The bell rang. A sign of freedom. There was the initial happiness, the first breath of fresh air. Rose Tyler said goodbye to her friends, and walked on home.
The Doctor watched her in the shadows, not daring to approach. This young girl didn't deserve to know. He was here to see her face. If only for a moment, if only from a distance.
She didn't need to know that he was sad because Martha had left and he was alone again. She didn't need to know that he was also relieved, because all he wanted to do was drown himself in grief over a past and future of losing this yellow haired teenage girl with a schoolbag slung over one shoulder.
He had been overcome with a sudden desire to see her. But now that he was here, the loneliness was stronger than ever. The happiness on her innocent face was more like a knife in his chest than the comfort he had expected.
She'll see him again. He was in her future, though she didn't know it yet.
But the Doctor's future with Rose was impossible.
He returned to the TARDIS, his face hardened and his hearts sinking in his chest. He knew better than to believe that this feeling of alone would ever disappear.