(Note: This is a sequel to, and contains spoilers for, 'Mirror Universe Turnabout' and 'Infiltration')

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Running on the treadmill to the left of mine in the gym this morning is James T. Kirk, captain of the starship Enterprise. I try not to stare, but I still find myself casting frequent glances sideways. He has no idea the person running alongside him is also James T. Kirk, and also once captain of the starship Enterprise. Just not the same Enterprise or the same James T. Kirk. But then why would he suspect anything? At the moment not only don't I look like him, but I'm not even a man. The name I currently go by is Jenna Lawson, and I'm the new Security Chief on the USS Enterprise.

In my own universe, a woman named Janice Lester swapped our bodies and fitted me with an undetectable 'bio-collar' that prevents me from telling anyone who I really am. Janice is dead now, killed by my science officer Mr Spock when he decided to take command of our Enterprise. Unfortunately, she was still in my body when she was vaporised, so I've travelled to this universe to steal the body of their Jim Kirk. Now that I'm here all I need to do is figure out how.

"Good morning, Captain, Jenna," said a female voice.

I turned to see Nurse Chapel get on the treadmill to the right of me, acknowledging her greeting with a smile and a nod.

Christine is one of those people who most surprised me by how different she was from her counterpart. This Christine Chapel is much more to my taste. If I thought she had any sexual interest in women I'd seduce her, but where my Nurse Chapel is happy to go with anyone, male or female, who shares her interest in pain, this one appears to be wholly heterosexual. Unfortunately. In fact it's obvious to everyone that she has the hots for Spock - obvious to everyone but him, that is. I'd first encountered her the day after coming aboard, when I reported to sickbay for my medical as part of my induction...

"Good afternoon, Lieutenant Commander, and welcome to the Enterprise," Dr McCoy had said, greeting me with a smile I wasn't expecting because, though a brilliant doctor, my McCoy was a cold, emotionally distant man more comfortable cutting up cadavers than dealing with the living.

"Nurse Chapel will get the basics off you and then I'll give you a thorough examination."

"Hello, Jenna, it's nice to see another woman joining us in a senior position."

That was my first sight of her, and I was enchanted. She might be physically identical to the Christine Chapel I knew, but she was a lot more wholesome and, well, yummy.

She recorded my height, weight, blood pressure, etc., then McCoy did the in-depth stuff.

"Hmmm," he said when his scans were complete, "I see you've had cosmetic surgery. Repair work?"

"Elective," I replied. "I didn't like my face so I had it changed. Chalk it up to female vanity. The procedure is fully covered in my Starfleet medical file, complete with before and after pictures."

This was a lie. I'd had to change my appearance because there was already a Janice Lester over here, counterpart of the woman whose body I'm stuck with, and she's known to Jim Kirk. Needless to say, the 'before' image in my file does not show Janice Lester but was selected from those turned up by an image search for faces that could plausibly have been mine before the surgery. It was as fake as everything else in my Starfleet records.

"Nothing wrong with a little vanity," McCoy had chuckled. "I once thought of doing something about the bags under my eyes, then decided I looked more authoritative with them. While you're in here is there anything we can do for you? I notice you haven't taken any contraceptive measures."

"No need," I said, "since I'm not sexually interested in men."

I'd given Christine a significant look then, and she had blushed, but we had still become reasonably friendly in the two weeks since, often sharing our mealbreaks with Uhura. Which was a whole other level of strangeness. I knew every inch of Uhura's body, having shared a bed with her counterpart for several weeks when I was her concubine.

Jim Kirk finished his session on the treadmill, grabbed his towel, and stepped off it.

"Ladies," he said, acknowledging us with a nod as he draped his towel around his neck and made for the exit.

I left myself a few minutes later, and headed out of the gym, towelling myself off as I walked to the turbolift. I would shower when I got back to my quarters.

Arriving at my deck, I emerged from the turbolift and was making my way along the corridor when someone rushed out from a side corridor, almost colliding with me. It was Kevin Riley.

"Sorry, sorry," he said, "my fault."

He scurried away, casting a wary look my way as he did so. I grinned. I had 'accidentally' broken Riley's arm during a friendly martial arts contest two days earlier. His counterpart had tried to rape me in our universe and it felt good to get some payback, even though I knew it wasn't really the same man. The fact that I was able to break his arm at all was also very satisfying. I'm smaller than I was, a woman, and with a lot less upper body strength than I had as a man. I've been training hard since this became my body, working to improve my strength and stamina, and all that effort was finally starting to pay off.

After showering in my quarters, I donned a bathrobe and sat down in front my mirror to do my hair and make up. I might be a woman now, but inside I'm still a man so this wasn't something I enjoyed doing. Since a certain level of personal grooming was expected of me, it also wasn't something I could really avoid. This done, I turned on my computer terminal. I had spent most of the time I had to myself getting up to speed how the culture and history of these people differed from my own.

"Computer," I said, "tell me about Zefram Cochrane."

"Cochrane, Zefram," said its mechanical voice, "born March 12, 2030 in Billings, Montana to Charles and Janice Cochrane. In aftermath of World War III, constructed humanity's first warp-capable vessel, the Phoenix, in Bozeman, Montana, using a Titan II nuclear missile dating from 20th century. On April 5, 2063, made Earth's first warp flight. Phoenix warp signature was detected by Vulcan survey ship, T'Plana Hath, which then made peaceful first contact with humans, including Cochrane, at Phoenix launch site."

