Title: Mutualism
Rating: R
Pairing: Kirk/McCoy
Word Count: 3,874
Warnings: Mirrorverse fic, knife play, medical experimentation, part of the Symbiosis Trilogy (see also Commensalism and Parasitism) which is the prequel to Infection
Summary: Pike leans back, his eyes hard. "It's simple, McCoy - if he dies, you'll be soon to follow."

Author's Note: If anyone's curious, this is the syringe McCoy uses on Kirk after the Klingon attack.

+++

"What did you do to end up here?" Kirk asks after he hands back the flask and McCoy's introduced himself. McCoy figures there is no harm in telling - Kirk is likely more dangerous than he looks, and he will just be able to look up the information when the shuttle lands in San Francisco, anyway.

McCoy takes another sip before pocketing the flask, deciding that Kirk must be insane to have taken a drink from it to begin with. For all Kirk knew, McCoy could have laced it with poison that he's built an immunity to - an early attempt at taking out competition. Or maybe Kirk is just as paranoid, has built up his own tolerance - either way, clearly insane. McCoy chooses his wording carefully, wanting Kirk to think that he is just as dangerous.

"I killed my father." Which is the truth - McCoy just neglects to mention how his father had been dying and asked for it. Starfleet caught wind of what he had done, recruiting him with the threat of imprisonment, and in the Terran Empire, jail means certain death. "You?" He asks, glancing to the side to catch the maniacal gleam in Kirk's eyes that doesn't sit well with his boyish grin.

"The same," Kirk responds, slinging an arm around McCoy's neck in a way that probably looks friendly or possessive but is tight enough to be a threat while not affecting McCoy's breathing. "It's gonna be you and me, Bones, and none of these bastards will be able to stop us."

+

They are fucking almost immediately.

The first time is after a night of drinking. Kirk's been getting burned by everything with a pulse. McCoy has been out with him before, knows that Kirk will usually just take what he wants and not accept no for an answer, but tonight he's letting it slide, keeps ordering more drinks for the two of them before going off to attempt to pick up a lieutenant or a captain that is on leave or between assignments because James T. Kirk doesn't believe in fucking down - there is nothing for him to gain from it.

They stumble back to the barracks, mostly holding each other up. McCoy falls into his bed, Kirk's legs tangling in his own, pulling him down on top of McCoy, and when he shoves a hand down McCoy's pants with a smirk, McCoy knows that this has been the plan all along. They leave bruises and bite marks along each other's bodies, using hands and rutting against each other because neither is willing to roll over and submit.

One night Kirk shows up at McCoy's room, bleeding from a fight and, judging from the cuts, at least one of the attackers had pulled a knife on Kirk. McCoy fixes him up while describing how a few centimeters in a different direction or a few millimeters deeper would've left Kirk dead. McCoy knows Kirk is clearly crazy when he notices the younger man starting to get turned on, but with Kirk in his hands, completely at his mercy, McCoy is too. All it will take is the wrong hypospray cocktail or too high a setting on the laser scalpel he's using to cut away Kirk's clothes and Kirk will be dead. McCoy doesn't kill him, though - just fixes him up and sends him on his way.

He really shouldn't have been surprised when Kirk shows up the night after that, pressing the hilt of his knife into McCoy's hands, and demanding, "Cut me."

All that pale, perfect skin is too tempting a canvas for McCoy to say no.

+

He doesn't like to think that Kirk is a weakness, merely an opportunity that he is taking advantage of, but McCoy doesn't think their…arrangement is common knowledge until he's called before Captain Pike.

McCoy puts a fist against his chest in an abbreviated salute. "You summoned me, sir?"

"It'll be in your interest to keep Kirk alive, McCoy." Pike's elbows are resting on his desk, fingers steepled.

"I'm his doctor, sir. It's my duty."

"Well now it's more than that. Kirk is brilliant and dangerous - he'll do a lot of good for the Empire but he'll make a lot of enemies, a lot of people will be vying to take his position." Pike leans back, his eyes hard. "It's simple, McCoy - if he dies, you'll be soon to follow."

McCoy feels his mouth go dry but won't let any emotion show on his face. He isn't under Kirk's protection. "I can take care of myself."

"I'm sure you can, but that's not what I mean. There'll be a clause written into Kirk's file that wherever he's assigned, you're assigned with him. Should he die, you're to be executed for failure to follow orders. Have I made myself clear?"

"Yes, sir."

"Good. Dismissed."

+

Autopsy reports go from being a requirement to a necessity. McCoy reads up on all the ways Terrans have died in space and on each of the planets the Empire has visited, even the uninhabited ones. He studies methods of resuscitation used throughout history, finding older medicine intriguing - defibrillators, especially, the ancient way of restarting the heart with electricity. The technology has long since been replaced - a heart is just muscle, getting muscle working again is easy. The brain is more difficult to repair, so sensitive to trauma, but it is electricity the brain interprets, so couldn't the same concept be applied?

