Disclaimer: This short story uses characters and places of the Star Trek universe, which belong to Paramount.
English is not my native language so please feel free to point out any error.
There was a tensed atmosphere on the bridge. The officers' hands were clasped behind their back or on their console as the captain paced around them.
He was relaxed, compared to them. His blue eyes showed a mischievous light, enhanced by the red lightning of the alert sounding all around the ship, since the very beginning of this situation.
Spock took a glance towards the turbolift which had begun moving one point two minutes earlier. At that moment, none of the officers had noticed the sound, but he estimated the time before this to happen of zero point seven minutes. By this time, the captain would know about the person incoming, and Spock had to say that he had apprehension about his reaction.
Sensing a boring look on his back, he reported his attention back on his console which showed nothing else than nothingness. All his screens were as blank as the space that surrounded the ship. They were lost in the vast expanse of emptiness, with nothing more than a mad captain, a frightened bridge crew and hundreds of innocent officers below deck.
A strange robotic voice sounded behind him and startled him off his thoughts. However, he showed nothing of it and concentrated his attention on his console, trying for the umpteenth time to recalibrate his sensors and avoid his captain's gaze.
"Is there a problem, Mister Spock ?
— None, Captain. I was simply thinking about the possibility of the arrival of an officer on the bridge.
— Amusing that you talk about it, Mister Spock…"
The red lightning passed again by his eyes and Spock noticed a metallic reflection in it. A metallic reflection which was not there usually. Yet another strangeness that Spock had to point out.
Just as a dangerous grin split onto his face, his hand fell upon a small button on his chair, his gaze locked with Spock's.
"Did you think I wouldn't notice this muffled buzz sound ? I will simply state that this little monster of an officer that was trying to join this sanctuary is another human idiot, just as you and your shitty crew are."
There was a blocking sound, the motors stopped. He touched another button, taped a code on the keypad and a muffled shout resonated as the lift fell down several decks, if not all of them.
Spock heard Uhura and Chekov stifle small cries behind their hands but kept his unblinking gaze trained on his horrendous captain before him.
"You just killed one of your crew members.
— Indeed I did.
— You would never do this in your normal state. You are not our captain."
An evil grin broke again on his face, soon followed by a booming laugh which sounded nothing but crazy.
"So observant, Mister Spock ! I never saw anyone this smart. Don't you think your pretty little friends knew that by now ? I must say their brains are pretty empty compared to yours, but they are not that blind."
Spock felt his brow shot up behind his bangs. He was not prepared to encounter a telepath in Kirk's body.
And suddenly, the realisation struck him and a vague impression of horror hit his perfect vulcan control. Kirk was not a telepath, his mind was not made for it. To have such a mind melded with his would damage his brain, irrevocably.
He tensed even more if possible, hands clasped behind his back, containing his emotions and controlling his reactions.
"Leave our captain be in peace. You are paining him.
— Oh god yes ! exclaimed the robotic voice coming from the captain. His voice is so hoarse by screaming his agony with so much fervor… That's so good to hear !"
On this one, he hit again the button on his chair and left the bridge without so much as a glance towards them and a second crazy laugh.
All officiers fixated their gazes on Spock and he felt again his control slip. Their looks and their hopes were so intense and this stranger in Kirk's body walking around the ship, and this crew member dead in the turbolift… There was too much to handle right now and he didn't know if he could save them all.
His eyes crossed Uhura's on his right and she just nodded in understanding. His decision was taken.
He handed the con to Sulu as he walked out the bridge, pulling a phaser off a rack as he passed by. He clutched it in his right hand and followed the lead left by the impostor with stunned crew members. At least, they were not dead, judging by the soft rise and fall of their chests.
"Jim ! What the hell are you doing there, kid ?" he heard from the end of the corridor.
Spock ran to the sound of the voice, Dr. McCoy's if he knew it well and he had no doubt about it.
As he rounded the corner of the corridor, an horrifying sight granted him. McCoy, pinned to the wall by the false Kirk, tried to get rid of him by debating against his once-best-friend, which had a crazy look, just as the one he had had when pushing the button that killed one of his fellow crew members.
"Release the doctor, stranger," he ordered, his phaser drawn on his former captain. "Release him or I will kill you."
A mad laugh resonated in the empty corridors and the impostor turned towards him, his grip on McCoy stilled. "If you kill me, Mister Spock, you know that you'll kill him." His grin grew wider and Spock internally cringed at the sight. "Right ? His body won't stand the strain, and you won't have your pretty little doctor since I'll have him killed by then."
"You will also be dead, that is the only thing that matters."
"Yet, you haven't killed me by now. But if you really want to know, he is not that alive."
His voice was lower, with an accent from another world, metallic, and its resonance in the empty corridor only made it worse. Spock showed nothing of his horror and his grip on his phaser didn't shake for an instant.
"His shouts are almost dead, just like him. And when his mind and his soul will have collapsed, I'll just keep his body. It's quite comfy in there."
His grip on McCoy released for less than a second but the doctor took it upon himself to hit his best friend right in the chest before punching him in the groin, efficiently making the impostor double over with a pained cry.
Spock took this moment to change the setting of his phaser on stun and shot Kirk in the back. He collapsed on himself with a groan as Spock and McCoy exchanged a glance.
"What the hell was that ?" groaned the doctor as he crouched beside his friend to check his pulse. "Is it Jim in this body ? Is he dead ?"
"I do not know, doctor." replied the first officer, crouching by his side. "This impostor has settled himself in his body while on the bridge. I could not assert the state of the captain's mind."
They both stood up and began carrying Kirk off the corridor to his quarters, thinking about the situation and what they could do to save their friend.
Just as they rounded the corner of the officers deck, the impostor's eyes shot open, surprising the other two. He kicked his legs, he kicked his arms, and McCoy had no other choice than to let him fall, not having the force to keep him still.
Spock took him by his hips to prevent him from moving and the stranger took advantage of it to kick him in the groin, just as McCoy did earlier. He freed himself from Spock's grip and turned toward the doctor.
The vulcan took a deep breath, forcing his mind to put the pain aside and straightened up to pinch the captain on the shoulder as he had done so often.
The stranger should have not been sufficiently humanoid to fall under it as he just groaned and pushed Spock aside by a powerful nudge in the ribs.
This last move was their end.
Now free in his movements, the impostor moved towards McCoy, who tried to retreat to the relative security of the captain's quarters, which he knew had a phaser in it. In no time, Kirk caught him and his hands set down on the doctor's head.
For an instant, the horrendous face relaxed, a hand brushed McCoy's hair and the captain's voice resonated, his real one, the one they used to hear, just a little weak and hoarse. Not the stranger's one with its terrible metallic accents.
"Everything is good, Bones. I'm sorry it ends like this. But I can't resist anymore."
Spock couldn't do anything but watch in horror as McCoy's neck snapped with a quick movement of the wrist. Seconds later, the walls were covered in blood and flesh and brain as his head exploded.
And another type of blood and flesh and brain joined the mess on the wall as Spock shot the captain right in the head.
The Enterprise had lost both her captain and chief medical officer, their insides covering the officers' deck's walls.
Never could Spock get it out of his mind.