For clarification: The first part of this fill occurs during Season 1 of the new series, where the Doctor (number 9) and Rose are stuck in Downing Street with the lovely Harriet Jones, trying to get the Slitheen to gtfo the planet. The second part occurs shortly afterward in Arthur's time stream, but quite some time after the Doctor regenerates into number 10, meaning after the finale of Season 4. I also worked in a reference to "Time Crash", the 10 minute 2008 'Children In Need' episode.
Disclaimer: Doctor Who is property of the BBC and I hold no rights to anything affiliated with it, as much as I sincerely wish otherwise.
Arthur was in 10 Downing Street.
On a normal day, he would leave everything to the Prime Minister and go about his day, drinking his tea and eating fish and chips. Today, however, was not a normal day.
Today, the Prime Minister was not answering his private phone line, the line that Arthur had always been able to contact him on. Not only that, but Big Ben had just undergone abrupt reconstructive surgery, there were hundreds of television shows broadcasting rumors of an alien landing, and most of the people in England were mucking about, drinking and carrying on as if the aliens honestly wanted to see them all make fools of themselves.
He had managed to get inside the building without too much fuss from security, as most of them had some vague idea of who he was. Arthur somehow managed to sneak upstairs in a very James Bond moment, hiding behind doors and desks whenever possible. There were people rushing about everywhere and a man was walked rather fast and carrying a peculiar looking briefcase. The Englishman decided to follow him, until it was clear that he could only get so close to the entrance of the room that the man had just haphazardly ran into. He listened closely, but heard little.
The replacement for the Prime Minister seemed out of sorts to Arthur, but he stayed silent and hidden. He wondered briefly what sort of situation he'd gotten himself into. He waited and waited, and suddenly the man who had been rushing about with the briefcase returned, speaking in a rush to the General, who had just come from the Cabinet room.
"General Asquith, sir, we've had a priority alarm. It's Code 9 confirmed, Code 9."
"Code 9? Which would mean...?"
"...In the event of the emergency protocols being activated, we've got software which automatically searches all communications for key words, and one of those words is Doctor. I think we've found him, sir."
Arthur's eyes widened in disbelief. Oh yes, he knew about this Doctor. He had never met the man himself, but he was old enough to have heard the rumors and whispers that only time told. He had seen the ship that the Doctor traveled in, but only once as a young man. That strange machine... He had never forgotten that blue box and the way that it had disappeared into thin air, much like his friends often did.
"Hey! What d'you think you're doing there?" Arthur was unceremoniously yanked from his spot under the desk and pulled to his feet by an imposing soldier, one who clearly had no idea who he was. "Civilians are not allowed without proper authorization!"
"I'm no civilian!" Arthur protested loudly. The nerve! He, a civilian? And yet... He had no proper authorization to give, no badge or card stating his name or identity. Bloody hell.
The soldier quickly escorted him downstairs, past a flood of incoming people. He passed a man and a woman with blond hair who were being escorted into the building. He could hear the voices outside, the whispers of "Doctor". "That must be him...!"
Well. There was only one way that Arthur would be able to be involved in all of this, he figured. Be involved with the one person they need more than anything. He shouted after their retreating forms. "Doctor! Doctor, please!"
But the Doctor didn't hear him. The security officer tossed him out and abruptly shut the door to 10 Downing Street after him. He almost flailed his limbs in helplessness, a gesture that would have been far too much like a certain pasta-loving Italian.
So unlike his normal composure, he ran to the door and banged several times. "Let me back in! I need to speak with the Doctor! DOCTOR!"
Later that night, helplessly walking the streets of London gave him another glimpse of that blue box. It faded away just as he turned a corner, taking the sound of the universe with it.