Notes: Set post-RotF, and assumes that Mikaela is apprenticing under Ratchet.

It all started with an accident.

Energon was corrosive. Really corrosive. Not instantly-kill-you corrosive, but enough that it would, given enough time, happily eat through organic materials. Like skin.

Ratchet had warned her about it up front, as soon as he'd agreed to teach her anything about Cybertronian medicine. She'd nodded through all his stern warnings and the demonstration of just how corrosive they were talking about (a drop of circulation-grade energon ate a hole through half an inch of mystery meat from the mess in under five minutes. Coolant and hydraulic fluids and the solvents that cleaned everything up were more give-you-a-rash than melt-your-face, but they weren't anything she'd like to bathe in, either.) Mikaela had pointed out that she was used to working with toxic and corrosive fluids, that she'd use full protective gear, that she'd be careful, no problem.

Still, accidents happened.

It hadn't even been a serious accident. Half a year into her training, half a year of more hours than not of each day spent keeping up with a tireless mechanical mentor, Mikaela had been sorting spare parts. It was an arm that had seen better days, ending in a bent strut and torn wiring. She wasn't sure where the arm had come from and hadn't asked, just followed Ratchet's instructions to strip it down to its component parts and separate them according to how many hours of swearing would be required to make them work again. She'd watched Ratchet clean it before he'd set her to it. She'd watched the solvent run energon-bright, then clear again as it flushed out the circulatory lines.

Normally, that would have flushed through the periodic energon reservoirs that dotted every mech's frame. There were hundreds of them, integrated into every limb and system, providing a store of energy if the main fuel pump was compromised or the circulatory processors needed to reboot. There were three reservoirs in the arm she was working on. She'd identified them as she'd hauled off the outer armor. She'd even noticed that one appeared damaged. It wasn't leaking anything, though, so she'd thought nothing of it as she'd started disassembling everything else.

Being jostled as she tugged and pried and swore at the parts around it must have aggravated the damage. Damage which, she'd notice later, had jammed shut the valve that should have allowed the flush into the reservoir. The result was that when she pulled the reservoir free from its housing, the metal was fragile enough to split in her small human grip. Residual pressure splurted liquid over her glove, down her wrist and forearm. She'd been wearing gloves and long-sleeved overalls, of course, so she'd just sworn as she'd felt the break, thinking that the warm sensation dripping down her arm was the solvent. Then she turned her hand to look at the mess and two things happened at once: she saw the bright magenta glow of undiluted energon and felt the heat of the chemical burn starting on her wrist, where the stuff had seeped between her glove and her sleeve to find bare skin.

She'd jumped, cursed, and dropped the reservoir. She tore off the soaked glove, yanking at her overall zip so she could get her arm through the energon-contaminated sleeve and said sleeve away from her skin. This, of course, smeared energon in a line down the inside of her arm, but she was less worried about that than the now really alarming burning pain coming from her wrist.

Get it off was the only thing running through her head. Luckily, the largest body of liquid was right next to her and actually the proper thing to use: the solvent bath she'd been tossing gunked up parts into. The solvent bath itself was a welded tub about ten feet across and five feet high, with about three feet of solvent in it: easy carrying size for Ratchet but more like a small swimming pool for her. She'd reached her arm over the edge, but couldn't reach down quite far enough to dunk her whole arm.

"Ratchet!" she'd yelled, as she gripped the edge of the tub and hauled herself over. She landed awkwardly on the other side, feet sliding on something already soaking under the surface, but her grip on the side of the tub kept her from actually falling. She crouched down to dunk her arm in the solvent, the pain too much to worry about the fluid that was rapidly soaking into her overalls and her boots.

"Mikaela?" Ratchet appeared, optics spiralling in on her in confusion.

Mikaela gritted her teeth. "Energon burn," she said, holding up her wrist above fluid level. The pain wasn't getting worse now that the energon was dissolved away, but there were angry red blisters all along her wrist and trailing up her inner arm.

"Hold still," Ratchet barked. She froze first at the commanding tone, then in purely human fear of Big Things Coming At Her Really Fast as the medic's hand blocked out the overhead lights. He reached right into the bath, picking up her and the handful of parts she was standing on in a loose, careful cage of fingers. She sat down hard against his finger as he moved, jostling her, and then bringing her down fast enough to make the bottom drop out of her stomach. The lights reappeared as he gently opened his hand and pulled it back, and she blinked, just in time to see where she was (right under the water pipe that Ratchet used as a faucet) and screw her eyes shut before water pelted down on her head like a lukewarm waterfall. She gasped and scuttled back out of the worst of it, then yanked her overalls off, leaving her in her t-shirt, shorts, and very waterlogged boots.

