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In the early 20th century, a French criminologist named Dr Edmond Locard established one of the main basic principles of forensic science, known as Locard’s exchange principle.
Every contact leaves a trace.
Buck likes to think that the principle doesn’t just apply to forensic science. It also applies to life.
He finds the principle on a relatively normal Thursday in his probie year, just after two in the morning. He’s been on a Wikipedia binge for hours, flitting from page to page and retaining the useless facts that fill his phone screen. Dr Edmond Locard is someone he comes across when he starts looking into Sherlock Holmes after watching a half-decent documentary on TV. Locard’s Wikipedia page isn’t very long, but his principle sits near the beginning.
Every contact leaves a trace.
It sticks with him in a way he doesn’t expect. He kind of likes it, the idea that every time he touches something, he’s leaving something behind. Proof that he exists, that he’s here. He thinks about it every time he holds something, every time he opens a door, every time his hand brushes against a wall. When he can, when it feels right, he puts his hand on Bobby’s shoulder, and nudges Chimney with his elbow, and taps his foot against Hen’s. Anything to leave his mark on their lives.
Enter Eddie Diaz.
Eddie Diaz is his replacement, of that Buck is sure. Every time Eddie gets something right, every time his friends laugh at something Eddie says, every time Eddie is just perfect, all he can think about is the hot, burning anger that fills him up. His brain whispers that Eddie is better than him, that Buck is going to be replaced by this new guy and Hen and Chim and Bobby will probably just forget all about him.
There would just be traces of him left in their lives.
But then he actually spends time with Eddie, in that ambulance, and he realizes that maybe, just maybe, there’s space for both of them at the 118. That he’s not going to get pushed out just because there’s a new probie.
And suddenly, without Buck noticing, Eddie Diaz is one of the most important people in his life.
It’s not clear to Buck just how much he cares about Eddie until he’s pinned to the ground under the ladder truck. His first thought, when he actually takes in what’s going on, is that he wants Eddie. Not wants, needs Eddie, needs him holding his hand or even just next to him to settle something in his soul. He’ll be okay if Eddie’s there. When Eddie does eventually come over, Buck makes a point to brush his hand against his leg, his arm, any part of Eddie he can reach.
Every contact leaves a trace.
And if he dies under this ladder truck, he thinks, he has to leave a trace on Eddie’s life.
Of course, he ends up surviving that, so his actions don’t really matter. And yet, every single time he gets close to dying, or that someone he loves gets injured, the Locard’s exchange principle comes to mind.
He makes sure to hold Christopher close during the tsunami, just to try and make sure this little boy who he loves so, so much can remember him if he doesn’t make it out. He can’t do anything when Eddie’s stuck underground, but when he’s back on the surface Buck takes every single opportunity to touch him, under the guise of checking for injuries. When Eddie gets shot, Buck’s there, holding pressure and dragging him into the firetruck and trying to keep him alive, and, secretly, making sure his hands touch Eddie’s skin, just in case it’s the last time.
Maybe it’s a little much, but, well. Buck’s always been a little much.
After he gets struck by lightning, after he’s clinically dead for three minutes and seventeen seconds, he gets more purposeful about it. He’s always been touchy, everyone knows that, so when he hugs people a little more often he’s pretty sure they all just assume it has something to do with the whole dying thing. No one questions it. Buck just carries on making sure he leaves his mark on the lives of the people he loves.
Every contact leaves a trace.
The first time he questions the whole thing is when Kameron gives birth on his couch. He’s holding the baby, his hands huge in comparison to the tiny, little thing and he’s all too conscious of how he’s making his mark. Buck is leaving a trace on this baby, this baby that shares his DNA, and he thinks maybe it’s a little unfair. This kid doesn’t deserve to have his purity tarnished by all of Buck’s imperfections.
He gives the baby to Kameron, and he makes a point not to touch him again.
The next time he questions it is when he realizes that it feels different when he touches Eddie.
Not a bad sort of different, not necessarily. It’s just that sometimes when Buck’s hand happens to brush against Eddie’s, sparks scatter up his arm and his heart skips a beat. It’s not all the time, but it hits him out of the blue every now and then, the way a brief moment of contact with Eddie can stop him in his tracks. Buck’s pretty sure it started after the bridge collapse. Or maybe the cruise ship. Sometime around then, his little ritual of leaving his mark on people’s lives hit a snag.
Buck pulls away from Eddie a little, after that. Stops touching him as often, and only really initiates any contact before one of them is about to perform a particularly risky rescue.
And then Tommy waltzes into his life.
