"I have found our most desirable location, sir," said Captain Iskander of the Fourth Aguacorpian Battle Fleet, his aqueous scales rippling with pride. His commander looked at him doubtfully.
"Go on."
"As we discussed in our previous committee meeting, our next target planet will be Earth. I was tasked with finding a suitable location for the invasion, which, given the complexity of the Level Four civilisation inhabiting the body, was no mean-"
"We know all this," hissed Captain Pilli, rolling her eyes derisively. "Move on, Iskander."
He returned her glare. "As I was saying, each Earth country is inhabited by a separate civilisation. My first concern was one of armament. It would be wise to launch our primary attack on a civilisation where access to dangerous weaponry is fairly limited. This rules out many major countries such as this body here-" He gestured to the American Continent on a holographic model of Earth, which immediately faded from verdantly green to a dull grey. "- or any of these smaller civilisations."
"We don't want to take over some helpless backwater, Iskander."
"Thank you, Captain Pilli, for raising the matter. My next step was to eliminate all contenders below the peak level of human civilisation, Level Four, as it happens, countries with little or no access to natural resources, or areas with low populations, as we will need labour once we land." Several places turned grey, predominantly large patches at both poles. "The third consideration was climate. Many Earth countries are hugely volatile, and huge civilisations are found at geographical fault lines. These, of course, could not be considered options." Australasia, all of Africa, and much of Europe, faded to grey.
"This elimination process seems extremely time-consuming, Iskander, cut to the chase." This time, to Iskander's delight, Pilli's comments received a few irritated glances from those who were interested by Iskander's lecture.
"Finally, there is the matter of size. Given the distances we must travel, and the ever-present danger of avoiding Earth's protector, we would be better off using less resources on the actual takeover, and so taking a smaller prize. This lead me to Britain." The hologram highlighted a small island in North-western Europe and zoomed in.
"The British Isles. A small collection of islands, cut off from the mainland continent. A bursting population, but still plenty of open space suitable for farming. A technologically advanced race, but heavy firearms laws making the general public unable to resist hostile takeover. A mild climate, and very few natural disasters. English is a massively widely-spoken language, so citizens will make excellent, ah, ambassadors for us. Furthermore, their governmental, royal, military, and economical strongholds are all firmly rooted in and around one city."
"So where do we land, Captain?" asked the Commander coldly.
"London."
There was a heavy silence, and the Doctor emerged from the shadows behind Iskander.
"Well, that explain things," he said, grinning. "That one's been puzzling me for a while."
The young woman beside him heaved a sigh of relief. "Oh, thank God, there's a reason," she said. "I was beginning to this it was just a kind of cosmic tumour."
The aliens looked at her for a few minutes until the Commander stood, pointing a scaled green finger at the two.
"Apprehend them!" he commanded. The Doctor grinned, the wild, free joy of the chase singing in his nerves.
"And now we run," he said, his coat flapping behind him as he left the aliens in the dust, his companion hot on his heels.
At least that was one more mystery solved.
A/N:
I do not own Dr Who. I own the poor-quality alien OCs. As for which Doctor this features, I'm picturing Ten, and Donna, but make your own minds up. S'long!