Clara had known for an hour or two now that this was going to happen. The Doctor had explained earlier that he would regenerate today, and that it would be his last. Or that he wouldn't regenerate and he would just die. Yeah, the second one. That was until he (well, his face) had his last battle with the Daleks and proclaimed "NOBODY TELLS THE DOCTOR WHAT TO DO."
After the battle was over, Clara walked to the TARDIS, hung up the phone hanging on the front door, and walked in. No one was there. The Doctor's clothes were strung on the floor, and a bowl of fish sticks and custard was on the right side of the control panel. Clara looked through the TARDIS, but that's all she could find. She started to walk down the flight of the stairs to go find The Doctor, but she heard something. Less of something and more of someone.
Footsteps. It was The Doctor. Clara Smiled. "Doctor!" Clara said happily. "Hello." The Doctor replied weakly.
"You're young again," Clara said, "you're okay! You didn't even change your face." The Doctor replied with a laugh. "It's started. I can't stop it now, this is just the reset. A whole new regeneration cycle, whoo!" He took a sip out of the bowl of fish sticks and custard. "It's just taking a bit longer. Just breaking it in."
He moved over to the left side of the control panel, where the switches and levers are. He grabbed one of the levers but fell forward, causing the lever to move back more than it should have. He gasped in pain. "It all just disappears, doesn't it, everything you are, gone in a moment like… breath on a mirror. Any moment now… he's a-coming." Clara started to tear up.
"Who's coming?" She asked. "The Doctor." He responded. "You. You are The Doctor."
"Yep. And I always will be." He sounded even weaker when he said that. But to make matters worse, his hand started glowing; he was regenerating. "But times change, and so must I."
Clara turned her head, ready to cry. Then, The Doctor started to hallucinate. Amy was there. "Amelia!" He exclaimed.
"Who's Amelia?" Clara asked. The hallucination of the younger Amy was running in circles around the TARDIS, now covered in pictures drawn by her. "The first face this face saw." The Doctor answered.
"We all change. When you think about it, we're all different people all through our lives. And that's okay, that's good, you've got to keep moving, as long as you remember all the people that you used to be. I will not forget one line of this, not one day, I swear. I will always remember… when The Doctor was me." The older Amy Pond then came down the steps (still a hallucination). She smiles. "Raggedy man." She whispers.
She cups her hand on The Doctor's face. He does the same. "Goodnight." She says. The Doctor stops hallucinating and weakly removes his trademark bowtie, and it falls to the ground.
The Doctor smiles for a second, but then winces in pain. "No, oh no!" Clara exclaims, on the verge of tears. She goes over to grab him. "Hey." Those would be his last words with that face and body. "Please, don't change."
Sadly, the inevitable happens. In a flash of light, The Doctor changes. But this time, there was something wrong. His clothes are too big, he was smaller, he was… younger.
"Wait a second, something's wrong. I'm pretty sure I'm," he feels his face; hands still covered by his suit's sleeves, feeling minor acne, "a teenager?" He asked confused and feeling scared. Clara felt the same, looking at the smaller, much younger, and much more different Doctor. "Well, I'm gonna need some new clOTHes. Oh bloody hell, not the voice cracks."
Immediately after he said that, the TARDIS started to crash. "Hey Clara, do you remember if I ever told you how to fly this?" The New Doctor asked. Clara responded with a confused stare.