Spock had his orders. He wouldn't say anything but I knew he didn't like them. Until our two guests and I returned, he was required to maintain the Enterprise in geosynchronous orbit over the planet below, and no one else was permitted to enter the transporter room in the meantime no matter what happened. He was as in the dark about the nature of our secret mission as the rest of the crew. That had to rankle, particularly given the identity of one of my companions on that mission.

"Have you set the coordinates, Lieutenant?" asked Sybok.

"Almost, Science Minister," came the reply. Sybok had brought Lieutenant Cho along with him, and assigned her to stay in the transporter room during our absence. None of my own crew had a high enough security clearance to see us on our way, apparently. Then again, despite being captain of the Enterprise, I myself had only been given our destination with no indication yet as to why I had been chosen for this particular jaunt in the first place. Science Minister Sybok was a powerful member of the Council of Ministers that actually ran the Empire, so the fact that he had decided to take an active part in the mission was an indication of just how important he believed it to be. Still, I couldn't help wondering if he had chosen the Enterprise because Spock was his brother.

Sybok was standing on a transporter pad beside me, as was his beautiful young assistant Kara Summers, the other person who'd come aboard with him. Both were wearing the same cumbersome harness as myself, high-tech devices without which the mission we were about to undertake would not be possible.

"Transporting on my mark," said Cho, "three, two, one..."

She flicked the switches, pushed the sliders up, and a second or two later we were dematerializing and beaming down to the world below.

Just not the one in our universe.

The first thing I noticed when we rematerialized was the biting cold. As well as being sixth planet from its sun and so at the far edge of that sun's habitable zone, Sigma Draconis VI was also currently experiencing a mini ice age. In this it was like its counterpart in our universe. The area where we'd landed was composed of largely uninteresting rock and dirt, with very little vegetation to speak of.

"Let's get the equipment moving," said Sybok, manhandling the container that had been on the fourth pad and beamed down with us, sheathed in a harness of its own. Sybok's breath was misting on the air as he moved about, as was mine, but not Kara's. Sybok pressed a button on the side of the container - which was as tall as a man - and it rose several centimetres off the ground when the internal antigrav kicked in, thus enabling us to push it along without effort. Through all of this Kara had watched impassively. Where I was feeling the cold even through the insulation of my uniform she was showing no signs of discomfort.

"I'm amazed those harnesses actually worked," I said.

"They are impressive, aren't they?" said Sybok. "The finest scientists in the Empire labored over them trying to duplicate the transporter effect that took you and your subordinates to the universe of the Federation, to this universe. Three months ago they succeeded. This mission is a direct result of that success."

"You still haven't told me what the mission is," I said, "what exactly it is we hope to accomplish on this freezing lump of rock."

"All in good time, Captain, all in good time."

Sybok led us over to a nearby cave. No sooner had we entered than a steel door closed down over the entrance and the whole cave shook as it started to descend.

"An elevator!" I exclaimed, surprised. "So that's why we landed where we did, but where's it taking us?"

"Beneath us lies a whole underground city. In our universe it exists only as ruins, but here it still lives even though its population has declined and now numbers little more than a hundred."

"Dangerous?"

"Hardly. A bunch of indolent pacifists whose subterranean existence has so far protected them from outside interference, living on the marvels created by their ancestors. And they are marvels. You can guess how we knew to look for them in our universe..."

I nodded, fully aware of the amazing secret that lay behind the Empire's rapid expansion over the past century, a secret kept from everyone below the rank of starship captain.

"We were able to learn a lot from the remains in our universe, and will learn even more from the functional technology in this one."

The elevator came to a halt, the steel door slid upwards, and we found ourselves looking down a corridor and into the very surprised faces of several gaudily dressed men and women. Kara stepped forward and pressed a button on the large metal wristband she was wearing. I had assumed this was nothing more than an ostentatious piece of jewellery, one somewhat at odds with the starfleet uniform she was wearing, but it was obviously much more than that. In response to her action the men and women we'd surprised all crumpled to the floor.

"Are they dead?" I asked.

"Unconscious," replied Kara, the first time she had spoken since we left the ship, "as is everyone else in the city. They will stay that way until I release them."

