Disclaimer: I don't own Doctor Who on account of the fact that I'd probably screw it up real bad if I did.
Martha woke suddenly at the cry from the Doctor's room. She stepped into her slippers and pulled on a sweater; even in the summer the TARDIS was refreshingly cool. Quietly, in case he was still asleep, Martha tiptoed across the hall and peeked into the half-open door of the Doctor's room. Through the darkness, she could see him sitting up in his bed, sheets strewn about the room and his face and chest covered in sweat. His eyes were wide and full of tears, and he jumped when he saw Martha in his door.
"Can I come in?"
He nodded blankly, and Martha flipped on the lights and sat down on the edge of the Doctor's bed, putting her sweater around his cold shoulders.
"Thanks," he mumbled, looking down at his sweat-soaked hands.
"Bad dream?" asked Martha, putting her hand on his back. "I heard you say…" She was half-afraid to say the name. It always put him in an odd mood, although now that she thought about it, Martha couldn't imagine him any more odd than this.
But she didn't have to say it; the Doctor did. "Rose."
He was quiet for a moment. Then he shook his head and chuckled a bit. "Amazing girl. Really. You would've loved to have met her…"
Martha stayed silent, not sure what to say. She examined the Doctor's face, a mixture of love, regret, and nostalgia that made the friend she thought she knew so well seem entirely foreign to her.
The doctor turned suddenly to face her. "You remind me of her sometimes. So inquisitive, very bright, both of you." He returned to his dream-like silence, lost in the past, present, and future with Rose Tyler.
Something in the Doctor's last statement irked Martha. Normally, she felt that jealousy was a silly emotion, but this was justified.
"You mentioned her, when we first met." The doctor nodded. "You told me I wasn't replacing her." Martha paused. "But am I? Well, obviously our relationship is nothing like what you had with her, bud do you just keep me around because I remind you of her? So you can pretend like she's still here? So it's like a swap? 'Oops, there goes that Rose, let's pick up somebody just like her!' Is that how it is, Doctor? Is that what I am to you?"
Martha had pulled her hand away from the Doctor and her eyes stung with tears. All her energy was being consumed by trying not to break down.
The Doctor turned around and looked Martha in the eye. He took her hand in his and leaned in close to her, shaking his head slightly. "I did not lie to you. You have not replaced Rose, nor are you so akin to her that I would use you as a substitute. You, Martha Jones, are your own person, and you have no idea how happy that makes me. Rose Tyler was my companion, my friend. I loved her, I lost her. You are my friend – Not as Rose was, but as only you can be – and I love you for that." He paused. "I'm not going to lie to you; I'm not going to say I'll never lose you. But I promise that we'll make the best of what time we do have together."
The tears broke through Martha's eyes, and the Doctor brushed them away, placing his hand on Martha's cheek and kissing her on the forehead. "I won't make the same mistake I did last time. I love you, Martha Jones, and I need you to know that. Because maybe love lasts forever, but relationships end, and not always perfectly. I learned that the hard way, and I'm sorry you had to suffer for it."
The Doctor held out his arms and Martha fell into them, wrapping her arms around his neck. "I love you," she whispered, hugging him tight. He let her go. "Back to bed with you," he instructed with a manic grin, back to his usual perky self. "We've got a busy day tomorrow."
Martha crawled back into her bed, wondering who else had occupied it in the past and if they, too, had fallen in love with the Doctor. She sent them her prayers, proud to have followed in their footsteps, and fell asleep.