It was in the taxi back to Pete's mansion that she found it. She placed her hands in the pockets of her blue jacket and her fingers hit something metal. The sinking of her heart told her what it was before she took it out for examination. She stared at the key in her palm, almost seeing through it. Her throat still ached dully from all her earlier crying, she was passed that point now. Her grief had deepened, was burrowing down inside her soul; she had wept herself dry.
It was surreal, this journey into the unknown. Pete and Mickey sat beside one another, quietly talking like old friends. Mum remained in that sombre silence, respectful of her daughter's loss, but Rose knew that she was secretly happy. Daughter home safe and sound, and a new rich version of her lost husband to boot. But for Rose it wasn't safety, it was a trap. In the back of her mind somewhere she thought of Sarah Jane who'd warned her of the heartbreak that was to come. Yet another person she could never contact again.
The Doctor's world was fading with each mile they drove, the only evidence of its reality remained in her hand, a small silver yale key.
She took to wearing it on a chain around her neck, telling the folks at Torchwood that the energy in it from the Tardis seemed to sense alien activity. A lie of course, and a bald one at that, but no one ever pressed her on the subject. It made her feel like he was there, at least in some form, pushing her to keep fighting when she was afraid, when the doubts were enough to overwhelm her. It developed from a symbol of mourning to one of determination as soon as the idea of a dimension cannon entered her head. She'd need it one day, she'd unlock the doors of the Tardis once again if her life depended on it.
The night after flying Earth back home she showed it to him. They were both so disorientated from the battle with Davros, followed so quickly by returning back home. He was out of sorts she knew, she wasn't much better herself; but there was something that never failed to calm her. She reached into her pocket where it had been stored safely all day and pulled it out.
His eyes narrowed the slightest bit.
'Is that-?'
She nodded and smiled, holding it out to him.
He took it carefully in his hands, taking a moment to study it.
'You kept it, all this time?'
'Well I'd hardly throw it away, a thing like that.'
He turned the key over in his hand.
'Besides, I thought I might need it.'
He turned to her then, his eyes downcast.
'I'm sorry, I know you wanted to come back...properly.'
She shook her head. 'Nah, it's alright. I'm just sorry you got stuck here.'
'Yeah well, stuck here with you, that's not so bad,' he replied.
She beamed. 'You should keep that. I know it's nowhere near as good as the actual Tardis, but it's a part of it.'
He gazed at the key once again. 'Are you sure?'
She nodded. 'Yeah, I think it's done all it can for me. It's your turn now.'
He pondered this. 'Tell you what, we can share. It'll be a small step towards this mortgage business,' he replied.
The key became a family heirloom, passing from daughter to mother, father to son all through the next generations of Tylers for centuries to come. As time passed, the stories and rumours changed, though it continued to be treated with reverence by the human Doctor and Rose's descendants, even if half of them couldn't fathom what was so special about a dirty old yale key.