'Obsession' by Art Wallace

While the Enterprise is en route to deliver much-needed medical supplies, Captain Kirk encounters a mysterious, gaseous life-form. Kirk met the entity once before: while serving aboard the U.S.S. Farragut eleven years ago; upon entering the region of Tycho IV, the deadly gas-creature killed Captain Garrovick and half his crew by draining their bodies of red blood cells. To this day, Kirk feels guilty for hesitating before firing his phaser at the creature, despite the fact that the blast subsequently had no effect on it. Consumed with revenge, Kirk deliberately ignores his current orders and sets the Enterprise in pursuit of the creature, desperately hoping that he can destroy it before it can claim any more victims. After the entity attempts to hide on the planet Argus X, Kirk and an armed landing party beam down to search for the monster; amongst them is Ensign Garrovick, the son of Kirk's former captain. However, when the young ensign locates the monster, he too hesitates before firing his weapon, enabling the deadly cloud to kill a crewman and then make its escape. Kirk is furious with Garrovick; he immediately orders a return to the ship and gives chase once more. But when Kirk opens fires on the creature, it turns around, phases through the ship's hull - and makes straight for Spock...

William Shatner (Captain James Tiberius Kirk), Leonard Nimoy (First Officer Lieutenant Commander Spock), DeForest Kelley (Doctor Leonard H. ‘Bones’ McCoy), James Doohan (Chief Engineer Montgomery ‘Scotty’ Scott), Nichelle Nichols (Lieutenant Nyota Uhura), Walter Koenig (Ensign Pavel Andreivich Chekov), Stephen Brooks (Ensign Garrovick), Jerry Ayres (Ensign Rizzo), Majel Barrett (Nurse Christine Chapel)

Directed by Ralph Senensky

TX:
15th December, 1967

Notes:
*Featuring Captain James Tiberius Kirk, First Officer Lieutenant Commander Spock, Doctor Leonard H. ‘Bones’ McCoy, Chief Engineer Montgomery Scott, Lieutenant Nyota Uhura, Ensign Pavel Andreivich Chekov and Nurse Christine Chapel