"Obsession" is the thirteenth episode of the second season of the American science fiction television series Star Trek. Written by Art Wallace and directed by Ralph Senensky, it was originally broadcast on December 15, 1967. In the episode, Captain Kirk becomes obsessed with destroying a deadly cloud-like entity that killed a crew he was on in the past. Plot During a planetary survey, three members of a landing party from the Federation starship USS Enterprise are attacked, their blood drained of hemoglobin. Fearing the killer to be a gaseous entity he had encountered eleven years before while serving aboard the USS Farragut, Kirk neglects a rendezvous with the USS Yorktown to hunt for the creature. While the Enterprise crew fails to find the creature using shipboard sensors, the creature attacks a second landing party, draining hemoglobin out of two more crew members. Back on the ship, when Kirk learns that security officer Garrovick hesitated to fire his phaser upon the creature, he relieves the ensign of his duties. Chief Medical Officer Dr. McCoy, having reviewed the records of Kirk's previous encounter, confronts the captain over his obsession with the creature; as a young lieutenant, Kirk had hesitated in firing phasers at the creature, which subsequently killed half the Farragut crew. Though given a commendation for bravery, Kirk continues to blame himself for the deaths. Kirk maintains that the creature poses an urgent threat. Ensign Chekov interrupts to report that the cloud creature is moving away from the planet. The Enterprise chases the creature until it turns around and advances on the ship. Kirk orders weapons to fire, which has no effect. The creature then passes through the shields and into the ventilation system. Spock points out to Kirk that, since the creature cannot be harmed with conventional weapons, the captain has nothing to regret about his earlier encounter. Determined to combat this illogical human guilt reaction, Spock is trying to convince Garrovick he did nothing wrong, when the creature emerges through a vent in Garrovick's room. Spock, after forcing Garrovick to leave, tries to shut it out, and is enveloped, but his copper-based green blood repels it. Realizing that neither he nor Garrovick could have harmed the creature, Kirk orders the ensign to return to duty. The creature finally leaves the ship. Believing it to be heading to the Tycho system to spawn, Kirk and Garrovick beam down with an