Moment Fifty

Parking her motorbike in the staff carpark, Clara dismounted and turned to face the school building, mentally preparing herself for her first day in her new job as an English teacher. Few who looked at her would have guessed that she was the latest in the long list of people who had been the Doctor's companions. And it was very unlikely that anyone would have guessed that there were a million versions of her scattered along the Doctor's timeline.

She had, though he was unaware of it most of the time, saved his life more than once. She had even convinced him to take a malfunctioning Type 40 TARDIS when he and his granddaughter, Susan, left Gallifrey. The beginning of a journey which, Clara had recently learned, would end on the planet Trenzalore.

Trenzalore, the site of the Doctor's final stand, the place where he would one day be laid to rest. And the one place no time-traveller must ever visit was their own grave; to do so would cause a massive paradox. But the Doctor and Clara had recently not only found themselves on Trenzalore but inside the decaying remnants of the TARDIS, which would become the Doctor's tomb. Except there was no body inside, just a tangle of glowing strands marking the Doctor's journey through space and time.

It turned out that the whole thing was a plot by the Great Intelligence to corrupt the Doctor's timeline. In the form of the sinister Doctor Simeon, the Intelligence had entered the tangle of light, at which point the Doctor's very history began to change. All his victories became defeats. Every incident where he had originally survived became the point where he died.

It was then that Clara realised why the Doctor called her the Impossible Girl, why he claimed to have met her twice before, at different points in space and time. She was supposed to save him and the only way she could do so was to enter his timeline and repair the damage the Great Intelligence had caused. And, the moment she did so, the Time Winds had split her into a million copies, scattered along the Doctor's timeline. So she had saved the Doctor's life many times over.

She had seen all the Doctor's past selves, from the old man with long white hair to the pinstripe-suited man who had been the Doctor's incarnation immediately before the one she knew. She had even seen the one the Doctor refused to acknowledge. For there was one version of himself that he kept buried deep in his memory, the one he believed had destroyed his own people. Clara had glimpsed this version of the Doctor moments before her Doctor rescued her from the void in which she had found herself, though she hadn't seen his face.

Afterwards, she had questioned the Doctor about this. "He's the version of me who fought in the Time War," he had told her. "The one who killed them all." As he spoke, a haunted look crossed his face, a look which reminded Clara that he was far older than he appeared. But he wouldn't elaborate any further, saying only that it was a time in his long life that he tried not to think about. "What would be the point?" he had added, before setting the TARDIS co-ordinates and taking Clara home.

Clara had seen the Doctor several times since then and he had taken her on many adventures. In between times, she had finished her teacher training course - her job as nanny to the Maitland family was to help pay her tuition fees - and she was now about to start her first teaching job. She made sure her motorbike was secure, then checked she had everything she needed for the classes she was due to teach that day. Then, she headed in the direction of Coal Hill School.

At the same school about fifty years earlier, another young teacher was becoming increasingly curious about one of her pupils, an exceptionally bright girl who had several inexplicable gaps in her knowledge. The teacher's name was Barbara Wright.