Written by a wife and husband freelance team, this ship-based sci fi ghost story is the third season's apologist's favorite episode to throw in the face of all those who criticize the last year of the show.
Like a combination of the best ideas of the season, "Tholian" includes a major crewmember's apparent death, a very alien-like alien, and rich interplay between Kirk, Spock, and McCoy -- though the episode sets itself apart by including everyone as well, making it one of TOS's rare ensemble pieces.
The heart of the story is Spock, giving Nimoy one last great use of the character before the cancellation of the series. Another "Spock in Command" show with McCoy breathing down the Vulcan's neck, the episode sees Spock challenged by one problem after another, with the issues piling up faster than he can come up with solutions. The most imaginative of these is the titular web that threatens to trap the Enterprise, a far more striking visual than the simple space battle another writer would probably use, and an effective ticking clock to boot. (On the other hand, the most annoying issue is Dr. McCoy, with the writers overplaying his favorite pastime of pestering Spock. There's a fine line between being cranky and being a dick, and the doctor crosses it faster than Kirk jumping on a yeoman, though writers somewhat make up for it by finally providing the characters with a touching resolution that ingeniously uses Kirk - from beyond the grave - to bring them back together).
As with most of the third season episodes, this one's technically a bottle show, though not a cost saving one, even with no guest stars. The writers are savvy enough to borrow a page from "The Doomsday Machine" and "The Ultimate Computer" by using existing sets and models to represent another ship, but between the new space suits (only seen here and in one other episode) and the Tholian effects, this is one of the most expensive episodes of TOS.
Fortunately, Director Herb Wallerstein and cinematographer Al Francis both do a fine job in their debuts, with Wallerstein taking over for Ralph Senensky (who began directing this episode before being fired) and Francis taking over Jerry Finnerman (who quit). Together, they make an episode of Star Trek that's largely devoid of its leading man one of the more interesting offerings.
ENT picks up a loose thread left here in its fourth season episode, "In a Mirror, Darkly", which simultaneously serves as a prequel to TOS's "Mirror Mirror" and sequel to this, with a story that features Captain Archer discovering the Enterprise's lost sister ship.
There's a reason fans love to create new effects for "The Doomsday Machine" but usually leave this episode alone: the original effects are superb, earning an Emmy nomination. Nonetheless, CBS Digital does a great job with their replacement shots, paying homage to the original while adding some subtle improvements that don't draw attention to themselves. The new web is nearly identical to the original, but CGI allows CBS to give it the proper perspective, with the threads on the far side smaller than those in the foreground. (There's even some fine rotoscoping work to integrate a floating Kirk into the new shots).
Along with some new angles and better "interphase" shots for their upgraded constitution ships, the team also includes more detailed ships for the Tholians (though for some reason they give these alien vessels more neutral colors than the originals) but happily leaves the viewscreen shots of the Tholian commander alone. A brief battle between the ships is redone through CG and comes across as a little bit cartoony but does the job.