Not much prepares you for wrapping your hand around 5500 kelvin of pure, searing heat. Certainly nothing had prepared Erin Shepard. Two years of rigorous Alliance training, over a decade of live combat experience, hundreds if not thousands of hostiles eliminated, titles, medals , honorifics through the roof, a dead Reaper, the first temporary trip to the spirit world in human history, and more gunshots and bullet holes and life-or-death situations than any sane person would care to remember… yet nothing in her vast repertoire of life experiences had hurt quite as much as handling a live heat sink.

The flesh on her palm was charred and raw now, with a solid bar of hardened blood running through the center, tracing the shape of the cylindrical heat sink. Every second or so, it sent a pulse of blazing pain across her skin. Like getting shot nonstop by an automatic weapon. It had been that way for most of the 20-hour period since her return.

After killing the Praetorian, Shepard and her team had stumbled the last few hundred meters to the shuttle waiting to take them back to the Normandy. And thank God for endorphins, because she'd been in a haze during the whole trip, barely even aware that her hand had been seared to a level somewhere between medium well and well done. It wasn't until later, when she was walking down the halls of the crew deck, still in mid-daze and with Garrus and Tali and Doctor Chakwas and other open-mouthed faces crowding around her, that she really started to feel it.

After that, well… not even a solid kilo of medigel had been enough.

Now her hand was wrapped under layers of bandages. Doctor Chakwas had cut away a few chunks of skin that were never going to heal, but for the most part, all Shepard needed to do was let her body repair itself. Her hand still hurt -- a lot. But the pain was slowly dying down. As she leaned against the far wall of the elevator, feeling the metal shake its way through its mechanical descent, she raised her hand to her face and slowly wiggled her fingers. The movement hurt, but it came easily enough. She was glad. She'd been worried that she'd never be able to aim a sniper rifle again.

Aim. Not like she'd done any of that during the Collector mission. Her mind was still replaying their encounters, straining to figure out why she'd been so off target. After so many years, aiming and firing a sniper rifle was second nature to her, like clapping her hands or typing on a keyboard. She relied on the guarantee that she never missed, fired with 100% accuracy. Shepard's Constant. An integral part of the equations that always ran in her head, and if it was wrong… everything got thrown off.

She'd made quite a few stupid mistakes on the Collector ship. Mistakes she could not afford to make again.

"Err… Commander?"

And once again, she had become so wrapped up in her swirling thoughts that she'd failed to notice what was directly in front of her. The elevator had reached its destination -- engineering. The doors were wide open. And a confused-looking quarian was watching her stare blankly into space.

"Sorry," Shepard said, giving her quarian friend a warm smile. Even with everything that had happened, she was legitimately glad to see Tali. "I'm just a bit distracted. I haven't performed that badly on a mission since… well, since Alliance Training."

Tali promptly started shaking her head. "Badly, Shepard? You killed a Praetorian with a heat sink. You bailed Garrus out of a very tight spot, from what I hear."

"Which he was only in because of bad shooting on my part," Shepard said quickly. She raised an open palm to the quarian. "Look, I appreciate what you're trying to do, but I don't need postgame analysis. It was a bad run, and you and I both know it."

"Fair enough, Shepard. I'm honestly a little relieved to know you're not perfect."

Perfect! Oh, what a riot! "Far from it."

She followed Tali through the engineering section's central station, giving Daniels and Donnelly a collective quick hello. Then the quarian led her down a few short, crisscrossing catwalks, their footsteps sending long echoes down the deck's tight spaces. After about a minute, Tali stopped. She knelt down in front of a section of wall that had been removed with surgical precision, revealing all the sleek metal machinery behind it.

"Is this what you wanted to show me, Tali?"

"Yes." Tali tapped the floor next to her, gesturing for Shepard to crouch. The Spectre did so. She began studying all the tubes and gears and instruments that sat tightly packed within. Most of the machinery was running hot, lit up with bright, flickering displays and gauges. But one device, a large metallic sphere about a meter in diameter, had its readouts all dark.

"It's a fairly simple problem, but I wanted to make sure you knew about it before I did anything." The quarian pointed a finger at the darkened sphere. "See this?"

"I'm guessing that's the problem?"

"Exactly. This is a cold fusion cell, one of six installed in various locations across the ship. This is what powers our engines…"

Shepard was hearing her, but, well… she wasn't really hearing her. The Spectre's mind was elsewhere. She was back on the Collector ship, her body planted square behind a thin sliver of cover. Harbinger's body swirled with white light. She popped out of cover and shot it once with her sniper rifle, then five times with her pistol. The creature began the long and arduous process of dying. Everything fine so far.

Then Garrus's voice hit her ear. "Shields down!" He sounded so panicked, so… scared. Her heart froze mid-beat. Her mind ran silent -- only one calculation, repeating itself over and over again. Garrus needs you. And you need him. Protect. Protect. PROTECT!

That was when it had all gone to hell…

"Err… Commander?" Tali always sounded so unwilling to interrupt, to cause any sort of inconvenience. Even with her commander staring blankly at into empty space as she explained the problem with the fusion cell, the quarian was the very picture of courtesy. "Are you okay?"

