The weary and tired Doctor just manages to get to the TARDIS console. The ordeal against the Cybermen had taken its toll on the poor old man's body. The Doctor knew what was about to happen and he was scared. He had never done this before. He had only heard various things about this process and the Time Lords made it very clear that something, anything, can go wrong. As he tried to move his hands to operate the TARDIS, the console controls started moving on their own. The TARDIS knew that her thief was tired and was trying to assist him through this ordeal in any way she could.
Suddenly there was pounding on the door of the TARDIS. The Doctor could hear Ben and Polly's panicked voices and he silently cursed himself. How could he have forgotten his own companions? He moved as quickly as he could, his legs, no, his entire body, feeling like they were filled with molten metal. Nevertheless, he kept moving toward the door. He could not, in good conscious, leave his companions behind, no matter how terrible or tired he felt.
With tremendous amount of effort that almost rivaled the will of God Himself, The Doctor managed to open the door and let Ben and Polly in and quickly closed the door again, just as the TARDIS dematerialized into the Time Vortex with that old, strangled wheezing sound.
"Good gracious, what was I thinking, leaving the brakes on?" thought The Doctor.
Polly quickly realized how ill The Doctor looked. "What happened to you, Doctor?" she asked, worriedly.
"Oh, I'm not sure, my dear." lied The Doctor. "Best not to worry them." he thought, in his delirium. "Perhaps from an outside influence. This restless, old body of mine is wearing a bit thin."
"What do you mean 'wearing a bit thin'?" she asked, now more worried than ever.
"Oh, don't worry, child. Don't worry, don't worry." said The Doctor, feeling more tired than ever.
He then moved back to the console unit, leaving his companions with confused expressions on their faces. Something was wrong. As far as they knew, The Doctor had never acted this tired before. Never.
He moved his hands over the console, feeling the TARDIS with his old hands for the last time. At least with these hands. The TARDIS was still flying herself, so her precious thief could rest during the process. He then lifted his hand up to his head feeling dizzier and dizzier. He could not take it anymore.
He collapsed.
"DOCTOR!" yelled Polly, as she rushed to the old man.
"What's happened?" she asked, fear filling her very soul.
The Doctor did not reply. "So this is what it is like to die." he thought. He immediately felt extreme sympathy for all of the other beings of the universe who did not have this amazing ability the Time Lords were gifted with. Suddenly, a blindingly bright light engulfed the old man's body. It remained like that for a few moments before it slowly dimmed and then faded out of existence.
Before Polly and Ben stood The Doctor, fully healed.
But at the same time, it wasn't The Doctor. At least, not their Doctor.
Before them was a completely different man. This man had darkish, long, rumpled hair and had small bags under his eyes, along with a slightly pointed nose. Though noticeably younger, he was still an old man.
Then suddenly, The Doctor's eyes opened. They had not changed one bit. They were still a bright, piercing blue.
He raises his head with his hand clutched to it. He quickly groaned as he got up. Then suddenly, he chuckled for a little bit.
"It's over." The Second Doctor said with his new voice.
Thus ended the life of The First Doctor, and the life of The Second Doctor began.
