Episode Title: "OBSESSION"
Production Number: 47
Original Air Date: 12-15-67
Captain Kirk leads a landing party on a planet, where they find a mineral vein of pure tritanium, which is 21.4 times as hard as diamonds. As the landing party collects a tritanium sample and prepares to return to the Enterprise -- to make its rendezvous in eight hours with the U.S.S. Yorktown, which is carrying perishable vaccines for the Theta Seven colony -- Kirk detects a familiar di-kironium cloud creature, but he is unable to stop it from injuring Ensign Rizzo and killing two members of the landing party by absorbing all of their red corpuscles.
Eleven years ago, 1,000 light-years away on Tychos Four, young Lieutenant Kirk first encountered the cloud creature while serving on the U.S.S. Farragut in his first deep-space assignment. Nearly half the Farragut's crew and Captain Garrovick -- 200 crewmen -- were annihilated. Kirk blamed himself for the deaths becaused he hesitated before firing at the creature.
Ensign Garrovick, the new Enterprise security officer and son of Captain Garrovick, leads another landing party on the planet to find the cloud creature, but he hesitates beforing firing his phaser when the creature attacks his men, and it escapes. Later on the Enterprise, Kirk relieves Ensign Garrovick of his duties and confines him to his quarters.
When the cloud creature leaves the planet and travels through space at warp speeds, the Enterprise pursues it. The creature turns and approaches the Enterprise, eluding the ship's phasers and entering the Enterprise through the number two impulse vent.
After the cloud creature attempts to attack Commander Spock, but is not able to absorb his green Vulcan blood, the creature leaves the ship and moves away at warp speed. Kirk orders the ship to head to the Tychos star system, which he believes is the cloud creature's home. On Tychos Four, Kirk and Ensign Garrovick set a trap with a container of hemoplasm to attract the creature, and a bomb containing an ounce of antimatter that will destroy the creature when it approaches.
Factoid: The element di-kironium exists only in laboratory experiments.
Factoid: Captain Garrovick was Kirk's commanding officer from the day he left the academy.
Factoid: An ounce of antimatter is more powerful than 10,000 cobalt bombs. A matter-antimatter blast will rip away half a planet's atmosphere, and shockwaves could damage a vessel in orbit.
McCoy: "One man has a chance for survival. The other is dead."
Guest Characters: Nurse Christine Chapel, *Ensign Rizzo, *Ensign Garrovick, *Henson, *Bardoli, Lt. Leslie dies
KEY: [brackets]=illusions *star=first appearance