Author's Notes: Once again, I am thrilled that people are enjoying this story! However, I feel the need to clarify some things about this fic.

What it is: The Doctor running around with the crew of the Normandy and changing the events of ME2/ME3.
What it is not: Sontarans and krogans having arm-wrestling contests.

What it is: The Doctor treating the Reapers as a threat and a problem he needs to solve, with the help of Shepard and everyone following her.
What it is not: The Doctor sonics a Reaper and it blows up. Or he calls UNIT and they blow it up. Or he brings in the Judoon.

What it is: A plot-driven fic with certain relationships highlighted and/or developed, whether they were explored in the game or not.
What it is not: "Shepard couldn't understand why she was drawn to this mysterious stranger. What she didn't know is that he had a thing for dangerous women... and Shepard was the most dangerous of them all."

Some of this will be clarified in the technobabble in this chapter, but it seems important to state this up front to try and avoid major disappointment later. Some of what I'm outlining is interesting ideas that don't really fit in this fic - and some of it would make the fic extremely boring. If you've liked what you've seen so far, please stick around!

2185
Normandy SR-2

Shepard entered the tech lab after seeing Samara to the observation lounge to find the Doctor and Mordin already deep in conversation.

" -independent development. Would be too similar to be coincidence, save for Reapers. Confirmed existence of Reapers and role in galaxy cycles explains much." Mordin shook his head. "Elegant explanation. Fits several theories. Can not understand denial."

"I see Tali took off," Shepard commented.

"Ah! Shepard. Good. Can get started." Mordin went to sit behind his desk so that he could start taking notes. "Begin, Doctor."

The Doctor nodded. "First, Shepard. I realize that you've already had the limits of what you know to be possible tested today with my TARDIS." Shepard saw Mordin pause and look up, mouthing the acronym as if trying to make sense of it. "This will seem completely fantastic, but I am telling you the truth. If you don't believe me, you can ask Liara."

"She said you were a colleague," Shepard said, remembering. "She trusts you?"

"Liara said that she'd melded with your mind in the past, to help process the knowledge you received about the Protheans," the Doctor said. "She did the same with me. To give me the knowledge of this universe."

Shepard blinked. "Say again?"

"This universe." The Doctor smiled. "I'm not from around here, as they say."

"Parallel universe?" Shepard thought Mordin's head would explode, he looked so excited. "Truly? Travel between universes possible?"

"Possible, yes. Advisable, maybe not," the Doctor continued.

"How?"

"Don't really know," the Doctor said. "There was a rift-"

"Back up," Shepard said. "I still don't know who you are." So help her, if he just grinned at her and said I'm the Doctor…

"I'm a Time Lord," he said. "My people are from a planet called Gallifrey. I'm not sure if the planet exists in this universe or not - my people certainly don't. Regardless, it's outside the Milky Way, and I'm aware that galactic civilization is just that: galactic. Constrained to a single galaxy.

"We're called Time Lords for a reason. For millennia, we have understood and mastered the Time Vortex, which allows us to travel anywhere in time and space. Our TARDISes are - were - my TARDIS is how I do that. Stands for Time and Relative Dimension In Space, Mordin." The salarian looked up, nodded, and went back to typing. "Basically, the interior exists in a dimension different from the exterior. Bigger on the inside, as I like to say."

"So why does it look like a blue box?" Shepard asked. She thought she might be starting to get it. Maybe.

"Well, technically, she can look like anything, but the chameleon circuit has been stuck on the police box for … ages now. I keep meaning to fix it."

"She? Does your ship also contain an AI, Doctor?" EDI asked.

The Doctor jumped a little and looked over his shoulder. "Hello, AI. Listening in again?"

"I am everywhere in this ship. However, I can refrain from making further comments if it disturbs you."

"No, go ahead." The Doctor turned back to look at Shepard and Mordin. "A TARDIS is a living ship. Alive. Though not really capable of talking like, ah, EDI here. In some cases, able to take actions on her own, although that really only happens when things go completely pear-shaped. You shouldn't worry about it. As far as you're concerned, she'll do what I tell her to."

"Why are you here?" Shepard asked.

