A/N: Ep-fic of Uncertainty Principle, because there's some classic scenes between Don and Charlie that needed to be addressed in this verse. Also, because this ep has some of my favourite lines.

Hmm, so that's two votes for a Charlie/Edgerton, which I must admit is my first love… see 'Sniping Equations' and 'Parallax Method' for my slash-writings of those two. Though a triangle with both Colby and Edgerton could be promising… I think I know where I'm going to be taking this. *evil grin*

Melissa: Thanks for the constructive feedback, it is muchly appreciated. And I agree with the clearance comment. It probably wasn't made clear, but in my verse, Charlie's clearance works that she is able to be read into pretty much any case if she goes through the proper channels, not that she can wander anywhere she feels like. But most of the misconception probably comes from that line in 'Vector', when Don says that she doesn't need a visitor's pass to visit his office.

Also, the line about the dead and the wounded comes from the novel 'Dragon' by Steven Brust, and is an assassin's recollections of a battlefield. I just loved the idea of Charlie being so upset by the bloodshed that she mixed up the living and the dead.

Anyway, on with the story.

Chapter Three: Nothing Is Certain. In which Charlie panics, and Don fights to keep her attached to reality.

-BD-

Charlie dished out the pie while Larry moved about the kitchen finding cutlery. It didn't distract either of them from their conversation, and neither did the television on in one corner of the kitchen.

"Well, wait," she said, thinking aloud, "let's clarify. This is the red-headed post-doc? Red-headed and he plays baseball?"

"Yeah, yeah," Larry nodded, smiling a little. "Really brilliant theoretician." He'd known that Charlie would jump at the chance – if a man had a sharp mind and fit body, she was putty. Then he noticed what was on the television, and froze.

"Well forget the undergrads, I'll help him!" Charlie's grin faded when she saw the horror on Larry's face. She spun to look at the TV when she heard the word 'shoot-out', and felt her heart drop.

"Don…" She ran for the car. She needed to find Don.

-BD-

Smoke was all around her, from the car bomb. Everywhere she looked there was chaos. Dead people were moaning and writhing on the ground, and the wounded were lying still, and she was fairly sure it was meant to be the other way around, but she could feel her logical mind struggling to take everything in.

A man in a uniform tried to block her path, but a word from Terry was enough to let her through. Part of her wished that the man had been more adamant, and that she could run back home to the safety of the garage – and that part was growing louder by the second. But she had to see Don first.

At her feet, a bullet lay in a pool of blood, and Charlie knew that the image would stay with her in her nightmares. She only became aware of Terry beside her after the agent called her name thrice.

"Where's Don?" It sounded as though her voice was coming from miles away, and Terry looked at her for a moment.

"He's over there," she indicated one of the ambulances, and Charlie's heart fell again. Don was hurt?

Terry quickly ushered her to where an EMT was putting a bandage on Don's arm. He looked more confused than anything else.

"Hey Charlie, how you doing?" If she hadn't felt so guilty, Charlie would have laughed. That was such a… such a Don statement. He was the one with a bullet wound, and was worried about Charlie, who hadn't even been there.

"It's okay, really," Don assured her after seeing the look on her face. "It's fine." He seemed concerned about something, so Charlie hastened to explain.

"They weren't violent, there were two of them," she hated how desperate she sounded. "Why sixteen robberies exactly the same, and then this?"

"Well first of all, nobody ever tried to stop 'em, right?" Don asked Terry. "And it wasn't just two guys, y'know, they had four backing 'em up."

"Yeah," Terry agreed. "We had no way of knowing until we confronted them."

"You're good to go," the EMT finally told Don, and he stood up quickly. First stretching a little, he then pulled Charlie into a brief hug, before holding her at arm's length.

"Nothing to Dad, okay?" He tried to look her in the eye, but she was avoiding his gaze. "Charlie. Don't say anything to Dad, I'll take care of it. Okay? You heard me, right?"

Charlie nodded, but the truth was that she was struggling to hear anything over the roar of numbers in her head. She needed to get back to the garage.

-BD-

Don, however, had insisted that she come back to the FBI office. He seemed reluctant to let her too far out of his sight at the moment. So she sat in the war-room, staring at the board with her equations written on it, trying to see where she'd gone wrong.

"Charlie? Did you hear what I said?" Don's voice was low and soft, as though speaking to a frightened child. He'd seen the look in her eyes before, and it scared him. She was close to losing it.

Add to that the fact that she'd been sitting silent ad perfectly still for the last half hour, and he was beginning to freak out. Charlie was never silent for this long.

"I'm just, ah… thinking," she whispered, before turning to face him – though she kept looking back at the boards. "Understand that this is an entirely new problem now. The pattern I was working from was a false pattern."

"No," Terry shook her head. "No, not so much false as incomplete. You were able to accurately predict today's robbery."

