Plot
On stardate 3619.2, the starship USS Enterprise has sent down a landing team to a planet to search for tritanium deposits, an element "21.4 times" as hard as diamond. While surveying, Captain Kirk notices a sweet honey smell, and has his men go on alert. He tells them to scan for dikironium and immediately fire at anything that looks like a gaseous cloud. The landing party fans out to explore.
One group picks up a signal but loses it, at which point they are immediately attacked. Kirk and his Science Officer, Mr. Spock, rush over to the scene, finding three crewmen down; two dead, the other, Ensign Rizzo, near death. All of the men have had their red blood cells drained of hemoglobin. Kirk fears that the killer is the same space-travelling vampiric entity he had encountered 11 years before while serving aboard the USS Farragut. As a young lieutenant aboard that vessel, Kirk had hesitated in firing the ship's phasers at the creature, which promptly attacked. The encounter killed half the Farragut's crew, with Kirk and the ship's first officer among the few survivors; Kirk felt great guilt over what happened, even though on the subsequent official record, the XO had praised the young Kirk for his actions in battle, describing Kirk as "a fine young officer who performed with uncommon bravery."
The landing team quickly returns to the Enterprise. Once aboard, Kirk becomes obsessed with finding this entity, feeling guilty that it caused so much damage and then eluded him. He ignores his priority orders, to rendezvous with the USS Yorktown and transfer urgent medical supplies destined for Theta VII, until Chief Medical Officer Dr. McCoy can confirm the cause of the men's deaths. McCoy, however, is at a loss, and complains that he doesn't even know where to begin. Kirk tells him to reference the Farragut's medical logs and start there. On the bridge, Kirk has Spock look for a gaseous entity that is able to conceal itself and pass through solid matter. Spock is intrigued about such a creature, fascinated that it can move through space and time at will. He suggests that it must be able to control its molecular structure, but Kirk cuts him off, stating he doesn't want a scientific analysis and just wants the creature found.
Kirk calls one of the security officers, Ensign Garrovick. to the bridge. The ensign's father, Captain Garrovick, was the commanding officer of the Farragut, and also lost his life to the entity. Kirk believes Garrovick's son will be just as driven in finding the creature as he is and assigns him to the next landing party that will hunt the entity down. As soon as they prepare to leave, Dr. McCoy reports that Rizzo has died of his injuries.
Kirk returns to the planet and orders his landing party to fire at the entity as soon as it is spotted. He informs his men that the creature attacks quickly, so any hesitation will be deadly. The team spreads out and Ensign Garrovick soon encounters the creature. He aims his phaser, but hesitates to fire, becoming frozen with fear as his partner is killed. The creature moves off just as Kirk arrives. Back on the Enterprise, Garrovick admits that he hesitated before firing his phaser and is confined to quarters.
McCoy has seen enough and confronts his Captain on his reckless obsession over the creature. He summons Spock to consider Kirk's fitness for command. Kirk justifies his actions by maintaining that the creature is an intelligent predator and a serious threat to inhabited worlds. He warns that if it manages to travel to a populated system, there is no telling how many it will kill. McCoy finally lets off as Ensign Chekov reports that he is tracking the cloud creature moving away from the planet.
The landing party quickly returns to the ship and Kirk orders a pursuit of the cloud at Warp 8. The ship gives chase, but Chief Engineer Scott doubts the Enterprise can sustain the high speeds for long. Kirk orders a reduction of speed and curiously, the entity slows down as well. It slows as if preparing for a confrontation and allowing them to catch up. Kirk orders battle stations and as the entity cloud comes into range, he fires weapons. The phaser streams and torpedoes pass right through the being without incident. Not only do the weapons have no effect, but neither do the deflector shields - the creature passes right through the ship's hull and into the ventilation system. Kirk orders a shutdown of life support, limiting the crew to two hours of air. He orders Scotty to close all vents and flood the system with gas.
After a conference, Spock points out to Kirk that since the creature is immune to conventional weapons fire, he needn't blame himself for hesitating to fire in the earlier incident. Firing promptly on the creature would have made no difference, and the resulting deaths by the creature were impossible for Kirk to prevent.
Meanwhile, the creature exits the vents and kills two crewmen. It then enters the room of Ensign Garrovick (he had accidentally activated the ventilation system bypass) and tries to attack him, but Spock had already entered shortly before the creature's entrance. The creature gets a sample of Spock's Vulcan blood, which repels it due to the alien composition of Vulcan blood. The creature quickly lets Spock go and flees the ship. Dr. McCoy comments that Spock's green blood "must have left a bad taste."
The creature makes its way to the fourth planet in the Tycho system, and the Enterprise heads there in pursuit. Kirk wonders if the planet is the creature's natural home, since that is where the Farragut originally encountered it. Kirk plans a trap for the creature while Spock comments that the creature may be feeding in order to reproduce. If the entity is not stopped they could be facing more of them.
Kirk and Ensign Garrovick beam down to the planet with a case that holds an antimatter bomb. The two lure the creature with blood from the ship's medical supply, but it quickly gobbles up the "bait" before they can lead it to the bomb.
Kirk decides to use himself as bait and remain behind to detonate the bomb. He orders Garrovick back to the ship, but the young officer refuses to abandon his Captain and a fight ensues before Kirk can explain that he does not mean to sacrifice himself. The two ready the bomb as the creature draws near. With the creature ready to envelop them, Kirk and Garrovick beam away and the antimatter bomb explodes, annihilating the entity and much of Tycho IV with it.
Once safe aboard the ship, Kirk takes Garrovick aside to thank him and tell him a few stories about his father, Captain Garrovick.
Read more about this topic: Obsession (Star Trek: The Original Series)
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