A/N: This is AU and takes place directly after Tooth and Claw.
It started with just a scratch.
A tiny scratch that Rose had gotten trying to get away from the werewolf. She had turned the corner with the wolf pursuing her and narrowly missed getting her arm biten off by the enraged beast. She escaped with just a scratch from the wolf's tooth when he tried to clamp down on her arm as she swung it back behind her and she thanked God that was all she got. But almost from the time they got back to the TARDIS, Rose began feeling funny inside as if her insides were shifting around. She ignored it, thinking it was just her imagination, especially since she and the Doctor had the conversation about Queen Victoria being a werewolf.
But she began having weird dreams where she saw herself as a wolf running through the woods with a wolf pack, bringing down deer and slaughtering them. Again, she dismissed it as just a byproduct of their recent adventures. But then they landed on Uqia and set out to explore some ancient ruins there. They had ventured far from the TARDIS in their search for the ruins and had to spend the night with some locals in their huts. The men and women were segregated and Rose had to stay in a woman's hut while the Doctor slept in a men's hut across the village from her. That night, there had been a full moon and Rose woke up, feeling funny. She thought she was gonna be sick and stumbled out of the hut into the woods, thinking she had to throw up. Instead, to her horror, the moon came out from behind some clouds and she began to change into a werewolf. The change was both painful and terrifying and she tried to get as far away from the village as she could in case she got the urge to turn back and slaughter everyone.
She spent that night, roaming through the woods in her wolf form, howling at the moon and stalking small prey. She wasn't as big as the Scottish wolf had been, she was a bit sleeker than he was but she was still just as powerful. While she was the wolf, she enjoyed the freedom and exhilaration of running through the woods on all fours, her powerful paws propelling her through the forest. Her sharp eyes could spot the slightest movement, even from small vermin that scampered through the undergrowth and she had never felt more alive. But when dawn came and she changed back to her human form, she was terrified of the Doctor finding out what had happened but even more afraid of hurting him or changing him into a wolf like her. She managed to make it back to the hut, butt naked, and had to beg one of the native women to let her have one of her hide dresses. Once she had the dress then she had to lie to the Doctor and tell her that she felt like going native. To her relief, her lie was convincing and the Doctor admired her hide dress, unaware of the reason behind why she was wearing it. After eating breakfast, she and the Doctor thanked their hosts and made their way back to the TARDIS.
Since then she had been living in dread that they would land somewhere, step out and there would be a full moon. That was the danger of time travel now, a full moon that could happen anytime, anywhere that would cause Rose to lose control and change and attack the man she loved. She realized that in order to protect him, she would have to leave the TARDIS for good. So, with a heavy heart, she walked into the console room one day and saw the Doctor sitting on his jump seat, reading a book. She walked over to him and the moment he saw her, he lowered the book and smiled.
"Ah, there you are, ready for another fabulous adventure?" he asked her.
"Doctor, I've been thinking…" Rose said. "I think it's time for me to leave."
The smile fell off his face.
"What? You mean for good?" he said.
Rose nodded and he stood up.
"Why?" he said, taking her hands. "I thought you loved traveling with me."
"I do…I mean, I did. I don't anymore," Rose stammered.
"Rose…is it because I've changed?"
Rose hadn't considered that but she realized his regeneration made a great excuse for her to leave. She nodded.
"I…I don't like being around you anymore. I prefer the other Doctor," she said, trying to make it sound convincing.
"But you haven't given me a chance yet. I know it takes some getting used to after I change but if you just wait…"
"No, I want to go home. I need to get on with my life anyway," Rose said. "I'm sorry but I want to do this."
It was hard fighting the urge to hug him when she saw the forlorn look on his face. She felt like an ass doing this but she knew the Doctor loved her and she didn't want him knowing that the woman she loved was a werewolf who might turn on him at any time. She figured that there was no cure apart from death and she didn't want to spend the rest of her life in a cage. For both their sakes, she had to leave and find somewhere where she could live and not harm anyone. She could see the Doctor's anguish and practically hear his hearts breaking but before she could say anything more, he went past her to the console and without another word began inputting the coordinates for home. She turned and noticed he was now angry but she dared not say anything in case either one of them changed their minds. So she went and sat down and waited for the Doctor to take her back home.