Despite taking as many precautions as possible, Doc doesn't quite anticipate the full strength of the reaction. He carefully drops the lump of calcium into the beaker of solution, and backs the hell up when the whole thing splatters on him "Damnit."
At another worktable, Marty's observing something fizzing away. He pauses his stopwatch and looks up sharply when he hears Doc swear, knowing something must've gone wrong. He calls out, "Doc, you alright? What happened?" By the time he gets up to see what the hell's going on, Doc's already on his way to the bathroom.
"Nothing to worry about. I just got a bigger reaction than expected," Doc says casually, like getting partially covered in potentially hazardous materials is a regular occurrence. Not for the first time, Marty wonders about some of the stuff Doc gets into when he's not there to "assist". "I need to clean up before this stuff burns me," he shouts from the bathroom. "Could you get me a clean shirt while I'm in here?"
"Yeah, no problem!" Marty heads straight for Doc's dresser and rummages around the drawers a bit before grabbing a plain gray t-shirt. He clutches it nervously in one fist while waiting for Doc to finish. Accidents like chemical spills come with the territory of scientific experimentation, Marty knows this, and if Doc says it's no big deal, well. Marty has little reason not to believe him.
The thing about it, though, is that Doc's just so good at what he does (50+ years of experience probably has something to do with that) that little incidents like this tend not to happen all that much. No, it's usually something bigger, like that time with the microwave and the dish soap-vinegar concoction (Marty swore he could smell burning metal for over a week after that). When something goes sideways in the lab, he's so ready for it to be something major that even minor things like this make him flinch and fret a little. At this point, Marty figures he's always going to worry about that old man (and he's surprisingly okay with that).
Doc comes out topless with a towel draped over his shoulders, still drying himself off one-handed. He's got his old shirt and his lab-coat bunched up in the other hand. "A trip through the wash should take care of this," he says. "It's a good thing we weren't working with anything stronger, eh?" When he looks up, he's taken aback by how intensely Marty's staring at him. "Marty?"
Marty would love to say something, anything, to assure Doc he hasn't gone mute, but he's currently too distracted by all the thin, spidery lines branching across Doc's upper body. They seem to start from his wrists and trail up his arms and across his shoulders and upper chest, like pale brown fractals. That they look a lot like lightning bolts makes something uncomfortable, almost like guilt, prickle at the base of his skull.
When Marty's voice comes back to him, he very pointedly asks, "The hell are those?!"
"What?" Doc looks down at himself, worried the contact with the solution did more damage than he thought. "I'm not sure what you're talking about, I made sure to check myself for any chemical burns earlier--"
"No, Doc, I'm talking about those..." He flounders for a bit, searching for the right word, "...lines all over you! How the hell'd that happen? When the hell'd that happen?"
Doc looks at him with confusion all over his face. He's about to question Marty's suddenly poor memory when a thought occurs to him. "Marty," he starts slowly. "You've seen me with my shirt off before, and I explained what the scars were. Do you remember that one experiment a few years back, when I got all that fuel on me and had to hose off?"
If anything, Marty seems even more confused now. "Of course I do! It took days before this place stopped smelling like a gas station. Doc, I'll probably remember that one until the day I die, but I did not see anything like that on you."
Doc stares at him for a few moments before looking away. He sighs, shakes his head, then huffs out a short laugh. "It appears you're not the one with a memory problem here," he says mostly to himself. He heads off for the washer and dryer, searching for any other things that may need laundering. "Incredible! I invent a fully-functional time machine, manage to travel to the future and back in it, but I can't seem to remember basic principles of causality." He tosses his shirt, lab-coat, towel, and the few other things he found into the machine and preps it for a cycle. "Absolutely incredible!"
Still standing in the middle of the garage holding Doc's shirt, Marty has no idea what to think. Doc can tell as much on his way back, and decides to relieve his friend's profound confusion. "They're Lichtenberg figures, Marty. At least, the remnants of them. They're typically caused by a rupturing of the capillaries beneath the skin when an individual is struck by lightning. Shirt, please." He holds out his hand, gesturing for the gray t-shirt Marty's still holding on to. Marty hands it to him, still dazed.
"Thank you." He pulls it on over his head, then continues his explanation. "I received the injury on November 5, 1955. The night I sent you back to the future. I assume nothing like this happened in the original timeline, correct? That would explain why you don't remember seeing the scars; that version of me didn't have them."
Marty shakes his head slowly. "No, nothing like that happened originally. This is the first time I'm seeing any of this." That prickly, guilty feeling's like a dull ache now, and he struggles to ask his next question. "How'd you get them, exactly?" He can't quite meet Doc's eyes when he asks this.
Doc closes his eyes in concentration, playing the memory like a movie. "I believe it was just before you left 1955. I'd had some additional trouble with the cabling closer to the street. I had no choice but to bridge the gap myself by holding the loose ends together. Belive me, it wasn't my intention to become part of the circuit, but I couldn't let go soon enough once the lightning hit."
Marty shrinks inward on himself. For the sake of his sanity, he avoids thinking about that night too much in detail, but he does remember the weird sight of Doc holding onto the lightning-rod cable where it plugged into the wire strung over the street. At the time, he didn't know what to make of it, why Doc would be holding on to something about to be electrified. Truthfully, he'd just put it down to an adrenaline-fueled hallucination, and left it at that. He knows now, though.
"So it's my fault, basically," Marty says. "You got hit by lightning because of me."
"Marty, that isn't what happened at all--"
"Doc, you got electrocuted for chrissakes!" Marty has to turn away, the guilt's so heavy on him. "It wouldn't've happened if you hadn't had to help me out. I'm sorry."
Marty's startled when he feels Doc's hands come down on his shoulders. Doc turns him around so they're facing each other, and looks him in the eye. "Marty, please. This--" He gestures to his chest. "--was no more your fault than it was mine. There was storm and there were trees everywhere, I should've accounted better for the possibility that some of them might get struck or blown over and interfere with the cabling. And I'd do it all over again in a heartbeat, because that's how much you mean to me. I'd do anything for you, Marty, you know that."
Marty leans his head on Doc's shoulder, desperate to hide how much he's blushing. "Yeah, I know." He's content to let Doc idly pet the back of his head, and Doc's content that Marty's content.
"Wait a minute," Marty says after who knows how long they've been standing there basking in each other's presence. "Didn't you mention something about getting a rejuvenation treatment or something while you were in the future?"
When it's clear Doc's not going to say anything more, Marty goes on. "Well, couldn't they have removed the scars? I mean, if they can add on 30 years to your lifespan, minor plastic surgery shouldn't be too hard."
Doc can't help but laugh. "Yes, they could have removed them. If I had wanted them to, that is."
Marty looks up at him like he had lost his mind. "Why the hell wouldn't you? I mean, not that you look bad with 'em or anything."
Doc firms his grip on Marty's head to quiet him down and keep him from digging a hole for himself. It's nearly half a minute before he responds. "I kept them so I'd always know that night was real. As good as my memory is, there's nothing quite like an external, physical reminder."
Marty scoffs at him, face still buried in his shoulder. "What, my letter wasn't reminder enough? You even taped it up."
"Yes, well, I prefer to think of these as battle scars gained in the line of duty while protecting a dear friend."
Marty playfully shoves at him. "You're weird, you know that?"