Traveling with humans brought problems, he had known that before; they had no idea of habits, traditions or dangers of the whole wide universe.
Therefore they were traveling with him. Him, the Doctor, the Time Lord from Gallifrey who was over 900 years old, had seen almost the entire universe, encountered numerous species and people of history. He let them travel with him because he still believed that he was able to look after them.
Problem was, his companions all had one thing in common: they had real problems with simply listening to him and his advice. Like "Don't touch any flowers or other plants." or "Don't take any gifts from strangers - you could be married within seconds." Or, the one he most often gave: "Don't drink or eat anything on other planets without asking me first."
Of course it was also the most ignored advice.
Even Rose, his lovely, clever, far-too-brave Rose Tyler still hadn't learnt to listen. Or maybe she had just become a bit too careless after traveling with him for such a long time.
Whatever it was. Rose really had done it. She had drunk something she shouldn't have drunk. And now she was invisible.
It was a strange thing. Rose could touch things - though living beings didn't feel her touch - but not be touched. Of course she wasn't visible, so it wouldn't have been very useful had she been touchable - when someone is invisible he or she shouldn't be found by poking around in the air. They hadn't tested if she still could eat things - they weren't yet back on the TARDIS and the Doctor had forbidden her to eat or drink anything else from the planet - and they really hoped it wasn't so bad that she couldn't because she would possibly starve before she was back to normal.
Which was another problem - the people on this planet weren't exactly helpful. No one cared. No one talked to them. They - or more the Doctor - could ask around and they would ignore him, like he wasn't actually there. Like he was as invisible as Rose.
The only good thing was, he could still hear her. Her voice, that was, not her footsteps, not even her breathing. Only her voice. Which was also a bit spooky because he really had never any idea where she was until she said something - and that happened mostly when she complained and lamented. But then he just rolled his eyes at her whenever she did it; meanwhile, he didn't even need to say "I told you not to..." anymore. He would look at her - at the direction her voice was coming from - and she would shut up.
He had tried to give her anything she could carry around so he would have an idea where she was. But everything she carried for a longer period of time somewhere on her body would eventually fall from her like it had lost grip.
They gave up asking around on the planet after a while. They hadn't even succeeded in getting a bit of the stuff Rose had drunk before she had become invisible. Defeated, they returned to the TARDIS, hoping that she or her laboratory would be able to do some wonders. But even the ship that had seen universes couldn't help. And the Doctor began to lose his hope.
"I really would like to hug you now," Rose sighed, in an attempt to give him some comfort.
"Me too," he murmured, barely audible.
What frightened her was that he was walking around almost the entire time with a sad expression on his face. The smile she loved so much was just gone. She had asked him, Why are you so sad?, and he had answered that he wasn't sad, but frustrated that he couldn't solve the problem.
But in fact, he was sad. Sad that he couldn't see her smile and her eyes. That he couldn't see those eyes sparkle with this happy ever-present sparkle.
In the beginning, it had only been there whenever he brought her to a new place and time. Now it was always there; always when she was with him. And he was also sad that he couldn't see her bouncing and running around, or sitting in the pilot's chair, reading or just watching him.
The days stretched terribly, into weeks, into months. They didn't know how long exactly it was because after they were back on the TARDIS, they lost their sense of time - even the Time Lord. Probably it felt much longer than it was, but in their hearts, it felt like the number of hours increased exponentially.
They parked the ship on some unnamed, empty planet and the Doctor kept searching for the possible cause for Rose's invisibility. He might have lost hope, but he was not about to give up. He studied all books in the TARDIS' library that contained information about the planet they had been at and about potions that made invisible and the cures, but nothing they found and tried helped.
She got used to watching, observing him. She watched him when he was working, when they ate something - luckily, she still could eat - when he was in the library and read just another book; she even watched him when he was sleeping. It was nice for a change to have the chance to watch him without him realizing it.
And because he mostly had no idea at all where she was, especially not when he needed her to be quiet, one thing she did was to at some day start to sit at his bed every time he went to rest a bit. He looked peaceful when he slept; he didn't sleep much, but when he did, it was already relaxing to just watch him.
And then, sometime, she lay down beside him. From then on, she would lie beside him every time he went to bed. As soon as he would have fallen asleep she would cover his hand with hers and kiss him softly, on the cheek, on the temple, on the forehead, even on the mouth. It probably wasn't more than a light breeze of air against his skin, and yet he smiled everytime, even in his unconscious state, like he could feel the touch. She never said a word and she was sure he had no clue she was there.
One day, one time when he had just laid down and was watching the seemingly empty air beside him, thinking about his Rose and how much he missed seeing her, that her being invisible nearly was like her not being on the ship anymore, and how much she meant to him, his heart made his mouth form the words "I love you, Rose".
Rose was, now as usual, lying beside him; unknown to him directly in his line of sight, so it was for her like he was declaring his love while looking at her, even though his eyes didn't focus on her. Touched by his words, words she had longed to hear for half an eternity, she answered, "I love you too, Doctor," and though it was said in a very low voice, without any intention to let him hear it, since she was sure his words hadn't been meant for her to hear, he sat up, a mixture of happiness, relief and shock on his face, and looked down at her.
He looked at her.
It took them both a few moments to realize it; that he was looking directly at her and that he could see her. Rose never got the chance to sat up herself, because he fell down onto her and wound his arms around her, hugging her so close that it almost felt like he was going to crush her body. But she couldn't have cared less. She hugged him back, holding onto him for dear life, and they savored the feeling of finally being able to feel each other again.
And when he finally released her from his embrace, he took his face in her hands, caressed it gently, and then he kissed her, slowly, softly, and then more passionate, kissed her with all the love he felt for her, and all the happiness his body and soul contained after having her back to normal. He kissed her and she wished that she would never again need to breathe, because the feeling of his lips caressing hers was indescribable and she didn't want to lose it anymore.
When they parted nevertheless and looked at each other, it was then that realization hit them - realization about what had them reunited, and that it had been the two most powerful things in the universe, more powerful than any cure or magic could ever be:
Truth and Love.