All the ideas and plans McCoy keeps in his head, away from the prying eyes of the other research doctors who will willingly steal his research or kill him for it once it amounts to anything. More importantly, though, Kirk doesn't need to know, and he can hack into anything as not even a computer can deny him. McCoy doesn't need Kirk to know the power he holds over McCoy, doesn't need to be lording over him with threats or promises.

McCoy starts to think he isn't in that much trouble when Kirk isn't getting into fights anymore - the other cadets having learned their lessons, finally, and the new recruits not even brave enough to try. The rare few that do receive threats from McCoy, and he hasn't had to kill anyone yet.

Then the incident with the Narada happens, and McCoy has to keep refraining himself from counting how many times Kirk has almost been killed in a single day.

"Doctor McCoy," Pike groans from his biobed, waking up as the anesthesia wears off. McCoy could've killed him, but it would've been too suspicious, and with the uncertainty of how long he will remain paralyzed for, it is more than likely that someone else will do it for him. "Have you spent your time in the agony booth for bringing Kirk onboard while he's on probation?"

McCoy stiffens. "You yourself wrote that wherever he's assigned, I'm assigned. It means nothing if the reverse isn't also true. Sir." He adds the last as an afterthought.

The slow smile that spread across Pike's face is all teeth. "I'm assuming he's still alive, then, if you're here."

"Captain Kirk is on the bridge at the moment - would you like to see him?" This time it's Pike who stiffens, giving McCoy a sense of smug satisfaction. Pike hadn't expected Kirk to actually end up in charge of the ship, and the captain seems to understand now that he wasn't going to be getting her back. "Chapel, make sure Captain Pike stays sedated until we're back dirt-side. He's in a lot of pain."

+

Kirk's blood is still on his clothes when McCoy finds the lieutenant who had been in charge of the security team that went down to the planet with the captain and Sulu. The planet had pre-warp technology and would've been completely uninteresting to the empire if it wasn't for sciences identifying a plant containing a compound that serves as a combined hallucinogenic and hemotoxin. They hadn't expected the natives to consider the damn plant holy, resulting in half the security detail dead and Kirk with, of all things, a fucking spear sticking out of his abdomen.

As much as he likes cutting into Kirk, emergency surgery isn't his situation of preference.

McCoy moves quickly, hypoing the lieutenant in the neck to drag the man to one of the autopsy rooms. He would've preferred one of the surgical suites, but there wouldn't have been any privacy. At least the autopsy table is hydraulic, lowering completely into the floor so McCoy doesn't have to attempt to lift the man onto it. He sets restraints at the lieutenant's wrists, ankles, and neck and gets to work.

He's calculated the dosage of the sedative perfectly with the man's metabolism and how long it takes McCoy to work as the lieutenant wakes up, blearily, to see his chest completely open, ribs spread, only a local anesthetic keeping him from feeling the heart attack that McCoy has induced. There are significantly easier ways he could've done this, and likely will in the future, but this man was almost responsible for the captain's death and then McCoy's own.

"If you'd just done your damn job, you wouldn't be in this mess," McCoy says, keeping his voice casual, like he is just treating a twisted ankle before sending the lieutenant on his way.

He watches, almost bored, as the man dies.

McCoy waits fifteen minutes before grabbing a laser scalpel, setting it high enough to cut through bone with a beam length short enough that, with a controlled hand, he won't accidentally damage the brain.

Six hours later, a beeping from his PADD signals that he needs to return to medical as Kirk is waking up. He thinks about coming back to the lieutenant later but knows the experiment is worthless without a fresh corpse. Freezing the body will damage too many cells. McCoy tosses his bloody scrubs into the resequencer, comming the infirmary to tell them he will be there momentarily and that some orderlies are needed in autopsy to take care of the mess.

+

He is in the captain's quarters at 1850. The order had been for 1900, but McCoy's never taken orders well, even from Kirk. If he is ten minutes late he will be spending some quality time with Kirk's agonizer, but he's yet to say anything about McCoy being early aside from an innuendo and a glance that is almost affectionate. There is only an irritated glare this time, though, and McCoy knows why he is here. He is more surprised that it took Kirk this long to notice - it's been nearly two weeks.

"You've failed in your duties, Doctor. Did I order you to leave it?"

The use of his title in private is almost worse than his last name. Kirk has always called him "Bones" like a child might name a puppy - a mark of ownership. It had been McCoy's turn to leave his own. "Did you call me to your quarters to kill me instead of a public reprimand?"