"Right, but there's new factors now. Okay? They aren't two unarmed guys, so the assumptions we made about them are invalid."

"Okay, all right," Don tried to calm his little sister. "All right, look, so you take the new factors and you make a new equation. Right?"

"It's not that easy," Charlie muttered as she flung herself up from the chair and started moving. Don had thought the stillness was bad, but now her agitated movements seemed even worse. "It's not that easy because, ah, there's something else that has to be considered."

"Like what?" Don decided to keep her talking. At least she wasn't shutting down on them yet.

"Heisenberg's uncertainty principle," Charlie said, sounding defeated. "Heisenberg noted that the, ah… that the act of observation will affect the observed, in other words when we watch something we change it… and ah… for example, like an electron. You know, you can't really measure it without bumping into it in some small way." Her pacing and gesturing were unnerving to watch. She was really worked up about this. "Any physical observation requires interaction in some way with a form of energy, like light."

Charlie was standing on the other side of the transparent board, with the equations seemingly written over her. Don didn't care much for the image. She kept talking about electrons, though Don didn't understand it.

"Wait, wait, hold on, you know I got, like a 'C' in physics," Don tried to lighten the mood a little. Tried and failed. "Just take me through how this relates to the case."

"Don," Charlie finally came out from behind the board and approached him again. "You've observed the robbers, they know it, that will change their actions."

"Okay, so they change their MO," Terry spoke and Don turned to look at her. He'd almost forgotten that she was there, he was so focussed on Charlie. "But we don't have to go back to square one. We know some things about them, we have a basis for making some conclusions."

"Yeah, right, good, but I, ah…" Charlie looked at Don with those large, expressive eyes, and he saw the fear. Fear of what, he couldn't exactly say, but she was definitely terrified of something. "I can't help. I can't do this." She turned and started trying to erase the equation, but Don was quickly at her side, holding her wrists gently.

"Charlie, listen to me. What you saw was bad. Trust me, I know. So I want you to go home and get some rest, get a little sleep, then maybe you can work on this when you're feeling better. Okay?" Charlie lifted the eraser again, but Don confiscated it with a small smile. "Go home, Charlie. And call me if you come up with something."

"Call me if you need to talk," Terry murmured as Charlie left, leaving the two agents to look at each other.

"She didn't look too good," Terry commented, and Don shook his head.

"I knew she shouldn't have gotten into this. It's too much for her."

Terry watched as Don went back to his desk. It was clear to see how much he loved his little sister, but Terry couldn't help thinking that he was a little over-protective. Charlie was a grown woman, after all, even if she had been sheltered most of her life.

-BD-

Night had fallen, and Don was worried. Both Alan and Larry had tried to talk Charlie out of the garage, and both had failed. He hated doing this, but he needed to bring her back to work on the bank case.

"Charlie, look, you helped us find these guys once before, you can do it again."

"Why, so you can get shot again?" Charlie muttered as she hung another blackboard. Don was taken aback at the venom in her voice.

"No! Baby-doll, look, I appreciate that you care about me, but it's not gonna happen!"

"Statistically," Charlie said, not looking at him, "you're dead now. Do you understand what that means? A man aimed a gun at your head and fired." A shiver went down Don's spine at that. He had tried not to think about it too much – but Charlie obviously had. "The fact that you survived is an anomaly, and it's unlikely to be the outcome of a second such encounter." Her voice was close to breaking, and Don couldn't stand it. He gently pulled her away from the board and wrapped his arms around her, trying to calm her a little before he continued.

"Listen to me. I'm still here. And we need your help. We don't have many leads. If you can help us predict when and if these guys are gonna hit another bank… this is the only shot we've got." Don's voice was soft and pleading. He never used that voice on anyone except Charlie, but now not even that seemed to be working.

"Please… understand…" Charlie broke away, still alarmingly close to tears. She went back to writing on the board, even though her hand was shaking so badly she could hardly hold the chalk. "Sometimes… I can't choose what I work on. I can't follow through on a line of thought just because I want to, or… or because it's needed! I have to work on what's in my head, and right now… this is what's in my head."

Don stood, watching as Charlie seemed to shut down and become little more than an equation-writing machine, and felt the same cold anger as the last time they'd been through this. But this time, the anger was mostly directed at himself – if he'd been more careful with what Charlie worked on, maybe they wouldn't be doing this now.

Charlie didn't seem to notice, but Don sat in the garage, watching over her until she finally collapsed on the old couch and slept. He gently covered her with an old blanket, then returned to his chair and resumed watching. When Charlie got like this, she could turn into a danger to herself without even realising it.

-BD-

Terry stood, watching as Don made his way to the koi pond in an attempt to reason with his little sister. Alan offered her a cup of coffee, smiling slightly. He liked Terry.