"Damn it, sorry, Tali." Shepard shook her head. "You lost me again."

"Maybe I should just skip the tech speak…"

But she wanted to hear it. "No, no, I'm sorry. Explain it to me. I'll listen. " People often forgot that Erin Shepard had an engineering degree from the Alliance Academy. Sure, she ended up becoming infantry -- and she was certainly no Tali -- but Erin knew her way around advanced machinery well enough.

Of course Tali obliged her. "Well, the cold fusion cell takes material -- liquid hydrogen, mainly, since that's the lightest -- and slaps it with a high-speed wave of free electrons. This eliminates the atoms' repulsive charge and forces them together. They fuse into deuterium. And that creates massive amounts of energy."

"Which powers the ship," Erin said, remembering many long lectures at the Academy.

"Exactly. Huge returns on very little investment." Tali's eyes drifted to the darkened sphere, and the quarian brushed its metal surface lightly. "But this one stopped responding earlier today."

Erin felt a dull pain of frustration in the pit of her stomach. Really? Malfunctioning equipment was the last thing they needed right now. "So we need a new one, I'm guessing. Omega's not too far. Would you be able to find one of these there?"

"I'm sure I could, but…" Tali seemed to hesitate. "I don't know if that's necessary, Shepard. I've seen this sort of thing before."

"What, malfunctioning fusion cells?"

"Fusion cells, coolant systems, intake valves… all sorts of things, breaking for what seems like no reason. It used to happen all the time in the fleet." Tali let herself fall into a sitting position and curled her legs close. "Take this fusion cell, for example. It's Devlon make. Model EPC-64. It's a very good fusion cell -- one of the best, even. Say what you want about Cerberus, but they spare no expense on their operations. There's no reason for it to break down like this."

Erin wasn't sure she followed. "Maybe it's a defective piece?"

"Maybe, but I don't think so. I had EDI run some diagnostics before I called you. She couldn't find anything mechanically wrong with it. From what I can see, this very capable fusion cell stopped working for no reason at all."

Nope, Erin definitely did not follow. "So… what does that mean? Can you fix it?"

What happened next was… curious, to say the least. Erin Shepard could see very little of Tali's face through the helmet, and only fuzzily at that. But at that moment, Tali's eyes seemed to brighten in a way she'd seen only three times before. Once on the first Normandy, when the quarian had been talking about her people. Once after the battle with Saren, as she said her final goodbyes before she left to return to the fleet. And on Haestrom, when she realized that the geth were gone, the Collosus was dead... and Commander Shepard had saved the day once again. Yes, Erin knew that look. The Spectre was absolutely certain that her quarian friend was grinning.

"Sometimes, Commander, a paragon of a machine begins to falter," Tali said. "Not because there's anything wrong with it. But because it's not complete."

All those years of Alliance training, all that live combat experience, all those hostiles she'd dispatched and all those bullet wounds she'd taken… that was just barely enough to keep Erin Shepard from reeling back at the implications of Tali's words.

She was incomplete. That was the answer. She suddenly knew why she'd been so distracted, why her aim was so off its mark, why she couldn't shut off all the rushing thoughts in her head. It all had a single root cause.

Garrus Vakarian. The final, inevitable answer to Shepard's Constant.

And you didn't need a calculator to figure it out.

When Erin's mind cleared enough to really think about it, she stared at Tali'Zorah for half an eternity. Did the quarian have… any idea what she'd just said? How swiftly and… and violently she'd redrawn Erin's perspective? She had to. This couldn't have been an accident. But holy shit, those naïve quarian eyes! No. Nobody could have stumbled into such a moment all by accident. Tali had to have known what she was doing. She must have picked up on some subtle cue, analyzed some little statement the Spectre had made and realized the significance behind it. Erin often forgot how well the quarian knew her. As well as anyone, really.

Anyone except for Garrus.

With all her concentration focused on Garrus since finding him on Omega, she'd forgotten that there was another worthy of her trust. A certain quarian down in engineering. "Tali…" she said in a very low voice. "Do you have any idea what you just did?"

The quarian perked her head ever so slightly. "What do you mean, Commander? I was about to suggest that we get a Valco-Mendofsky model fuel injector for the fusion cell. If we feed it liquid hydrogen directly, I think it will work just as well as it did before."

No. The quarian was lying. She knew very well what she'd said, and now she was just playing innocent. Well, Erin Shepard wasn't about to fall for it. "I don't know if you realize it, Tali. But you've told me exactly what I needed to hear." Erin lightly placed her good hand on Tali's shoulder and stared straight into her quarian friend's eyes. "I hope I can return the favor one day."

Then she was gone. Every neuron fixed on a single root destination.

Once Shepard had left the engineering deck, EDI's spherical form appeared over one of the nearby console pads, soaking the hallway with a soft blue glow. "Tali'Zorah, I do not understand," the AI said. "There is nothing wrong with any of the Normandy's cold fusion cells. They are all working at optimal capacity."

The quarian grinned smugly to herself. "Not what this was about."