Part 1.a: Two of everything

Ever since Donna Noble started travelling with the Doctor she inwardly waited for a prove of his alienness. Of course there was the TARDIS – nothing could be more alien than this ship – but in the end it was just a prove for the old girl's alienness.

Donna kept waiting for such a prove from the Doctor himself, not consciously, but continuously. None appeared and slowly she came to terms with seeing the Doctor as a very smart, very weird human who had somehow grabbed hold of all those alien tools. She wasn't very disappointed about it, maybe she was even relieved. If he had turned out to be the Martian as which she kept addressing him just for the fun of it, it might had crept her out.

And then all that changed. It was a day when they had been running and the Doctor had been talking, a day like every other, that introduced Donna to how alien her Martian really was.

The high-pitched noise of laser guns being fired bounced off the walls around the Doctor and Donna. They were running through the narrow corridors of some kind of mouldy dungeon in a galaxy far away from home.

"Run!" the Doctor shouted unnecessarily over his shoulder. Donna did her best to keep up with him.

"I am!" she replied angrily and completely out of breath and then: "We need a better plan than that."

"Run faster!" he proposed. Just when he'd said that he suddenly stopped, his sneakers were sliding over the slick ground and he had some trouble with keeping his balance, but finally he stood completely still and looked to the right side.

Donna reached him seconds later and all but crashed into him. The Doctor caught her and pushed her into a very tiny niche Donna wouldn't have recognized if not for the Doctor. He followed her close behind and pressed Donna against the greasy wall.

She pulled a face, disgusted by the feeling against her clothes and skin.

"Sorry. Shush!" He whispered firmly before the sound of footsteps arose. The Doctor pressed himself even closer against Donna and hence pressed Donna closer against the wall. She twitched her mouth nauseated, but kept quiet.

The group of guards dashed past them and Donna held her breath. Moments later, that felt like ages, the group was gone and the sound of their feet faded away. Their luck that the Doctor had been right. Those aliens had been living in the dungeons for centuries and were all but blind; additionally the smell around here drowned every other hint of their existence.

Donna waited a solid minute after the noises had vanished before she asked: "Are they gone?"

The Doctor waited another second before he replied whispering: "S'pose so, yes."

They exchanged a relieved smile and when the Doctor didn't made any further attempts to move Donna frowned.

"Err, Spaceman..."

"Yeah?"

"Would you shift your arse?" The slick mould felt moist against her skin and she was sure it had started to seep through her top. Donna pressed her hands against the Doctor's chest to make him move and pulled back like she'd burned her hands.

The Doctor awkwardly took a step back before he recognized the shocked expression on his companion's face.

"What's wrong?" he wanted to know.

"Your-" Donna started, not sure how to say it and just pointed at his chest and then looked down at her palms. "Two," she managed to say before turned her hands for the Doctor to see.

A frown appeared on his forehead and he looked down at himself.

"What?" he asked, puzzled.

"Two!" Donna repeated, her tone nearly accusing as if he had done this on purpose. "There were two... things beating in your chest!"

"Those two are called hearts, thank you very much," he replied slightly offended.

"Hearts," Donna said, emphasizing the 's'. "How come you've got two of them?!"

"I didn't steal one, if that's what you're afraid of," the Doctor mocked. Then he realised that Donna wasn't in the mood for jokes and simply shrugged. "I've been born with two – All Time Lords are," he told her matter-of-factly.

"Why?" Donna asked with a squeaky voice.

"What do you mean 'why'? It's quite useful, so why not?"

Donna didn't say anything to that. She looked at her palms for a moment and then wrapped her arms around herself. The distant sound of more footsteps reminded them of where they were and the Doctor and Donna crept back to the TARDIS without having to hide again.

The blue box was safely trundling through the vortex, far away from any slick dungeons and the Doctor and Donna were sitting in the kitchen over two cups of steaming tea.

"Do you want to talk about the... hearts-thing-y?" the Doctor carefully asked. He hadn't expected that this awareness would startle Donna like that.

Donna shook her head, hesitated and nodded then. Finally she shrugged.

"You could have told me before," she complained.

"Sorry, I'm quite used to them, so I tend to forget arranging caution labels." Gladly the Doctor accepted Donna swatting at his arm, it meant she could deal with his jokes again.

"And you've always had two?" Donna asked.

"Yup," he popped the 'p'.

"Blimey! Doesn't that drive you insane? I mean... are they even beating at the same time?"

The Time Lord chuckled at Donna's newly discovered curiosity for his anatomy. He usually was the explorer more than the explored subject, but this conversation was more enjoyable than he'd expected.

Deciding that learning was still best done by doing, the Doctor stood up and walked around the table, offering Donna his chest to find out herself.

She frowned suspiciously at him at first, but then she really wanted to know and raised her hands to his chest. Carefully she rested her palms against the Doctor's shirt. She felt the soft throbbing against her hands. Not quite at the same time but in an oddly harmonious way, they beat in time.

"Blimey," she repeated and when she recognized she was still touching him, she pulled her hands away.

The Doctor gave her his enthusiastic smile he always wore when he could teach her something new so Donna decided this was the best time to ask.

"What about pulse? Would a doctor - a real doctor, mind you - know you've two hearts when he feels your pulse?"

"I am a real doctor!" he retorted offended. "And it doesn't need a medic, like me, to feel it."

Petulantly he offered her his wrist to try. Donna grinned and reached out, taking his pulse with her index and middle finger like she'd seen in crime movies.

And really, she could feel two pulses.

"What if you lose one?" she suddenly wanted to know. "Does it kill you? Do you need both?"

"I need both," the Doctor replied calmly. "But it doesn't directly kill me to lose one. It weakens me, though and it's not a pleasant feeling."

After all this new information, Donna sat back in her seat and the two friends to a long sip from their tea.

"Do you feel better about them now?" the Doctor finally wanted to know. He still saw the shocked look in Donna's eyes when he thought about the dungeons.

"Of course, dumbo! I'm not that easily scared. It just came as a surprise," she replied. She didn't want to admit that it had shaken her image of the very human-like Doctor and she had been scared, no matter what she said. Now that she knew more about it, she could accept it.

All this must have been written on her face because the Doctor suddenly decided he could go a little further. Confidently he pointed out: "I don't get why everyone's so interested in the hearts all the time."

"Do you mean that's not everything you've got two of?" Donna wondered.

The Doctor waggled his eyebrows suggestively at her and Donna felt a flush creeping onto her face. No! He could impossibly mean he had two... Oh, she didn't even want to think about that possibility!... Would that imply Time Ladies also had two...

"Oh, you Space-pervert!" she exclaimed and hit his arm. "I could have lived without that information!" She hit him again and an expression of confusion and agony appeared on his face.

"What's wrong with having two livers?!" he wanted to know and tried to escape another of Donna's blows. The ginger hesitated and gaped at him, unbelieving.

"Livers? If you were talking about those, what was that smug look for?"

"Having two livers is pretty great," he justified and the look reappeared on his face, telling Donna he really had been talking about them.

"W-what did you thought I meant?" he asked, sounding rather sheepish.

"Never mind it," Donna replied shortly.

"But, you-"

"Never. Mind. It. Doctor."

They exchanged an embarrassed look and both had to stifle a grin. Donna took a deep breath then and announced: "I'm just glad that we could sort all that out. No nasty surprises anymore."

"You think that's everything?" the Doctor asked amused. "Time Lords may look like humans but there are countless differences. Organs are just the tip of the iceberg."

Donna gave him an intimidated look.

"What else is there, Martian?"

He put an even smugger look on.

"You'll find out, Miss Noble. Sooner or later."