The doctor woke up one morning, he thought, after feeling unusually tired the day before. Sleepily, he walked out to the control room, where Clara was waiting. As soon as she saw him, she looked frightened.
"Who are you and what have you done with the Doctor?" she demanded.
"I am the Doctor! Look at me! We've been traveling for so long now and you don't even know me?"
"No, I don't. If you are the Doctor, then I suggest you look in a mirror."
The Doctor complied and went to the bathroom, which had a full length mirror in it. He couldn't even believe his eyes. He was in his 10th form. "No, no, no, no!" he almost screamed, "not yet!" He quickly calmed himself and began to prepare a lie for Clara so she wouldn't get too terrified. He walked into the control room and told her simply that he was a little under the weather and he was degenerating a little, a common symptom. She believed it.
The Doctor then began to take Clara to many different planets, and just letting her wander about because he made sure they were safe. He stayed on the TARDIS for those excursions, tinkering with the device he used to turn himself human once before.
Clara boarded the TARDIS once again after visiting the planet of the snails and asked, "Why are you not trying to save the day, as normal? And why are you never getting off the TARDIS anymore?"
"I just don't feel like it," he felt a pop in the device and it began to glow a shimmery gold, "I did it! Finally!"
"Did what?"
"Nothing…"
"Tell me right now or I'll never speak to you again."
"Ok fine," the Doctor gave in, "The truth is, I'm dying," Clara looked annoyed, knowing that he could regenerate, "It's not like that! All Time Lords are unique, and some end up dying very early for an unknown reason. There's no cure, and I am one of those Time Lords that will die from it. No regeneration."
Clara looked horrified and crushed at the same time, "But you're the last full Time Lord. The species will go extinct without you."
"You don't think I know that!" the Doctor said, letting his temper from his 9th regeneration get the best of him, "That's what I've been doing in the TARDIS all this time, and I think I've found a way to save the species. The only problem is that I need a test subject."
"What does it do?"
"It creates Time Lords and Ladies out of people."
"Isn't that what Cybermen do to make their species?! What are you thinking? It's sick even for you!" Clara retaliated.
"I promise you that it isn't. People that do this will keep all of their memories from before, and they will only gain a second heart and the ability to regenerate, as well as heightened intelligence and connections with the time vortex. It's as safe as I could possibly make it. Could you please just help me?" he begged.
Clara eventually agreed and stepped over to the machine. The Doctor put a helmet on her and threw a switch, which caused her mind and heart to race. When she thought she couldn't take it anymore, all went black.