Sorry, this one is very, very, angsty. Also I don't own Star Trek. In case you were wondering.
Mccoy was frantically hitting buttons on the machine that was attempting to sustain the life of his captain. The first three injections of Khan's blood had failed, bringing back a spark of life, only to extinguish it faster than before. Mccoy's usually steady hands had started trembling as he inserted the final needle into Kirk's neck.
"Damnit Jim, don't you dare, not after all this."
The monitor turned green and his heart leapt, until it faded back to red and started sounding out a steady pulse that meant James T. Kirk's resilient heart had given out, once and for all. Bones sank down into a chair, cradling his head in his hands. Moments later, Spock burst through the doors.
"Doctor Mccoy, you were able to successfully use the blood-" He broke off, his gaze dropping to Mccoy's slumped figure, the blinking red lights, and finally resting on the still figure of his captain. He walked slowly to Jim's side and stared down at the slack face and pale skin. As the last bit of life faded from Kirk's body, Spock felt the ground spin, like he was losing his footing in reality. He sunk to his knees, a sudden agonizing pain bearing down on him.
"I'm so, so sorry Spock, the blood wasn't, it wouldn't-" Spock breathed in harshly and Mccoy fell silent.
Bones had been the only person Spock had confided in when he realized his mind had begun bonding with James Kirk.
"Could you please... give me a moment?" Spock's voice was a feeble attempt at calmness. His eyes still hadn't moved from Jim's still face.
"Of course." Leonard couldn't stand being in that room for another second anyway. The doors behind him began to shut, but not before he heard a constricted sob leave the once unflappable Vulcan.