"Make sure you rinse completely," Ratchet's voice said, over the patter of water.

She certainly wasn't going to argue, moving back under for as long as she could hold her breath. When she emerged, the skin of her legs no longer crawled, and her wrist, though painful, was manageable. She clawed her hair back out of her face one-handed and surveyed the damage: angry-red welts all along her wrist and up her arm, the sheen of rising blisters, and it hurt in that horrible way that burns always did, but all in all, not too terrible. She'd gotten worse once when she'd laid her arm against a hot tailpipe.

She looked up at Ratchet, wringing water out of her hair. "Sorry. Didn't mean to freak out on you, there."

Ratchet hmmphed, the rings of his optics shifting fitfully in a way she knew meant he was scanning her within an inch of her life. "Your response was perfectly justified. How do you feel?"

"Okay," she said, wringing water out of her shirt. "Wrist hurts, but not bad. It's just blisters. Thanks for the express lift down here, by the way."

"You are welcome. Dizziness, headache, changes in visual perception?"

"Uh. No?" She looked up at him, puzzled, as he transformed down into his vehicle mode and opened the passenger-side door for her. "You're gonna make me go to the hospital for some blisters? Seriously?"

"No," Ratchet said, "I am going to make you go to the hospital for observation for the possible toxic aftereffects of several skin-permeable compounds dissolved in that solvent you took a bath in. Get in."

Mikaela swallowed. "Oh. I hadn't thought about that. I guess that wasn't too smart, huh?" She hauled herself up into Ratchet's cab with her good hand. She squelched into his front seat, but figured that he'd asked for it.

The door closed behind her, and Ratchet blipped his siren as he pulled out of the medbay. His voice rang through the cab, surprisingly gentle. "On the contrary. Undiluted energon could have caused you serious injury in the time it would have taken you to climb off the berth and retrieve cleaner solvent. You reacted appropriately."

High praise, coming from Ratchet. The afternoon sun shone in warm through the window. It felt good on her wet skin. She rubbed her opposite arm with her good hand. "Do you really think I'm gonna get sick?"

Ratchet hmmed thoughtfully. "Unknown. Several of our fluids have no known safe human exposure limit. The bath would have significantly diluted them, and your exposure was short. My expectation is that lasting damage is unlikely, but I would rather have you under a human medical facility's care, just to be sure."

She sighed. "I really feel fine. Really."

"The effects could be delayed," Ratchet replied placidly.

She winced at the thought of showing up at the base emergency room, soaked to the skin, in an ancient American Chopper shirt and cutoffs. She caught a look at herself in the rearview mirror (still dripping, her hair tie lost somewhere, and a smudge of grease on one cheek that had somehow survived the deluge when she'd washed off.) She stared at herself in the mirror for a long moment, then snorted a laugh.

"Mikaela?"

Now that she'd started thinking about how she must have looked, jumping into the solvent bath, or Ratchet plopping her under the faucet like a muddy puppy, she couldn't quite keep the laughter down. "I'm...I'm ok, I'm fine. It's just...come on, you have to admit, it's pretty funny. I look like a drowned rat."

Ratchet paused. "I will take your word for it."

That, for some reason, only made her laugh harder.

"Are you certain you are all right? Euphoria could be a neurological side-effect of-"

Mikaela slashed her hands apart in a "no", gasping, "I'm not sick. Honest!"

"Oh, I don't believe I can trust your judgment, as you are obviously having some sort of seizure-"

"I am not!" Mikaela wasn't sure if she should be alarmed or not. It wasn't as if Ratchet didn't have a history of jumping to conclusions about human physiology.

"-and are an unreliable witness." He vocalized a sigh. "Probably neurological damage. Such a shame..."

Mikaela opened her mouth to protest and then stopped, his tone finally sinking in. Ratchet didn't always do humor well, and it was often as dry as the desert outside, but she was getting better at identifying it. "Oh, totally. I mean, really, you break one assistant, and who's gonna want to give you another one?"

Another sigh. "And I was SO hoping for a few more impertinent humans for Christmas. And an ultrasonic saw. And a pony."

She was sure that the emergency room people had wondered why she was laughing as they pulled up to the entrance.

There'd been nothing wrong with her. Her wrist and arm had hurt for awhile, but she'd wrapped it and worked around it. Ratchet had watched her more closely for several weeks. She'd gotten used to him scanning her at least once a day, but eventually things had returned to normal, and she'd almost forgotten about the incident completely.

Months later, though, when Ratchet told her he couldn't teach her any more, she was absolutely sure that he hadn't.