An Enterprise away party beams over to the USS Defiant, found dead in space, to assess why its crew had gone mad and apparently mutinied itself to death. When the Defiant begins dissolving and enters an "interphase realm," the party hastily returns to the Enterprise, except Kirk, who is left stranded on the Defiant when a lack of power causes his transport to be delayed.
Spock plans to retrieve the captain when the Defiant returns to normal space from its interphase cycle, provided Kirk's atmosphere suit can keep him alive long enough. The situation grows more complicated when the Tholians intervene, ordering Spock to leave the area, which they claim as their own. Whether it's Spock's interphasic theories, the Tholians' energy webs, or McCoy's medical research to cure the insanity that has spread from the Defiant to the Enterprise, "The Tholian Web" provides a good example of Trekkian tech plots being juggled in relatively interesting fashion. And although the "interphase" plotting rules are conjured at will, they're somehow still believable on the story's terms.
What gives this episode its lasting power, however, is the way Spock and McCoy work with and challenge each other—as McCoy questions Spock's dangerous plan to retrieve the captain at the expense of the ship's safety. Eventually, it is Kirk's final recorded message that reveals the way Spock and Bones require each other for guidance, nicely highlighting the cemented relationships within the Big Three.
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