"I was actually contacted by the Illusive Man," the Doctor replied, which startled Shepard. "And no, I don't know how he did it, but he called the TARDIS and asked me to come and help you. He directed me to Liara for a more complete explanation. I'm not sure why."

"Liara did give him my body," Shepard said slowly. "Maybe he thought she'd be inclined to help again. Though he told me she was working with the Shadow Broker and couldn't be trusted." She snorted. "I'm sure he had his own reasons for doing it. I expect I wouldn't agree with them, which might be why he didn't tell me."

"As you might have guessed from my speech back in the shuttle bay, I have no particular love for Cerberus," the Doctor continued. "If I decide to stay, Commander, it's because I think you're worth helping. Which I do. I've fought species like the Reapers before. I hope that I might be able to make a difference here."

"So how did you defeat them?"

The Doctor sighed. "With lots of running and being clever."

"That's not very helpful," Shepard said.

"No, it isn't. I need to know about the Reapers before I can defeat them," the Doctor said. "The last time I faced the Daleks… I failed. They got away. Before that, I had … help." He paused, as though collecting his thoughts. "They brought me on board their ship to gloat. My old enemy, Davros… he needed to see me fall."

"I don't think that's going to happen here," Shepard said. "The Collectors tried to kill me - did kill me - two years ago. If it wasn't for Cerberus, I'd already be dead. I don't think they're going to be so kind as to invite me onboard for milk and cookies."

"I turned their own weapon and systems against them," the Doctor said. "And there was quite a bit of prophecy. And monologuing. And a countdown. And … well, that's more detail than you need to know."

Well, that was very unhelpful. "So, is there any way we can get the TARDIS on board a Reaper?" Shepard asked, turning her focus back to what could be predicted and verified. "If we could figure out a way to destroy it from the inside, your TARDIS could get us there."

"Geth cannot be hacked. Too many programs running on platforms. Always fight it off. Assume Reapers must have similar defenses." Mordin shook his head. "More data needed. Possible, but need to know Reaper weaknesses. Right now … none."

"I think Professor Solus is right," the Doctor said. "Right now, there's no guarantee that a trip onto a Reaper wouldn't be one-way. And destroying one Reaper wouldn't be enough." He paused. "Hang on. Isn't that what you're doing for the Collectors? It was all kind of confused. I'm not sure I got that."

"We have the ability to follow the Collectors," Shepard said. "We're on the path towards finding their home world. It's not a great lead, but it's the only lead we have. We know there's a connection between the Collectors and the Reapers. We can't get to the Reapers-"

"So you go to the Collectors," the Doctor finished. "Right. Well. I will do my utmost to make sure that everyone comes back from the Collectors alive."

"But what about time travel?" Mordin insisted.

"Time travel is a fact in my universe." The Doctor began pacing. "The fabric can adapt, change, rebound. Different universe, different rules. I don't know your rules. I'd need a lot more time to figure that out. And," he said, forestalling the objection he thought Mordin was about to make, "making that time, by me hopping around, risks destabilization, and decreases my chances of getting home." He frowned. "Usually the sort of thing I'd just go off and do, but I don't fancy being stuck here permanently, and rifts were always tricky things."

"Rift- not stable?"

"I need to check the readings from the TARDIS, but it appears to be stable at its current configuration. Which means that one TARDIS can pass through without doing damage. The more travel, the less stable it becomes. General rule for rifts." He stopped and leaned back against the wall. "It doesn't appear as though the ability to travel rapidly between places is a problem. Given what your mass relay system does. It appears similar enough. Time is another issue."

"I may be able to assist with that," EDI said. "I can process information at an advanced rate. Since this is a new topic, I am not sure how effective I will be, but it may be possible for me to figure out the limits of what the universe will permit for time travel. I will need to interface with your ship."

The Doctor pulled a face. "But you're Cerberus."

"I operate within Cerberus parameters."

"Then… no. I'm sorry, EDI. You seem nice, but I already said I won't give Cerberus data on my ship."

"Very well."

"Guess I'll just have to do it myself, then," the Doctor muttered.

"If I can be of any assistance -"

"Yes, yes, I'll let you know," the Doctor said, waving a hand around.

"Mentioned your species - your planet - not in this reality," Mordin said. "Know this - how? Other differences?"