"Your device is that powerful?"

I let out a low whistle.

"We have to start issuing them to everyone in starfleet."

"I'm afraid it's one-of-a-kind, Captain, as is Kara herself. It's keyed to her body and won't work for anyone else. As for copying it, the technology is so far beyond our own we wouldn't know where to start."

"What do you mean when you say Kara is one-of-a-kind?"

"Haven't you guessed yet?" chuckled Sybok, "Despite appearing human to even the most sophisticated scanners, Kara is in fact an android, the most advanced we've ever come across."

"I was the last surviving sentient being on my homeworld when Sybok found me," said Kara, "and working for the Empire has given my existence meaning again after too many centuries when it had none. Kara was the name my creators gave me, but Sybok told me I would require a surname to fit into your society. So I chose Summers, because it was summer when he found me."

"Enough talk," said Sybok, "time to get to the hall of the Thinker. We have a tight schedule to meet."

The Thinker turned out to be a learning machine, similar to some prototypes I'd seen in the Empire but vastly more sophisticated. Kara stood before it, lowered its transparent dome over her head, and appeared to go into a trance. Sybok explained what she was doing.

"Kara is uploading large amounts of data from her mind, including how to successfully remove and replace a humanoid brain, a skill only she possesses in all the galaxy."

"Wait, what?" I said, non-plussed by this revelation.

"All will soon become clear. Now help me get the unit set up."

Sybok opened the container we'd brought with us. Inside were a couple of cases, but most of the space was taken up by a unit that looked like a featureless box with several hoses coming out of it, and a large dome on top.

"We need to connect it to the equipment here so that it appears to be a part of the system. The hoses have no actual function but will further the illusion."

I did as Sybok directed and we soon had the unit installed. We then stowed all the harnesses in the container and hid it away. As we finished so Kara lifted the dome from over her head.

"It is done," she said.

"Good, good, then make your way to the craft. You know what you have to do."

"Yes, Sybok."

"What craft would that be?" I asked when she had gone.

"The only one of its kind we know of," said Sybok, "ion-powered, like the city."

"Developing ionic-powered drives has been a goal of starfleet for decades. We must have it!"

"And you will, Captain - just as soon as our little charade here has run its course. However, before that can happen you and I have to process the populace."

"All of them?"

"All of them. Between the two of us, and with the aid of some of these people's antigrav gurneys, we should just about manage it before Kara gets back."

Processing the populace involved dividing them into three groups and using the Thinker to essentially brainwash them. One group of males were reduced to a primitive state, outfitted with furs and spears and made ready to be deposited on the surface. The remaining men were rendered subservient to the women, while the women were made dominant. Everyone had their intelligence reduced to that of children and false memories implanted. The templates for all this came from Kara's upload and the scenario I knew we had to follow. We then opened the cases that had been in the container. One held belts with large circular devices attached, the other copies of Kara's wristband.

"They're non-functional," said Sybok, "but giving all the women one to wear disguises how unique Kara's is."

"And these?" I said, holding one of the belts aloft.

"Each contains one of our agonizers. These people don't administer painful punishment, but the scenario requires them to do so."

Processing close to a hundred people took a lot of time and I was pretty exhausted by the end. We did however finish just before Kara's return, as Sybok had predicted we would.

When Kara entered the chamber she was wearing the same clothing worn by the women of this world rather than her starfleet uniform and carrying a transparent sphere filled with a clear liquid.

Floating in the liquid was a brain.

"Whose is that?" I asked, almost afraid of the answer.

"It belongs to my brother," said Sybok, "or, rather, to his counterpart in this universe. We knew where the Enterprise would be, so it was a simple matter to have Kara intercept it. Now my dear, if you would...?"

Kara placed the sphere in the dome on top of the device we'd brought from our universe. It was a perfect fit.

"The machine will feed sensations into Spock's brain to give him the illusion he's running the city's systems," said Sybok, "while all the time it's performing its true function."

"You know that my counterpart and his crew will come here looking for Spock's brain, of course?" I said.

"I'm counting on it. The plan won't work if they don't."