I can't imagine any of them having been too happy with what he did – at least some of them would surely have tried to stop him! He won't be making an actual appearance (as of yet), but of course, being the genius he is, he will have factored a way to let them know what happened after Mickey, Jack and Martha left him. I have also had to improvise slightly with the conditions under which Donna would "burn up", otherwise this story wouldn't be possible …

--

Jack's mind was in turmoil … at midnight he had seen Donna Noble, in Cardiff, without the Doctor, and she had completely denied any knowledge of him. This was a woman who had flirted with him unashamedly the first time they met, for Christ's sake!

It was now the early hours of Wednesday morning and he was lying in his bed in the Hub, going over and over things in his mind. Did it have something to do with the Doctor-Donna situation? Sure, something could have gone wrong with that … but Jack had no experience of such a fusion – who knew what it could have done? All he did know was that she had genuinely forgotten him – and that in itself was just plain weird. He hardly thought himself forgettable, after all.

Even so, Jack had other pressing matters to be thinking about. Things were hectic at the Hub – Martha was away on an operation in conduct with UNIT and wouldn't return for another fortnight. Oh, he still bitterly resented that organisation, but knew that it was for the good of the world that he let her fulfill her duties. Stubbornness didn't exactly go hand in hand with leadership and as a result they were all swamped with paperwork and other menial tasks. Medical problems had to be consulted via fax and video-communication and that simply wasn't enough. Ianto's coffee machine had broken (leading to caffeine withdrawal symptoms all round) and if Mickey called him "Captain Cheesecake" once more, he thought he might have to set a weevil on him. Even Gwen was starting to infuriate him. And an annoyed Jack was an angry Jack.

But if there was one other problem that Jack knew he had to solve, it was what the hell was going on with Donna.

--

He woke at six o'clock the next morning and climbed up the iron ladder and out of his quarters. Having drunk a glass of water to clear his head, he strolled into his office and sat at the desk, ready to write a disgruntled email to the head of UNIT enquiring as to whether they could get Martha back early. Having finished this, he sorted some of the papers on his desk and found a couple of articles about weevil attacks that needed to be filed away. Picking them up, he sauntered away to the archives and filed them in the "W" of the "Accidentals" section. Flicking through this draw, Jack noticed that there was also a section named "Noble." He stared at it, wondering if he had seen it before … surely he hadn't? He was immaculate in his keeping of the files and knew he would have remembered if there was a section on a particular individual …

With a deep breath, Jack picked the file out of the draw and began to rifle through it.

--

The file was divided into two sections, each with a date. The first date read: Wednesday 3rd December 2008. That was today's date!

The second was inscribed: Friday 3rd December 2010.

Utterly bewildered, Jack looked inside the first section. There was one envelope, creamy white cartridge paper, uncreased and as new as if it had been placed there that day; an impossibility, as he would have known had anybody entered the Hub – the trespassing system was too complex to be swindled and he'd even have been alerted had any member of Torchwood come in. Turning the envelope over so that he could see to whom it was addressed, he saw that his own name, 'Captain Jack Harkness' was written in letters cut out from newspaper articles. Even more dazed, he opened the letter. Never, in all of his wildest dreams did he anticipate what he would read.

--

Dear Jack,

I am writing to dispel any confusion that I know you're feeling right now. I thought it would be best to do so before you manage to put your foot in things too much; I know the situation well and if this does not reach you by the time I plan for it to, the fate of the world could be at stake. Last night you saw Donna Noble. She failed to recognise you and you are clueless as to why. I am here to clear that matter up, as well as explain some important things she and I need you to do.

I will start by saying it is of the utmost importance that you do not open the letter under the section, "3rd December 2010" until that date. These are messages written in hindsight and the structure of time and space could fracture if you were to do so. Even the one you are reading right now is risky, but your good friend, the Doctor, has assured me that it remains within the limits of matters such as this.

The last time you saw him, the Doctor was with Rose, Jackie, the human Doctor and Donna on board the TARDIS. As you know, the first three passengers were returned to a Parallel Universe. As far as you were concerned, Donna remained a companion of the Doctor. It is regretfully that I have to inform you that day was the last journey she took with him. Due to the fusion of her mind with a Time Lord's in her physically human state, she would have "burned up" had the Doctor not taken action. The physical, chemical and biological details are complicated, but you will come to understand them one day. As a result, he was forced to wipe her memory of him and her travels with him. If she is told anything about the Doctor, extra-terrestrial life or her adventures as his companion then her mind will burn up and she will die.

I am aware that she has come to work in Cardiff, the location of a rift in Time and Space. Therefore, it is your duty to prevent her from remembering that which will kill her. One day, you will understand why this letter was written, and why, of all the people there fighting for the Doctor's Secret Arm that day, it is you who has been chosen to be her shield. Until then, I wish you good luck.

A Friend.

--

Jack folded up the letter and put it in his breast pocket. All that is holy … that was quite a message. The letter had been written on a typewriter, giving no indication as to its sender. Who else had been there that day, who could have known all of those details about what had happened, who had been where, travelling with whom? His mind drew a complete blank.

Suddenly, he heard the hum and whir of the siren by the door – someone else was arriving at the Hub, probably Mickey (always an early starter). Hurriedly, Jack clammed the filing cabinet closed and darted up the stairs to his office. It would be a long day and a sleepless night, that much he was sure of …