"Probably lots," the Doctor said. "I've never seen you, or a turian, or an asari before. The other times it's happened, it's been more like … alternate versions of the reality I know. Subtle changes. Sometimes having big effects, but you can usually point to one thing, or two, well, a small handful. As far as I can tell, things are completely different here. Humans are the only constant." The Doctor glanced over at Shepard. "You lot. You're everywhere."

"Opinion shared by many species," Mordin said with a nod. "Especially batarians." He paused. "Other rifts? Other universes?"

"Batarians - those are the blokes with four eyes?"

"Yes."

"Right. As far as other universes go, I'm not quite sure. It used to be that my people could stabilize things so that neither universe was in danger if a rift appeared. But I'm the last of my kind."

"Truly?" Mordin looked even more interested now.

"He's not a museum piece, Mordin," Shepard said dryly.

"Of course not." Mordin looked offended at the thought.

The Doctor sighed. "Yes. I'm the last. There was a war, between my people and the Daleks - the enemies I mentioned earlier, with similar goals to the Reapers. Well. From what I understand, the Reapers don't kill everything. They leave some organic species to flourish for the next … cycle?"

Shepard nodded. "Humans were around 50,000 years ago. Just not very advanced at that point."

"The Daleks really do want to be the only life form in the universe. And they have some ability with time travel as well - so the war wasn't just costing lives, it was tearing reality apart. Creating abominations, monsters that should never have existed." He looked down at the floor. "I ended it."

Shepard had seen that look before. "You killed them both," she said. "Destroyed your people and the Daleks alike."

"I had to."

Shepard walked over and laid a hand on the Doctor's shoulder. "I'm sure," she said gently. "You were saving the rest of the universe by doing so. It must have been a hard decision."

The Doctor looked up at her, and Shepard thought she understood him a bit better. When they'd first met, she'd thought how absurdly young he looked. Then again, Tali, who was barely an adult by quarian standards, was one of the brightest and most competent people Shepard had ever met.

She'd been wrong about the Doctor. He was old. Maybe his people were like the asari, not visibly aging the way humans did. However it was, he might be the oldest person on the ship.

"I suppose," the Doctor said, "you would know something about that."

Shepard nodded. "Not on the same scale, but … I had to choose between saving the Council's ship, and saving Alliance forces. Human forces. There are those," and her mouth twisted as she thought of that obnoxious reporter, "who say that I failed my species by saving the Destiny Ascencion. But I'm a Spectre. I can't be just a human. I have to look out for everyone."

"Also know something about that," Mordin said, then took one of his long breaths. He was going to start on the genophage, Shepard knew, and she shook her head quietly. There was time for that later. Mordin started, then nodded to Shepard in understanding.

The somber mood was broken by Mordin's attention-seeking cough. "Your ship. Possible to see?" He looked over his shoulder at a beeping instrument and reluctantly added, "Later?"

Shepard smirked as the Doctor's face lit up again. He just loves showing off, doesn't he? "Of course, Professor Solus. It would be my pleasure to have you aboard when you have time."

"I'd like you to introduce yourself to a few more people onboard, sometime," Shepard commented. "Joker's our pilot. You might be able to trade tips with him."

The Doctor pulled a face. "Flying a TARDIS isn't really like flying a normal ship."

"Then I'm sure Joker would love to hear about it, since the Normandy's not exactly normal either," Shepard said. "You saw Miranda and Jacob briefly - they're both Cerberus people, but they've been useful. Zaeed and Grunt are down in the cargo area. Zaeed's human, and Grunt's a krogan. And Dr. Chakwas would probably appreciate anything you could do for the med bay. We seem to need patching up a lot."

"Following Shepard may be hazardous to your health," Mordin put in.

"There are lots of things I do that are hazardous to my health," the Doctor said. "So, nothing new."

Shepard just shook her head. "That ship of yours - is it okay to stay in the shuttle bay? It's not going to fly out when we take the shuttle out, is it?"

"No, no, she's much too stable," the Doctor said. "Besides, I can always put her down somewhere else temporarily."

"Then … welcome to the Normandy, Doctor."

"Do you know," the Doctor said, "I'm getting the sense I might actually like it here."