A/N: So because of my major Deep Breath feels, I had to write a fanfic. Just to make this clear, I KNOW THIS IS NOT COMPLETELY ACCURATE because of the whole regeneration cycle thing. I do think that 11 explained regeneration to Clara at some point, but obviously he wouldn't explain it this way because he knows he would die instead of regenerate. It was just more feels-y and fun to write it this way.
Also, I don't actually ship Whouffle, but I really love 11 and Clara's friendship and think it's really adorable.
Clara panted hard as the Doctor slammed the TARDIS doors shut behind her. Her whole body ached from running, and all she could do was watch while the Doctor pounced upon the TARDIS console and readied the ship for takeoff.
"That didn't go like I thought it would," the Doctor commented, pumping some sort of lever on the console.
Clara took a deep breath in order to speak. "Pretty much sums you up, doesn't it?"
"Oi!" He shot her a look of annoyance. "I had it all under control."
"Yeah, until you started flirting with the Queen!"
The TARDIS groaned and whined its usual noise, throwing Clara into the railing as it took off. The Doctor approached her with his brow furrowed in his "I'm always right and you're wrong" superior look.
"I wasn't flirting," he retorted, talking with his hands. "I was gaining information!"
Clara righted herself and stood toe-to-toe with him, her arms crossed over her chest. "You flirt all the time."
"Do not."
"Do too!"
He copied her posture. "I'm not flirting now, am I?"
Looking down, Clara considered his statement and how deep inside of her it hurt, just a little. She shrugged as she met his eyes. "I'm different."
The Doctor didn't smile. Instead, he stated in a matter-of-fact manner, "Of course you're different. You wouldn't be here if you weren't different."
The TARDIS rocked and Clara grabbed on to the Doctor's shoulders to steady herself. When they locked gazes again, something had changed in the Doctor's eyes. He seemed sad and reflective, his mind a million miles away.
"Doctor?" Clara asked, releasing her grip on him and taking a step backwards.
"Clara." He focused straight into her eyes and inclined his head toward her. "What would you do if I changed?"
The question hung in the air for several silent moments. The TARDIS engines sounded very loud all of the sudden. Clara wondered at the question and what he meant by it.
"What do you mean?"
"What if I was different, completely different? New face, new voice, new person; everything?"
Clara was worried by the earnestness in his eyes. "Well then you wouldn't be you any more, would you?"
"That's just it." The Doctor closed the distance between them. "What if I was entirely different, but I was still me?"
Clara tried to imagine a different Doctor, a man with a different face and characteristics, but found she couldn't. Her Doctor was so familiar that it was impossible to picture him any other way.
"Why are you asking me this?" she wondered instead, deflecting the harder question until she could come up with an answer.
He backed up a few steps, his expression somber. "It's called regeneration. The way the time lords…" He paused, as if looking for the right words, and then half-smiled. "Cheat death."
Death. He thought he was going to die?
"But-"
His gaze was trained on the floor. "If something were to happen to me," here he met her eyes, "and something very well might, instead of dying I would change. I would become a different person."
The Doctor's words were beginning to scare Clara. Why was he talking about dying? And how could he change into someone else?
She began to feel a little breathless as an emptiness settled itself into her gut. "Why are you talking about dying?"
"Because," he said quickly, adding the next part in a gentler tone, "everything has its time, and everything dies." He met her gaze with a resigned stare. "Including me."
"But you can't!" Clara blurted out as she imagined a life without the Doctor. She grabbed his arm as if she could keep him from disappearing and leaving her forever. "You're the Doctor! You can't."
His eyes suddenly turned very soft and earnest as he stepped forward and placed both of his hands on the sides of her face. She couldn't look at him now that she had considered the possibility of losing him.
"I can."
Clara felt tears sting her eyes. Really, she thought to herself, it was ridiculous that she was getting so upset. But this was her Doctor, talking about dying and regenerating.
"Clara."
Her name brought her eyes back to his. He looked at her imploringly.
"That's why I need to know. What would you do if I changed?"
Clara Oswald stood looking through blurry vision at a man who was so different than the Doctor she thought she knew. His grey hair, old and weary face, and hard eyes were the complete opposites of her Doctor.
Will you help me?
His question rang in her mind, as did another from a conversation between her and the Doctor from what seemed like long ago.
What would you do if I changed?
Clara took a step forward. The Doctor eyed her strangely as she leaned to one side and then the other, examining his wrinkled face. There was no trace of the Doctor anywhere on him.
But in the middle of that busy street she listened, and could just faintly hear the sound of two heartbeats pounding rapidly. Racing.
The man you are with right now, the man I hope you are with, believe me, is more scared than anything you can imagine right now…
Clara felt his gaze following her.
Just see me.
She met his gaze and saw those same ancient eyes that had seen thousands of years pass by. They were hard on the outside, but deeper down she could see the suppressed fear.
What would you do if I changed?
"Thank you," she pushed out past the lump in her throat.
His thick eyebrows furrowed in confusion. "For what?"
"Phoning." She threw arms around him in a hug because she finally realized what her Doctor had been trying to tell her: it was still him.