Rose was never fantastic at maths. She had always preferred what she had called life sciences and her teacher had called 'meaningless chatter and useless social interaction that was not conducive to study', one of the reasons she had ended up leaving with no A levels. Still, she had a grasp of the basics, and it wasn't hard to see that the current equation wasn't balanced.
Jenny was the main figure, the detached one, the one they couldn't divide by. She was the x, the sole pronumeral in a field of numerals. She was the only full Time Lord, the only one who knew how to handle the ship (which frequently required more percussive maintenance than the TARDIS ever had), and if the x was factored out, they'd find themselves standard.
And on board was Rose and Martha and the Doctor, and 1 + 1 + 1 = 3, and three wasn't a good number in close quarters. 1 + 1 = 2 made sense, and if that equation was to be true, then someone had to be 0.
She had accepted the fact that she and the Doctor would never be together again. She didn't want to any more - it wasn't her Doctor. It wasn't the Doctor at all. And it was Martha who had helped him, Martha that had healed him, and Rose watched the two of them with something approaching jealousy.
She knew now how Mickey had felt when it was the three of them on the TARDIS. Martha had effectively been in the new Doctor's life longer than she ever had, and now they had their own stories, their own injokes.
They never did it consciously, which was perhaps worse than them knowing how Rose felt, being on the outside and looking in. She wished she could apologise to Mickey. And she still talked to Martha, and she still looked after the Doctor, and somewhere, their little pronumeral noticed.
"So," Jenny asked, settling back in her captain's seat, "What's the matter?"
Rose glanced over at the girl from her own seat (when the crew had increased in size, so too had the seating, which was why an overstuffed armchair, a striped deck chair and a beanbag were now secured to the floor) and smiled wryly. "Who said anything was the matter?"
"Because you've been staring at Dad and Martha like they did something wrong all week," Jenny said matter-of-factly.
Blinking, Rose sighed and swung her legs over the side of the armchair, settling back and staring at the ceiling. "It's nothing," she said, sounding only slightly uncertain, "It's just... it's stupid."
Jenny patted a foot. "I bet it isn't," she informed Rose, "Is it because you feel like they're ignoring you?"
Rose sat up abruptly. "What makes you think that?" she frowned, studying the girl. Jenny looked the picture of calm, relaxed, comfortable, and in control - of her ship and of her life. She deflated. "Yeah," she confessed, "I'm feeling left out."
"Who's feeling left out?" Martha said cheerfully as she headed in to the small cockpit, giving Jenny and Rose a cheery grin.
"No one," Rose said abruptly, rising from her seat and brushing past the other woman on her way out. She heard Martha call her name, and although it hurt in the pit of her stomach, she ignored her and ignored it and went straight for her bunk.
She didn't quite close the hatch on it, and the conversation rising from the cockpit was still fairly audible. Rose couldn't quite help but listen in.
"And we're so different," Martha was sighing, "We've experienced such different things in life. In comparison, I've seen nothing. I'm just a passanger."
The irony didn't escape Rose. She had been talking to Jenny because the Doctor and Martha were too close, and now it sounded like Martha was looking for advice on how to get closer.
"You're not a passanger," Jenny scolded, "You were never just a passanger. You're my crew, as much as the Doctor and Rose are. In this ship, you're on equal footing."
A sigh. "But there's still all that history - and the Doctor and Rose are so close. I don't know if I could get between that. There's still all that history together."
No, an angry part of Rose's mind said, You can't, he's mine! But she knew it wasn't true any more. It never was. (But if she knew that, then why did she still get a pang of jealousy - cold, hard, right in the guts - every time Martha looked at the Doctor?)
She missed Jenny's answer - she could imagine the girl leaning forward, conspiring, "There's no problem. Dad doesn't love Rose any more. Go ahead!"
She was probably close to the mark - Martha's next comment was a faintly scandalised, "Jenny!", and a brief smile touched Rose's face. But Martha continued - "But yes, I'm sure. I don't think I've been so sure of anything in my entire life."
