"Operation Con Game"
While the Enterprise is exploring a neutral sector near the Klingon Empire, sensors pick up traces of dilithium on a nearby planetoid. Mr. Spock reports that the planetoid is Class - M, with multiple life-form readings. Captain Kirk, Mr. Spock and Dr. McCoy beam down to the surface in order to confirm the existence of dilithium, and make contact with the inhabitants.
They arrive at an outdoor pavilion to find another group of Klingons talking with a tall, mysterious hooded being. The native inhabitants of the planet (small, purple child-like beings with large eyes) stare in amazement at the off-worlders. "Ah! Visitors from the federation!" a Klingon says, "You are too late, gentlemen!" "Too late, for what?" asks Kirk. "I assume you have come for the dilithium, Captain!" the Klingon replies, "But this treaty gives exclusive mining rights to the Klingon Empire!". "Treaty with whom?" the Captain asks. The hooded figure speaks. "I, the Grand Qaal of Eulus, have bestowed such rights! This substance which you call dilithium, abounds on our world. But it is of no use to us! The Klingons are welcome to it!".
After the Klingons have gone, Spock tells Kirk that the molecules of the dilithium are in a state of flux. "As if it were synthesized or, worse still, manufactured!" he says. Just then, McCoy tells the Captain that he has discovered another Earth human life reading in the area. "There's an impostor among us!" the Doctor says. "Obviously, he isn't one of these" he continues, pointing at the small, purple natives. "Synthetic Dilithium" Kirk shouts, "An impostor from Earth, the game is over, Mr. Grand Qaal!". He pulls the Grand Qaal's hood away to reveal Harry Mudd - arch thief, liar and con man extraordinary!
"All right, Mudd, start talking, and fast!" Kirk says. "Mind your tone, Kirk!" Mudd replies, "I haven't committed any crime against the Federation!" Spock tells the Captain that Mudd is legally correct, since he only deceived the Klingons. Mudd tells Kirk that he had met a scientist who perfected synthetic dilithium. The scientist knew it had no value, and so had sold it to him very cheaply.
"What about the Klingons?" Kirk asks, "They took a load of that stuff back to their ship! One hard jolt could blow them to space dust!". "Why, that would make me a hero!" muses Mudd. "Imagine, wiping out a Klingon warship, single-handed!". "And what about full-scale war that would start between the Klingons and the Federation!" retorts Kirk. He orders Mr. Spock to contact the Klingons, and warn them of the danger. The Klingons, being distrustful by nature, believe that Spock's warning is a trick, and ignore it.
Back on the Enterprise, Kirk tells Scotty that he and Spock are beaming aboard the Klingon warship, in order to remove the dangerous synthetic dilithium. "Aye, aye, sir" Scotty says, "But if ye don't mind my sayin' so, it's sheer madness!". The two men beam over to an abandoned storage hold on the Klingon ship, and begin their search. Spock is able to locate the dilithium using his tricorder, and he and the Captain disable several Klingon guards to gain access to the room where it is held.
On the planet surface, Mudd uses sign language to signal the native people to take McCoy prisoner. "Once my 'subjects' load the gold onto my space-buggy, you and me are taking a little trip, Doctor! Some place where I can lose you, forever! After all, you're the only remaining witness to my little, uh, transaction!". "What about Captain Kirk and Mr. Spock?" asks McCoy. "The Klingons aren't altogether stupid, you know" Mudd replies. "They gave me this transmitter in case trouble should develop!". Mudd uses it to call the Klingon warship. "This is the Grand Qaal!" he tells the Klingons. "The men from the Federation plan to invade your craft! They seek the dilithium!".
At that moment on the Klingon warship, alarm sirens go off. "Invasion alert!" says Kirk. He and Spock finish collecting the dilithium and are calling the Enterprise to beam them over, when several guards break into the room. "Complete that transmission and you're both dead men!" a guard says, training phasers on them.
The Enterprise quickly locates Harry Mudd's space ship, heading for an uncharted asteroid. Kirk orders the tractor beam to lock on, the small ship is disabled, and the Doctor and Harry Mudd are beamed aboard the Enterprise. Kirk has Mudd's gold transported back to the Klingons. "We thank you for the return of our gold, Captain!" the Klingon commander says. "As for the dilithium, you convinced us of its deadly nature, and we discarded it! In view of the fact that you may have saved our lives, I will overlook this criminal invasion of my ship!". "Why, how generous of you, Commander!" Kirk replies, "As we say on my native planet, you're all heart!". "