Kirk becomes convinced that a gaseous cloud that is killing his landing-party crewmen is an intelligent creature—and the same intelligent creature that killed 200 crew members aboard the USS Farragut, on which Kirk served 11 years earlier.
"Obsession" is perfectly titled, showing Kirk at the mercy of his past when he makes the questionable decision of keeping the Enterprise in planetary orbit to search for this creature even while a vital rendezvous with the USS Yorktown has been requested. There's obviously a lot of guilt residing in Kirk's memory of the encounter 11 years ago—which is nicely demonstrated when he levies harsh discipline upon Ensign Garrovick (Stephen Brooks) for hesitating to fire phasers upon seeing the creature. Meanwhile, McCoy and Spock confront the captain for his unexplained decisions in a powerfully executed scene that exemplifies just how well the "big three" work together when the dialog is sharp and performances on target.
The plot is tight, the use of all the characters is effective, and the punchy finale and exciting Sol Kaplan score sends the show out just right. But it's the study of Kirk's feelings of that terrible encounter long ago that sets "Obsession" well above average.