Rose sighed, tuning out of the conversation and rolling on to her back. Studying the top of her bunk, she tried to imagine life on board the ship. Jenny on her own, the x, the pronumeral. The Doctor and Martha, Martha and the Doctor. 1 + 1 = 2. And that made her the zero, the outside factor. Could she be a part of the crew when she was so much on the outside?
2 times x was 2x, a perfect equation. 2x times 0 was zero, and she wondered how accurately to their date of departure Jenny could steer the ship. Suddenly she desperately wanted to see her Mum and Dad and brother.
"Then go!" Jenny was saying in a louder than normal voice, "You humans, you don't say anything to the ones you like, and then you wonder why they don't return your feelings!"
"It's not exactly your traditional boy-meets-girl love story!"
"So? Why don't you just tell?"
"I will!" Martha exclaimed, and Rose sighed, blowing stray strands of hair off her face.
There was a knock at the side of her bunk.
Rose sat up, undoing the latch curiously and revealing Martha's sheepish-looking face. "Rose, can I talk to you?" she asked hesitantly, not quite meeting her eyes.
Rose nodded sullenly, sliding back to allow Martha room. It was polite of her, she supposed, to ask her opinion first before telling the Doctor. Considerate. But Martha usually was - Rose wasn't sure whether she liked her or not because of it. "What do you want?" she asked in what was hopefully not too sulky-sounding a tone.
Martha looked down, studying her fingers. "Well," she finally said, "I've been doing some thinking -"
"I know," Rose said quickly, "I heard you talking to Jenny."
This time, Martha looked up, startled. She looked wary and hopeful at the same time, scanning Rose's face for any sign of approval or disapproval. "And... you don't mind?" she breathed, knotting her fingers together.
Rose shook her head and sighed. "Nah, it's okay. Me and the Doctor - well, he's not really the Doctor I knew, so there's nothing there any more - he's just... someone I help, I guess, he's... well, a friend, I think. But no, it's okay."
Martha nodded slowly, biting on her lip. "I had... kind of hoped so," she admitted, "And I hope that doesn't sound awful, but I saw the two of you together and I just felt jealous, and it was wrong - I really had to think about what I was about to do."
"Well," Rose shrugged, "Sounds like you've made your mind up, yeah?" She nodded in the direction of the cockpit.
"Yeah," Martha said quietly, "I have." The smile she gave Rose was tentative, and Rose suddenly found herself unable to look away from the curve of her lips, the darkness of her eyes. She swallowed roughly, eyes not leaving Martha. Suddenly, her jealousy made sense, and now she was about to lose them both. It's only gratitude because she saved your life, some stubborn part of her brain insisted, It's nothing. Nothing!
"I think you better go," Rose said quietly, "Go to him."
Martha's brow furrowed. "Why?"
What? Rose frowned. "The Doctor - you love him, don't you?"
Eyes wide, Martha stared at Rose. "Rose, what - no - I wasn't talking about the Doctor!" she exclaimed, "For one thing, he's still my patient, and it'd be a massive breach of the rules if we were to be in a relationship! And second," she continued, "He's not the one I'm attracted to."
Suddenly, Rose's muscles were tense. Her palms were clammy, hands shaking slightly. "Then who?" she whispered, half dreading, half wanting the answer.
"You," Martha breathed, "Rose, I was talking about you."
Rose caught her breath. For once, she had absolutely no idea what to say - 'I thought so!' sounded a bit cocky, 'I want you too, let's snog' might have scared her off, and, '...Oh,' sounded just a bit too flippant. "Um," she said.
Martha stifled a somewhat hysterical-sounding giggle. "Rose," she said in a rush, "Before I lose my nerve entirely, can I kiss you?"
"Yeah," Rose whispered, and leaned forwards to meet Martha halfway.
1 + 1 + 1 + x = 3 + x. We'll be alright.