"I should kill you," Kirk replies, voice icy, but anything else he might have said doesn't come as McCoy approaches Kirk and drops to his knees. One of the first things he'd learned at the Imperial Academy was to figure out how to control your enemy. With Kirk, this means sex. McCoy unknots Kirk's gold sash with agile fingers, gripping Kirk by his hips to turn him around, restraining the urge to bite the man's ass as he lifts Kirk's tunic to look at the scar on the left side of his lower back. McCoy had completely erased the evidence of the spear's entry wound but not its exit.

He leans forward, licking stripes over the pink tissue, Kirk resisting at first, but then McCoy allows a predatory smile to stretch across his face at the shudders he can feel wracking through Kirk's body. McCoy keeps working at the scar with his mouth, licking and sucking and nibbling around its perimeter as he works his right hand around Kirk's waist, unzipping Kirk's pants so McCoy can work a hand in and roughly jerk him off.

McCoy draws out Kirk's orgasm, the other man shouting out, "Fuck, Bones," as McCoy feels the warm wetness on his hand. Kirk slumps back against him, perfectly relaxed, a state McCoy has only seen after sex or mass genocide. Nothing quite like fucking or fighting to take the steam out of a man.

He doesn't say it because he doesn't need to. That McCoy left the scar as a reminder to Kirk that next time McCoy can just let him die has been made clear. Kirk still doesn't know about Pike's addendum to their files. The moment he does is when McCoy loses all the leverage he has.

+

The next time Kirk almost dies, they're getting attacked by two Klingon ships that think the ISS Enterprise would make a worthy trophy. The ship is rattled hard enough with the torpedo blasts that panels explode, pieces of the inner hull go flying, and the infirmary, when it's over, is filled with bleeding and impaled members of the crew. The blood replicator can't keep up with demand, so McCoy is making calls on who has a chance of recovery and who doesn't.

McCoy doesn't know how they defeated the Klingons, just knows Kirk probably made a decision that showed off his insane brand of genius, but Enterprise is rocking a lot less and the blaring red alert alarms have stopped. McCoy's working on an ensign from engineering who had been near one of the panel explosions when Kirk comes staggering in, holding a hand against his throat with blood gushing through his fingers.

"That green-blooded devil couldn't even help you down here?" McCoy's at Kirk's side quickly, holding up the captain and helping him to a biobed as Chapel rushes over with a tricorder. He really hopes Chapel doesn't plan on trying to kill him because she's the most capable nurse here, and it would be a shame if he had to kill her.

"He could, but he's running the ship at the moment," Kirk replies, his face getting paler every second, and McCoy really doesn't like how more blood spurts out when he speaks.

"Shut up, you idiot, unless you want to exsanguinate."

"His carotid artery has been grazed, Doctor." Chapel jumps in, reading the tricorder's scans. "It's a small hole, but Captain Kirk's activity level has been keeping the artery from healing itself."

McCoy gets Kirk into surgery, knocking him out in order to work, and while the nature of the injury had kept Kirk from bleeding out in mere minutes like he would've if the cut had been more severe, the fact remains that he lost a lot of blood, and they still weren't getting what was needed fast enough. Two liters had been kept on hand strictly for the captain, but they'd already gone through it.

One of the nurses enters the surgery suite, looking scared. "We're out of the requested blood type, sir."

McCoy swears out loud, going to one of the cabinets that he keeps locked without his personal authorization. Inside is his most valuable possessions - ancient medical tools that are considered priceless, passed on in his family. He's never expected to have to actually use them, just wanted them somewhere he knew they'd be safe. McCoy pulls out a syringe that's over three hundred years old, connecting two tubes to the barrel and hollow needles at the end of each tube. Kirk may have all sorts of strange allergic reactions, but at least his blood type is AB+. McCoy sterilizes Kirk's arm, then his own, just inside the elbow, before sliding the needles into their respective veins, working the valves and plunger to draw blood from his own body and into Kirk's.

Two days later, McCoy brings Kirk from medbay down to his own quarters. McCoy is there when Kirk goes into the bathroom and catches his own reflection for the first time. He doesn't say anything about this second scar McCoy has left behind. Instead he comes back, grabbing a knife out from under his pillow, handing it to McCoy hilt first before stretching out on the mattress, looking like he's going to enjoy the next hour or two so much more than McCoy will.

+

McCoy tries to be discreet, taking one new test subject every other week or so. The death rate is high in Starfleet as it is, and McCoy doesn't need Kirk questioning why they need to stop to get new personnel more often than usual. One of the orderlies that came to clean up his mess, once, questioned what he's been doing. McCoy had called for another orderly to witness McCoy killing the first then asking the second if he had any questions, as well. McCoy then used that first orderly, because why should he let a fresh body go to waste? The second orderly came back to clean up after that one, too.