If Buck is honest with himself, which he very often isn’t, he knows he’s not attracted to Tommy in the way he says he is. What actually happens is that he’s spinning out about the shift that has happened with Eddie, and his brain just sort of latches onto the first person who he could reasonably date.
So. Tommy.
Buck’s not even that shocked by the whole revelation that he’s attracted to men. It’s always kind of been something that has sat at the back of his mind, waiting for the right time to be acknowledged, and he forces himself to think that Tommy initiated the realization. Not Eddie. Never Eddie.
His relationship with Tommy is, for lack of a better word, boring. There’s no drama, not once they get past Tommy thinking Buck’s not ready for a relationship with a man, and they fall into being together not easily, but maybe simply. It’s not comfortable, in fact to Buck the whole thing feels a little, well, itchy, but it’s good enough.
Until Buck realizes that he doesn’t apply Locard’s exchange principle to Tommy.
Every contact leaves a trace.
He doesn’t feel the need to brush their hands together, doesn’t hope that he leaves a mark on Tommy’s life. It’s jarring, the way Tommy isn’t part of the group of people that he counts as his family, his people. It’s like he has Eddie and Christopher and Maddie and everyone on one side, and Tommy’s on the other, with the girl who bags his groceries and the guy who lives next door.
It’s not until he’s been with Tommy for a couple of months that he finally thinks about the principle in reverse. Thinks about how Tommy is leaving his mark on Buck’s life, and it genuinely makes him shudder. And it shouldn’t, considering the fact that Tommy is his boyfriend. He should find some enjoyment from it.
But he doesn’t.
So maybe that’s why he goes a little all-in on his relationship with Tommy. They spend time together, Buck listens attentively to whatever Tommy is talking about, they text during their shifts. He attempts to force his brain to remain focused on Tommy, and tries desperately to stop his thoughts from straying towards Eddie. He’s semi-successful.
He doesn’t realize just how far he’s gone until Eddie calls him one night and tells him that he’s messed up. Massively. And suddenly, Chris is leaving and Eddie is breaking down and all Buck can think is that he should’ve noticed. If he hadn’t been so focused on Tommy, would he have been able to stop this from happening?
Maybe that’s the moment when, subconsciously, Buck accepts that relationship with Tommy is as good as done.
Sure, they last a little while longer, but Buck’s heart isn't really in it. He spends more time with Eddie again, even if it hurts every time he remembers Christopher is in Texas, and tries to make sure everything doesn’t fall apart. He asks Tommy to move in as what is probably a last-ditch attempt to fix things between him, and a small, selfish part of him is relieved when Tommy says no and breaks up with him.
He’s also a little glad that Tommy can’t leave any more marks on his life.
Every contact leaves a trace.
He starts touching Eddie more often, and the sparks and the warmth still pop up on occasion, but Buck just tries to ignore them. Pushes away whatever is going on, because Eddie’s struggling and that’s way more important. He thinks he’s helping, holding Eddie together, until he tells Buck that he’s moving to El Paso.
Time speeds up after that.
All too soon Eddie is driving away from him, and it’s like Buck’s heart is being ripped to shreds. He thinks he’s doing an alright job of masking it during his calls with Eddie, but the rest of the time he knows he’s not exactly the most fun to be around. He’s maybe a little terrified that someone else will leave, so he ramps up his efforts of leaving his mark on everyone’s lives. A nudge here, a hand on a shoulder there. Anything to make sure he’s ingrained in their stories.
The one night stand with Tommy is a lapse in judgment.
He hates that Tommy’s left yet another trace on his life, and once he leaves the next morning he gets in the shower and scrubs his skin until it’s red. Tries to remove any evidence that Tommy left behind, anything that could link the two of them together. He also tries to erase what Tommy said from his mind, the insinuation that he’s in love with Eddie.
That’s not what the weird feeling when they touch means, Buck thinks. He repeats it mentally like a mantra.
He goes back to leaving his mark as often as he can. Ravi at one point asks him about his ‘weird touching thing’, but Chim just tells him that Buck is super tactile and then Hen makes a joke about him being a golden retriever and then it’s all forgotten. He almost tells Bobby about the whole reason behind why he initiates physical contact so often one night when they’re on shift and neither one of them can sleep, but stops himself at the last minute.
Funnily enough, Locard’s exchange principle is the first thing Buck thinks of when he realizes that Bobby is going to die.
He can’t remember if he touched Bobby before they went into that lab. Buck knows that he probably did, that it’s something he does unconsciously at this point, but he can’t remember. It eats at him. Did he leave enough of a mark on Bobby’s life?
Did Bobby leave a trace on Buck’s?
Every contact leaves a trace.
