One of the highest quality productions in TOS universe. Kirk is trapped alone in a hidden, unpredictable, parallel universe. Spock deals with the building tension of Enterprise systems failures, Tholian attacks, Kirk's disappearance, and McCoy's ranting in such a calm manner it only adds to the tension. With the crew going berserk, Kirk's ghostly 'phasing in and out' and the Tholians closing the trap, the well-written interactions between McCoy and Spock are the one thing the viewer can hang their sanity on. There's a lot going on and the writers and production staff did a stellar job in telling the story.