Oh, On The Starship Enterprise

"You must get back to your universe. I must have my captain back. I will operate the transporter." the goateed mirror Spock said. "You have two minutes and ten seconds."
"In that time, I have something to say. How long before the Halkan prediction of galactic revolt is realized?" Captain Kirk asked.
"In 240 years." Mirror Spock replied.
"And what will be the inevitable outcome?" Kirk asked.
"The Empire shall be overthrown, of course." Mirror Spock replied.
"The illogic of waste, Mister Spock. Waste of lives, potential, resources, time. I submit that your Empire is illogical, because it cannot endure! I submit that you are illogical for being a willing part of it." Kirk said.
"You have one minute and twenty-three seconds left" Mirror Spock said.
"If change is inevitable, beneficial, predictable, doesn't logic demand that you be a part of it?" Kirk asked.
"One man cannot summon the future." Mirror Spock said.
"But one man can change the present!" Kirk shouted. "Be the captain of this Enterprise, Mister Spock! Find a logical reason for sparing the Halkans and make it stick. Push until it gives. You can defend yourself better than any man in the Fleet!"
"A man must also have the power." Mirror Spock said.
"In my cabin, there's a device that will make you invincible." Kirk said.
"Indeed." Mirror Spock said, raising an eyebrow.
"What will it be? Past or future? Tyranny or freedom? It's up to you." Kirk said. 
"It's time." Mirror Spock said.
"In every revolution, there's one man with a vision." Kirk said.
"Captain Kirk, I shall consider it!" Mirror Spock said. He then activated the teansporter and sent Captain Kirk, Dr. McCoy, Scotty, and Uhura back to the Federation universe.
Reappearing in the Federation universe, Kirk, McCoy, Scotty, and Uhura saw a beardless Spock. They got off the transporter.
"Spock." Kirk said.
"Welcome home, Captain." Spock said.
On the bridge of the USS Enterprise, Captain Kirk said "What I don't understand is how you managed to identify our counterparts, Spock."
"It was far easier for you as civilized men to behave like barbarians then it was for them as barbarians to behave like civilized men." Spock said. "I assume that they returned to their Enterprise the same time you did."
"That Jim Kirk is in for some changes if I read my Spocks correctly." Kirk said.
"Jim, I kinda liked him with the beard better. It gave him more character." McCoy said. "Of course almost any change would be a distinct improvement."
Kirk chuckled and said. "I always thought Spock was kind of a pirate by heart."
"I had the opportunity to observe your counterparts quite closely." Spock said. "They were brutal, savage, unprincipled, uncivilized, treacherous. In any way, perfect examples of Homo Sapiens, the very flower of humanity. I found them to be quite refreshing."
"I'm not sure, but we might have been insulted." Kirk said.
"Yes, I'm sure." McCoy said.
Later, in the rec room, Uhura sang "Oh, on the Starship Enterprise" while Spock played his Vulcan lyre.