A picture of Cochrane shaking hands with the Vulcans appeared on the screen, with their Cochrane looking much older than his years, just as ours had. Perhaps that was the point at which our histories diverged. When the Vulcans landed on my Earth, expecting a peaceful first contact, Cochrane killed their leader with a concealed weapon. He and his people then seized the T'Plana Hath. From that bold act, from that captured technology, the Terran Empire was born. The computer continued:

"Cochrane made flight playing Steppenwolf's 'Magic Carpet Ride' during lift-off."

The computer then started playing this, and I smiled - I've always liked classical music. On the screen a photo appeared of Cochrane as a teenager, with his father. I'd seen it before. It was one of the few photos from his childhood to survive the war in either universe.

In the afternoon I was called into a meeting of senior personnel in the conference room. Present were the Captain, Spock, McCoy, Uhura, and Mr Scott.

"Thank you for coming," said Kirk, when we were all seated. "Many of you have been wondering where we've been heading since we set off from Earth two weeks ago and up until now I've not been at liberty to tell you, such is the secrecy surrounding this mission. Since we're almost at our destination I'm now allowed to let you know, though by this point some of you will have guessed given our heading. But first, a brief recap for Lt Cmdr Lawson. Last year, while in orbit around the planet Halka, the Enterprise experienced a transporter malfunction during an ion storm that resulted in Mr Scott, Dr McCoy, Lt Uhura and I switching places with our counterparts in a parallel universe."

I tried not to show my surprise. So this mission was about us?

"Where we have the Federation, a political alliance of worlds working together," Kirk continued, "they had the Empire, an alliance of worlds conquered by Earth and absorbed into the Terran Empire. Cruelty and brutality were part of everyday life on their Enterprise with advancement in rank being achieved by assassinating those above you. Their Captain Kirk, my counterpart, succeeded to command of the ISS Enterprise by assassinating its previous captain, Christopher Pike. His first action as Captain was the suppression of a Gorlan uprising through the destruction of the rebel home planet. His second involved the execution of five thousand colonists on Vega Nine. Here, such a man would be a mass-murdering monster, but there he's a loyal servant of the Empire."

So that's what they thought of me.

"How did you find out so much about him?" I asked.

"With everyone believing we were our counterparts, I was able to access their computer records. Eventually, obviously, we found our way back to our own universe."

"You said you were switched with your counterparts. So if you were over there, then they were over here. Did they pull off the same trick and successfully masquerade as you?"

"They did not," said Spock. "They were taken into custody almost immediately."

I knew this, of course, but not what had given us away.

"I don't understand," I said, "how were you able to identify them as imposters so quickly?"

"It is far easier for civilised men to behave like barbarians, than it is for barbarians to behave like civilised men," said Spock.

Oh barbarians, are we? As Security Chief I carried a phaser on my hip at all times, the only one at the table who did so. Looking at them in that moment, at all those smug, sanctimonious faces, I felt an almost overwhelming urge to vaporise the lot of them. But I didn't, because despite their self-regard I knew the Empire was superior to the Federation. I was the living proof. Which of us had a spy in a position of trust on the other's Enterprise, after all?

"Our encounter with the Empire worried Starfleet Command," said Kirk. "It was obviously aggressively expansionist, and they were rightly concerned that if the Empire should ever discover a way of crossing over to our universe in force, we would become their next target. It was decided that if there was a way to make that crossing then we needed to discover it first in order to be prepared in the event of any incursion by them into our universe."

My ears pricked up at this news. Could it be...?

"As part of the Enterprise's recent refit," Kirk continued, "the deflector dish was fitted with a prototype device which the scientists who developed it claim will open a portal between our universe and theirs. Most of you have been kept in the dark as to our destination on this, our first mission since the refit. Ladies and gentlemen, that destination is Halka. That's where first contact between us occurred and that's where the scientists believe we have the greatest chance of a successful trial."

I remembered the Halkans. When I returned to my own universe that first time I discovered my counterpart had suspended the destruction for opposing us that Empire protocols mandated. Spock had reported 'my' actions to Starfleet Command and been instructed to wait until planet dawn over the principal target to allow me to carry out our mission. In the event we didn't, his orders were to kill me and to proceed against the Halkans as the new captain of the Enterprise. Needless to say, my first action on my return was to order the Halkans destroyed. One more race gone extinct because they foolishly chose to defy the Empire. Still, having them gone made it easier to mine their dilithium since you don't need to pay even lip service to any environmental concerns on the part of the locals when there are no locals. Spock had tried to argue for a logical reason to spare the Halkans, but there were none. That he had done so was my first inkling that exposure to the other Kirk, the Kirk in this room with me now, might have caused cultural contamination.

"Have we let the Halkan Council know we're conducting this experiment while in orbit around their planet?" asked McCoy.

"Of course," said Kirk. "We wouldn't be doing it without their permission. The Federation is not the Empire."

That was true. Permission? From those weaker than yourself? Madness! I would never understand these people. The only true morality, the only one that mattered, was that might makes right.