He's hooking up some wiring to the man's dead brain in the tenth month of the experiment when he hears the door hiss open, not even looking up at the orderly when he growls, "I haven't asked for you yet."

"I know I get off on killing, Bones, but necrophilia is really not my thing. I prefer my partners to be…responsive."

McCoy is impressed with himself for not starting at the sound of Kirk's voice. He also doesn't look up to meet the captain's face, just continues sending charges and reading his tricorder for any neural responses. There's no reaction from the brain, and Kirk's not getting one from McCoy, either.

Kirk continues anyway. "I received a message from Admiral Pike calling your actions to save my life last month 'highly entertaining.' Interesting choice of words, don't you think?"

"Which actions were these?" McCoy asks. "I feel like I'm saving your ass every waking minute."

"Don't play dumb, Bones, you know I don't keep incompetent people on my crew for very long."

McCoy remembers how Kirk had launched the last one out of an airlock before figuring Kirk must know everything already if he's coming to him now about it. McCoy knows the orders from Pike came last week, and the admiral isn't one to go out of his way to send private messages. He would've just sent an encrypted postscript to Kirk. "I've been wondering how long it would take you to find the clause Pike added to your contract."

"All this time, Bones, you've been acting like you own my life when, really, I own yours." Kirk looks like the cat that got the cream, way too pleased with his newfound knowledge. He approaches McCoy, backing him into a wall, not caring about the blood he's getting on his uniform as he presses against the other man. "Your life is bound to mine, Bones. I own you."

McCoy snarls, feeling the evidence of how turned on Kirk is by his perceived power against his hip. "You own me? You ever stop and think for a second that maybe this goes both ways? No, Pike hasn't written it in to have you executed should anything happen to me, but I can still die out here. Then who will save you? What other doctor has known you for four years, knows every single one of your allergies, and your body's weird way of processing anesthesia? Who else will transfuse their own blood into you before letting you die on the table?" McCoy tamps down the feeling of victory as Kirk remains quiet. "It would seem to be that I'm the one who owns you. Captain."

He waits for Kirk to pull the agonizer on him or, even worse, for the younger man to drag him down to the booth, as Kirk's eyes are unreadable and there's a stubborn set to his jaw that McCoy recognizes from Kirk's second attempt at the Kobayashi Maru - the one he'd willingly lost in order to figure out how to come back and win.

Kirk pulls away without a word, leaving McCoy alone with the ensign cut open on the autopsy table, leaving McCoy wondering what the fuck just happened and what was going to happen, which, in a way, is worse than the agonizer.

+

It's nearly a week later when Kirk shows up at his quarters, uninvited, and lets himself in.

McCoy is reading a PADD at his desk in nothing more than a pair of sweatpants, hair still wet from the shower. He's up and out of the chair, taking on a defensive posture, the second he sees the flash of silver out of the corner of his eye as Kirk takes his knife from the hilt. Kirk throws the knife to the floor, and it lands mere centimeters from McCoy's left foot with the blade embedded in the floor. The next thing he knows, Kirk's on him, wrestling McCoy to the floor, taking every punch McCoy throws at him like he doesn't even feel it as he sits on McCoy's legs and presses his hands against his shoulders, willing him down, willing him still.

When McCoy's stopped struggling under him, Kirk grabs the knife, and McCoy looks him in the eye, not wanting to be the kind that closes his eyes when faced with death. He's seen and caused it enough to not be afraid. McCoy figures he should be impressed that he's lasted this long keeping company with someone like Kirk. "Just be quick about it," he says, not begging, never begging, because Kirk just gets more pleasure out of signs of weakness and will make it last longer just to see his victim cry for mercy.

Kirk doesn't put the blade to his neck or his chest, though. Instead he shimmies a little, moving himself backwards to bring the knife against the waistband of McCoy's pants, cutting easily through the material without so much as grazing the skin underneath. Kirk gyrates some more to cut down the legs, twisting his hips so he can see what he's doing, and McCoy can't help but groan because, with all the motion in his lap, he's getting turned on. Kirk moves off him to take care of McCoy's pants, cutting down to the ankles of each leg, sending the material to lay flat on the ground like a blanket.

Kirk quickly takes off his own clothes, pulling a bottle out of his pocket that McCoy can't quite make out from where he's still lying on the ground. "Relax, Bones, I'm not going to kill you." That almost-affection McCoy sometimes sees is in Kirk's eyes, and McCoy doesn't quite know what to make of it, but with the both of them naked, he knows that thinking isn't going to be something he'll be doing very much of in the immediate future.

When Kirk gets back to his knees, straddling McCoy's hips, lubing him up before sliding himself down, already slick and stretched and ready, McCoy knows it's not caving, because James T. Kirk doesn't believe in giving in, but McCoy knows that it's the closest he's going to get.