He gets more deliberate and maybe a little obsessive about it afterwards. Hugs people too often and for too long. No one calls him out on it, but Buck sees the looks when they think he’s not looking. At least Eddie comes home, back to LA, and brings Christopher with him. There’s still a gaping hole where Bobby should be, though. Before rescues, Buck makes sure he touches everyone, and he finds himself looking for Bobby more than once, trying to check him off his mental list. He’s a little more deliberate about it now, unwilling to be in the same situation again where he can’t remember if he touched someone before the worst happened.
The whole space situation is a scare. He hugs Athena and Hen maybe a little too tightly when they’re back on Earth, along with everyone else. They’re all a little raw, after Bobby, and close calls make them all a bit overprotective now. Then there’s another scare when Hen collapses, and even though she ends up being fine, Buck’s brain isn’t quite convinced of that fact, so he goes over to see her a lot. But things do eventually start to settle, and Buck’s still making sure he leaves his mark, leaves evidence of himself intertwined with everyone’s lives.
And then Buck gets kidnapped.
The actual kidnapping isn’t the problem. It’s afterwards, when Buck collapses to the ground and Eddie is right there. Eddie’s hands are roaming over his body, checking for injuries, and the warm feeling and the sparks take over so suddenly that he almost doesn’t notice when something shifts into place in his brain.
He’s in love with Eddie.
He’s been in love with Eddie for a while, he realizes later when he’s on his way to the hospital. Since the sparks started at least, or maybe even longer. Maybe that jealousy he felt during Eddie’s first shift came from a place of attraction. Maybe he was attracted to Eddie, not Tommy, that day at the basketball court, but his brain just hadn’t been ready to accept it back then.
Obviously, he can’t actually tell Eddie about it, because as he told Tommy a year or so ago, Eddie is straight. He starts avoiding Eddie just a little when they finally get back to LA, goes back to touching him as little as possible to stop himself from blurting out the embarrassing truth.
And then, as Buck begins to spiral, he all but stops his whole touching thing.
He withdraws from everyone, his mind telling him that he doesn’t deserve to leave any trace of himself. He’s lying to them, and they should just abandon him and find someone better. They, of course, don’t do that when the truth comes out, and Buck finds himself surrounded by everyone he loves. They seem to take over the touching too, holding his hand or putting an arm around his shoulders. It means more than they will ever understand.
Every contact leaves a trace.
Buck does, though, gradually bring the touching back. He even considers getting Locard’s exchange principle tattooed somewhere on his body, as a permanent reminder that he’s left his trace on people’s lives just as they’ve left a trace on his. He almost schedules the appointment, but the shift before he means to call, a little boy barrels into his life.
Theo.
He honestly has no idea who Theo actually is until Connor and Kameron show up. It’s only then that it clicks, that this child in front of him is the same tiny baby he held all those years ago. He spends most of the dinner with them just sort of staring at Theo in awe, unable to believe that enough time has passed for him to be that big.
And just as Buck’s starting to imagine being some kind of uncle to Theo, it all goes wrong.
He knows something is up when Eddie’s trying to hold him back, trying to prevent him from getting to the car the rest of the 118 is working on. But when he spots who they’re working on, the world seems to blur around him. There’s this ringing in his ears that is blocking everything out, and he can feel his mouth moving but he has no idea what he’s saying.
Until Theo is pulled out of the wreck, miraculously unharmed.
He’s handed to Buck almost immediately, and he takes him over to one of the ambulances, so that Theo can’t see the actual crash. Buck knows, in his heart, that Connor and Kameron are dead, but when Theo asks he just brushes it off and changes the subject. He shouldn’t be the one to tell this little boy that his parents are dead. All he can do is hold him close, and hope that Theo feels all the love Buck has for him through the marks he’s leaving on Theo’s life.
Every contact leaves a trace.
And then, because the universe enjoys torturing one Evan Buckley, Athena gets shot.
Athena isn’t someone he gets to touch often, which just means he usually takes every opportunity to give her a hug. But then she’s wheeled past on a gurney, and Buck can’t help but wish he saw her more often. The waiting is as awful as it always is, and he slips into his pseudo big brother role as he helps Harry through it. Of course, that means they’re stuck on the top floor of the hospital when the guy shows up to kill Athena, which isn’t great, but they get through it. They manage to get back down to the ICU, and Harry takes out the gunman, and Athena is okay, and everything is good. They’re all hugging, and Buck’s taking the opportunity to make sure he touches everyone, but then he realizes.
There’s no warm feeling. There’s no sparks.
There’s no Eddie.
