"Zombies," said Admiral Cartwright. We weren't in the same room or even the same sector of space, but I could imagine him shaking his head.

"To all intents and purposes, yes," I said, sinking my fingers into a tub of cold cream, "but then if you go trying to break through to another universe without knowing its address you have no way of knowing what you're going to get."

Wearing only a nightdress, I was sitting in front of the mirror in my quarters and working cold cream into my face to remove my make-up prior to retiring for the night. It was also my first opportunity to report in to Larry Cartwright, the Empire's other spy in the Federation. This was one of those quiet periods when the crew were allowed personal conversations with family back home, and our covert communication was riding the carrier beam between the Enterprise and Earth, lost in all the chatter.

"Nevertheless, their device created a portal to another universe big enough for a starship to pass through. We haven't managed that yet. Our trans-dimensional harnesses only allow for travel by individuals."

"Yes, but at least we can find the universe we want to. The Federation doesn't know that deep in the sub-atomic structure of every piece of matter from a given universe is a vibrational signature unique to that universe. This is its 'frequency', and thus its address. Not having any matter from another universe, they had nothing to compare to their own and therefore no way of even realising such a thing even existed. We had the USS Defiant, a whole starship from their universe of the present day, that fell through a spatial anomaly into ours a hundred years ago. We've known about those vibrational signatures for decades."

A century of studying the Defiant's computers and technology had given us a lot of advantages over the Federation. We had long since cracked all their current encryption and codes, for example, and had sub-dermal communicators that they could not detect and which would enable us to communicate with each other without starfleet knowing about it. This was how I was reporting to Larry Cartwright now.

"The incursion from the zombiverse could have given them a lot of material to work with," said Larry, "they could discover vibrational signatures exist that way."

"Already thought of that," I said, using a wetwipe to remove my lipstick, "when Captain Kirk gave me permission to vaporise the zombie corpses and all their effects, I made sure those effects included their phasers and every bit of equipment of theirs we had captured. Debris from the shuttlecraft they crashed into ours at the portal interface drifted back over to their side before it collapsed. That left only Crewman Wilson who, tragically, had to be vaporized when he broke free of his guards while being transferred from his holding cell this afternoon. His bonds had somehow been loosened before then. I can't imagine how."

"Good work," chuckled Larry. "So you got everything?"

"Yes, apart from the tissue samples Dr McCoy took. I'm confident those will only be looked at medically and aren't likely to have their sub-atomic structure investigated the way, say, a surviving phaser might. No, other than those samples, only a strip of material I tore from the sleeve of one of the zombie corpses survives beyond the surface of Halka. I'll get it to you via our dead drop next time I'm on Earth and you can make sure it gets back to the Empire. I'm sure our scientists would be interested in having the vibrational signature of another known universe."

"I'll be sending it across with the schematics of the device they used to open the portal. The Federation may not know how to find a particular universe but we do. They don't know it yet, but they've just given us the means to invade them."

"Good. How is the infiltration proceeding?"

"We've brought six more of our people over, all cadets, to replace their counterparts in Starfleet Academy."

"Why cadets?"

"I may have replaced their Admiral Cartwright, but the Council believe that attempting to replace more high ranking people is too risky at this point. Better to get cadets in place and have them move up the ranks. By the time we're ready to invade, years from now, many of them should be starship captains. We're going to hollow out the Federation from the inside. Oh, and I have a bit of news that may interest you. Starfleet Command has decided to do away with the post of Security Chief and combine it with that of Chief Tactical Officer."

"It's no skin off my nose," I said, shrugging. "I should be done here by the time the change comes into effect. Before I sign off, did you get me the information I asked for?"

"I did," said Larry, "and it surprised me. Why did you want me to look up Federation records on Camus II?"

"Because my last mission undercover took me to our Camus II and I discovered a cache of extremely dangerous bioweapons there, which our James Kirk had to destroy," I lied. "As this affair with the zombies has shown, you can't be too careful with such things."

"Yes, well, the file was marked 'top secret' and no one of lower rank than admiral was going to get into it."

"What did it say?"

"It appears that your counterpart, the Janice Lester of this universe, discovered a mind-switching device on Camus II and used it to steal the body of their Captain Kirk. Their minds were eventually returned to the correct bodies, but not before she was able to briefly take command of the Enterprise. You can see why Starfleet is keeping the existence of the device secret, of course?"

"Just the knowledge of its existence could be politically and socially destabilising."

"Exactly. We'll have to decide what, if anything, we're going to do about it."

After Larry broke the connection, I wiped the cold cream off my face and stared at my reflection, studying the face of the woman gazing back at me. Could it be? Janice Lester had swapped bodies with us in both universes. In this one, officially, Kirk had got his body back. But what if he hadn't?

What if the man sitting in the Captain's chair on this ship right now was actually Janice Lester?

It would explain a lot of things, and give me much to think about. I smiled. Oh yes, so much to think about...

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The End

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Incidentally, Toxoplasma gondii is a real thing. If you're interested, check out an article called 'How Your Cat is Making You Crazy' at The Atlantic from a few years ago. Spooky stuff.