Who is the person behind Martha?

Clara Oswald (Wait! Clara Oswald?)

Martha turned to see a short brown-haired girl emerging from the shadows of the church basement. The first thing that she noticed was that she was obviously a white girl. The second thing that Martha noticed was that she was white all over; in short, she too was naked and wore nothing to obscure the sight of her bare skin save for her shoes.

"Who are you?" asked Martha.

"An old friend of the Doctor's," said the girl. "Clara Oswald."

"Hey, I don't know any Clara Oswald," said the Doctor as he too turned around to face the new girl. "And I can't honestly say that I recognize your face."

"That's because you haven't met me yet," said Clara. "But I recognize you. I'm a friend of one of your future selves and I used to see images of you in the TARDIS's video archive."

"Wait!" said Martha. "The Doctor here has a future self? More importantly, he has more than one future self?"

Neither Clara nor the Doctor bothered to answer Martha's questions. Instead the Doctor asked, "So, Clara, I'm guessing that the two of us meet in the 26th century?"

"Actually the two of us first meet in the 19th century," said Clara. "However, the majority of our adventures together take place in the 21th century. I'd tell you more but you know... Spoilers."

"Then if you're not a resident of Earth's 26th century," asked the Doctor, "then why are you dressed like Ms. Jones here? Do you have a condition like hers?"

"God, no!" said Clara with a tone that made Martha cringe. "I'm dressed this way because of her."

At that moment, another figure stepped out of the shadows. This time the figure was an older woman, fully dressed in an outfit that made her look like a refugee from a steampunk novel. This woman too was white but for some reason Martha couldn't put her finger on, she didn't seem quite human.

"Who are you?" asked the Doctor.

The woman smiled. "My name is Mis -- er, Song. Professor River Song."