As soon as he voices his concern, the elevator dings and the doors open, and there’s Eddie, Buck’s Eddie, bleeding out.
And Buck is just frozen.
Hen and Chim run over and start treating him, but Buck just can’t move. Eddie, his best friend, the guy he’s in love with, is bleeding out, dying, and Buck can’t move. He should move, should get his hands on him just in case this is the time he loses Eddie, but he can’t. He just stares, frozen in time as Eddie bleeds and bleeds and bleeds.
He doesn’t move until the doctors show up and take over, moving Eddie with startling efficiency. He just sinks down into a chair, and resigns himself to waiting for a while longer. The rest of the 118 join him, sitting vigil as they wait for news, occasionally leaving to go and check on Athena. Someone presses a cup of water into his hands at some point.
Buck’s not sure how long they wait, but it feels like an entire day has passed before they’re allowed to go and see him. Well, Buck is allowed to go and see him, because he’s Eddie’s emergency contact and apparently Eddie himself is awake and asking for him.
And when he finally sees Eddie, he feels like he takes his first deep breath since he found out Athena had been shot. Eddie’s awake and smiling at him and already complaining about having to stay in hospital, and Buck can’t stop himself from getting as close as he can, pressing their arms together. The sparks are welcome, because the sparks mean that Eddie’s alive.
He only moves away when Christopher arrives, letting him have a moment with his dad, but after a minute or two he can’t help himself and he returns to his chair as Pepa goes off to call the extended family. He tries to be more subtle about it, but he still makes sure some part of him is touching Eddie at all times.
“Dad, I think you scared Buck,” Chris says, looking at where Buck’s pinky rests against Eddie’s arm. “He’s being super obvious about his touchy thing.”
And Buck’s blood runs cold.
“Sorry,” he breathes, already moving away, but Eddie grabs his hand and puts it right back where it was.
The sparks spread throughout Buck’s entire body.
“Don’t be sorry,” Eddie tells him, “I like it.” There’s this fond smile on his face that Buck’s only ever seen aimed at himself and Christopher, the one that sometimes makes his breath catch in his chest.
Eddie doesn’t let go of his hand, which means that Buck’s pretty much useless for the rest of the conversation. His entire being is focused on how the two of them are holding hands, and he can’t string a sentence together. He barely remembers to say goodbye to Christopher and Pepa as they leave for the night.
“You know,” Eddie says once they’ve left, “we don’t mind your whole touch thing.”
“Really?” Buck whispers.
“Yeah, even though we don’t really know why you do it,” he replies. “It’s just your thing, you know?”
“It’s Locard’s exchange principle,” Buck blurts out, making Eddie’s eyebrows furrow. “Dr Edmond Locard, he was this French criminologist, and he came up with one of the main principles in forensic science. Every contact leaves a trace.” Eddie still looks confused, so Buck keeps going. “I found out about it when I was a probie, and I thought that maybe it applied to life as well as forensics. Like, every time I touched something or someone, I left a trace of myself. Evidence that I was there, that I was part of someone’s life.”
“Huh.” The fond smile begins to bloom again, and Buck has to look away before his words fail him.
“Yeah, I know it’s stupid, but I just really like the idea, so I make sure that I touch people a lot. Leave a trace so that they can’t forget me, I guess.”
“God, I love you.”
Buck’s pretty sure his heart stops beating for a second.
“Eddie,” he whispers, “what?”
“I love you, Buck,” Eddie repeats. “And I’m pretty sure I have for a while. I just didn’t realize it until I was bleeding out in that elevator, when there was the moment where I really thought I was going to die. And I just knew. I love you.”
“But you’re straight.” Buck manages to stutter out, his eyes wide.
“I don’t know what I am, honestly, but I know I’m not straight.” Buck just stares at him for a moment or two in complete shock.
“You’re not straight.”
“I’m not.”
“You love me.”
“I do. I love you so much, Buck.”
“Am I dreaming?” Eddie reaches out and pinches his arm.
“You’re not dreaming. Although the fact that you just asked that does give me hope that you love me too.”
That’s the thing that finally breaks Buck out of his shock.
“Eddie, yes, of course I love you,” he says, leaning closer. “I’ve known that since New Mexico.” Eddie’s smile turns sweet, and Buck can’t stop himself from beginning to grin.
“Great, we love each other, can you kiss me now?” Eddie asks. “I’d do it myself, but I can’t really lean with the stab wound and-”
Buck kisses him before he can finish his sentence. The sparks multiply until his entire body is engulfed by flames, all of his nerve endings celebrating the fact that he’s kissing Eddie. That he’s leaving his trace on Eddie’s life.
Every contact leaves a trace.