Keeping in mind I’ve never seen the episode...
“Captain’s Log, stardate... wait for it... right about... now! Starships run on dilithium. This makes it very useful stuff. Unfortunately, there’s not much of it, making it an inconvenient and expensive material. I submitted a proposal to Starfleet Command to build starships that run on flowers. Perhaps even dirt. There’s plenty of that around. We’ve found a planet that has plenty of dilithium. Heaps of the stuff. I am beaming back to the Enterprise with McCoy, and Fat Scotty, after opening negotiations with the Halkanini... Halkeni... Spock?”
“Halkans,” Spock replied over the communicator.
“That’d be them!” Kirk agreed. “Three to beam up! Wait, considering Fat Scotty... you’d better make that four.”
The transporter beam grabbed the three of them and deposited them on the Enterprise.
What they didn’t know is that it wasn’t their Enterprise. It was the Enterprise from the mirror universe!!!! (dramatic music)
“Greetings, Captain.” Evil Spock said.
“Hello Spock!” Kirk replied, stepping off the transporter pad. “You’ve got something around your mouth. Just there, yeah. It could be chocolate, okay.”
The mirror Spock frowned at his goatee. “How did the meeting go?”
“With the Halkanenians?” Kirk asked. “Great, couldn’t have gone better.”
“Did you give the customary ‘give us the dilithium now or I’ll beat you into unconciousness with my baseball bat’ threat?” Spock inquired innocently.
Kirk paused in confusion. “Actually, no. I didn’t. Could you please excuse us for the tinciest moment? Thanks so much.”
He quickly turned to McCoy and Fat Scotty. “Guys, get a load of that thing on Spock’s face! Do you think he’s been drinking chocolate milk, because I can’t figure it out!”
There’s something more important at work here, Captain.“ McCoy said. ”Look at our uniforms! They’ve changed!”
“Hey yeah, they have!” Kirk agreed, suddenly noticing his sash and twirling it around. “Cool!”
Meanwhile... in the normal universe... though normal is debatable...
“I’m beginning to think that something happened in the beam-up process.” Spock said quietly to Uhura and Sulu. “The crewmembers have been acting strange ever since. Dr. McCoy has now gotten an excellent bedside manner, Captain Kirk seems to be scoring with every female member of the crew as opposed to just scoring with himself, and Fat Scotty is... well... nice.”
“I haven’t seen him this friendly since we spiked the pudding at last year’s Christmas party.” Sulu remembered with a shudder.
“One theory.” Spock continued. “That these three are not the colleagues we know. They are not the people we have worked with for many years. They are, in fact, their evil counterparts from a parallel universe.”
Sulu whistled in a low tone. “How did you come to that conclusion?”
“I read about it in the first edition of the Junior Encyclopedia all about the Parallel Universe.” Spock said. “Volume one cost 15 dollars left then all the other issues.”
Meanwhile... in a parallel universe not unlike our own...
Captain Kirk sat in the captain’s chair biting his nails in a bored manner. “Is anything happening?” He asked.
“Actually sir, yes.” Evil Spock replied. “We are receiving a message from the surface.”
“The Harikarinans?” Kirk said in an excited manner. “Cool, put them through!”
An alien appeared on the viewscreen. “Captain Kirk, we have decided to reject your proposal. You will use the dilithium crystals for evil. We don’t like that idea. So we’re going to have to say no. Stop by any time, though!”
“Thanks anyway then, see you later!” Kirk said, waving at the viewscreen. “Oh well, maybe next time. Spock, contact Starfleet Command, tell them I said hi, tell them they look good and that they’ve lost weight, and that the Hekalonions didn’t want to give us the crystals. Also, you’ve got some chocolate milk on your lips. Just there. Still.”
Evil Spock glared at him. “Aren’t you going to lay waste to the Halkan planet? Even go there and bash the leader senseless?” He threw Kirk his baseball bat.
Kirk held the bat nervously. “How about if I give them twelve hours to reconsider and play really annoying thinking music. Is that mean spirited enough?”
At the same time, in the normal universe...
“Fire!!” Evil Kirk yelled. “Don’t you hear me Spock, you stupid half breed!? Fire all weapons!!”
“Och, don’t call Spock an evil half breed, Captain!” Evil Fat Scotty said. “You’ll hurt his feelings!”
“They refused to give the Empire what we demand!” Evil Kirk said, jumping out of his chair. “Look at them! They’re pitiful! They’re asking for it!!”
“Captain, I’m beginning to believe that you are not in fact you, but in fact your own evil counterpart from a parallel universe very much like our own.” Spock pointed out. “If that made any sense.”
“Well, duh.” Evil Kirk sneered.
“There, there.” Evil Fat Scotty said, patting Spock on the back. “Evil Fat Scotty still loves ye. Would ye like some chicken?”
If we could just go, for another moment, to the parallel universe!
“I’m sorry that you have to leave.” Evil Spock said. “But I realize now that you wouldn’t last a week in this universe. You’re from a different universe. You’re a hopeless coward.”
“Let me at him, I’ll stretch his pointy ears outta shape so much he’ll be able to go hang-glidin’ with them!” Fat Scotty yelled.
McCoy pushed against him for dear life. “Make the goodbye quick, Captain, I don’t know how much longer I can hold him back!”
“Hold me back, I could break you in half with the cheeks of my ass, come ‘ere you!” Fat Scotty yelled. He grabbed at McCoy, who quickly ran for terror out the doors of the transporter room. Fat Scotty ran after him with surprising speed for someone voted “most likely to become the center of gravity for all known things.”
“Our universes are very much different, except very much undifferent.” Kirk pointed out.
“If you insist.” Evil Spock said, raising an eyebrow. “Live long and prosper.”
“Right, right, the traditional Vulcan greeting.” Kirk remembered. He began to pull down his pants.
“Just stop it right there.” Evil Spock warned him. “I believe our universes are more different then we realized. Incidently.... What did you think of Marlena?”
“Macarena?” Kirk asked. “The traditional Vulcan greeting one?” He reached to pull down his pants again.
“No, not that one!” Evil Spock yelled. “I meant Marlena, the captain’s woman, with whom you could do... anything. And I mean anything.”
A slow realization dawned over Kirk’s face. “You mean I could’ve given her a ‘warp core breach’ and ‘fired photon torpedoes’?” He asked, using quotation fingers.
“‘Yes.’” Evil Spock said, using quotation fingers.
“Oh.” Kirk said slowly. “If you could excuse me for just a moment...”
At that moment, the doors to the transporter room burst open, and McCoy ran in. He grabbed Kirk and dragged him kicking and screaming to the transporter pad. “Come on, captain, we have to go now!!”
“We’ll have words, wee man when we get back!” Fat Scotty yelled at McCoy as he jumped on the transporter pad. “We’ll have words!”
“Captain’s Log, Stardate: Now back in the normal universe. I have been sent back to my normal universe. Status of shagging: still undetermined. Presence of evil counterparts — not anymore. Spock normal with face clean of chocolate: affirmative. The Halipomtiddleypoms did not overly appreciate our presence here, so Starfleet Command ordered us to leave. I also regret to announce that my suggestions of making flowers a standard power source for warp cores has been declared unfeasible. I just hope that my new proposal, mixing together tomato sauce and barbecue sauce to create power sauce, will have more success.”