"You know," he started, "Don and Charlie graduated high school on the same day."

"Thank you," Terry took the coffee, barely taking her eyes off of Don. "Don's mentioned it. A few times."

"Kind of puts an edge on that sibling rivalry thing, y'know? Even being different genders, they still somehow felt that they had to compete."

"Still do sometimes," Terry commented, and Alan nodded. "But having a kid like Charlie had to put some unusual pressure on the family. How old was she? When you first realised she was exceptional, I mean."

Alan paused. Both of his children had always been exceptional, at least to him. But he knew what she meant.

"She was three… when she multiplied four-digit numbers in her head." Terry glanced at him, surprised, before turning back to the window. "By the age of four, she needed special teachers, special classes. My wife… I mean, their mother and I… we put a lot of time into her education." Her more so than me, Alan admitted to himself. Charlie's mind had always intimidated him, even though he loved her dearly.

"It was Don," he murmured, "that had to get used to taking care of himself."

"Well, he might have gotten used to it," Terry commented, "but I'm not sure he's as good at it as he thinks he is."

"Well," Alan sighed, "it's hard for him to ask anyone for help. And it's especially hard for him to ask Charlie."

-BD-

Out by the pond, Don was at his wit's end.

"I mean, we think they have special military training, and now they've struck again and killed another innocent person, Charlie," Don tried. Charlie just moved a little, to find a better position to observe the koi. "Charlie, please don't do this," he was out-right pleading now.

"Do you understand the stakes we're dealing with here?" He demanded, starting to get angry. "More bloodshed? People's lives? I mean, if I have to do this on my own, it's gonna put me and all my team at greater risk! Is that what you want?"

"The old pattern's gone," Charlie finally spoke softly, though she refused to meet his eyes. "I told you."

"Well then there's gotta be a new pattern, doesn't there?" Don shouted as he grabbed Charlie's arm, ignoring her flinch. "Doesn't there? Isn't that how it works? You incorporate whatever's going on!"

Charlie yanked her arm away from him and stumbled backwards. The back of her legs hit the bench and she sat heavily, looking up at Don. She finally met his gaze, and he was kicking himself, because now he could see just how badly he'd frightened her.

"I'm sorry," he whispered as he stepped back, running a hand through his hair in frustration. "I'm sorry… I… I need your help. You understand?"

Charlie looked down at the ground.

"I don't think I… I…" she couldn't seem to finish the sentence.

"Charlie, look, I know it's hard for you, I understand that, but it's hard for… for everybody!" The anger was coming back as she gave him a look that quite plainly said 'no, you don't understand'.

"You know," Don started angrily, "I don't know how I got into the situation where I need your help to do my job, but I sure as Hell have!"

He turned and walked away, furious again. How on earth did Charlie manage to do this to him? What should have been a simple conversation had turned into an emotional rollercoaster, as he swung between guilt, anger, concern and frustration.

"I wish you'd just snap out of your precious bubble for once!" He shouted. Damn, she really made him mad sometimes.

-BD-

Charlie took a deep breath as she hung her visitor's pass around her neck. She had to do this, had to help fix things. Pausing in the doorway to the war-room, she met Terry and David's gazes with a nod. She was back to work.

Don's expression was almost comical in its shock. He obviously hadn't been expecting her to show up at the office any time soon, and Charlie managed a tiny smile. He'd been right – Charlie had been living in a bubble.

Now the bubble had burst, and it was time to face the world.

-BD-

That evening, Don sat at the table with his father and sister, sipping from a bear bottle and helping himself to a good dinner. He was reassuring Alan that everything had gone well.

"Yeah, Dad, we arrested every suspect, and only one shot was fired. And it wasn't at me, which was a bonus."

"Really? How'd you pull that off?"

"Well, we knew roughly where they'd try to hit the next shipment, and I knew they'd have an escape plan."

"Very clever," Alan smiled at his son.

"Yeah, I guess I was inspired by Heisenberg, just like Charlie here suggested." Don grinned at his sister, who smiled back.

"Heisenberg?" Alan looked confused. "You mean the physicist?"

"Yeah," Don nodded. Putting down his sandwich, Alan levelled a look at his daughter.

"Your brother goes into a confrontation with heavily armed felons, and you prepare him with a lecture on the movement of sub-atomic particles."

Charlie paused for a moment, then nodded.

"Yep." She gave him a cheeky grin. "It worked, didn't it?"

"…Yes. I guess it did." He smiled fondly at both of his children. "I'm telling you, if your mother could see you two now, she would be… so happy."

Don and Charlie looked at each other, knowing that it was true. Sure, they had their ups and downs, but they came through it okay in the end. Don reached across to ruffle Charlie's hair, and she rolled her eyes at him.

Alan laughed at the interaction. Ah, children. Evenings like this made all the heartaches worth it.