Star Trek's second horror episode works better than the first, partly because of its innovative A/B story (one for Kirk on the planet and another for Spock on the ship) and partly due to some fine guest performances.
The first half of the episode, as a tease by the writer, is a combination of tension and release. When Dr. Van Gelder sneaks aboard the Enterprise, he keeps dropping hints (intentionally and unintentionally) that something is very, very wrong at the penal colony he came from. But each time we find out something surprising (like the fact that he's not an escaped inmate but a fleeing doctor) Dr. Adams (the king of "nothing to see down here") explains it away in a way that allays our fears... or at least Captain Kirk's. It's a clever device, because if writer Bar-David tips his hand too soon and turns the episode into a chamber of horrors from early on, it turns into another "Man Trap" where the horror becomes monotonous and tedious. Instead, it plays out more like a James Bond movie, with Kirk and the hot scientist (well played by Marianna Hill) who can't get enough of him having plenty of interesting moments (including a rare Star Trek fantasy scene) while visiting Dr. Evil's torture chamber.
As this is all going on, Spock and McCoy spend the B story aboard the ship trying to get coherent information from Dr. Van Gelder. This includes an interesting moment: the writers need a way to get inside Dr. Van Gelder's subconscious and after considering (and discarding) Spock performing hypnotism, they invent a science fiction version of it, the Vulcan mind meld. What's surprising, looking back, is how well developed the idea is from the get go. It changes a little over the course of Nimoy's fifty year trek, but the basics of its theory and execution come fully packaged in its introduction. Meanwhile the idea of a planet-based A story coupled with a ship-based B story, of course, becomes a Star Trek staple.
Still, as a budget saving episode there's not too much of visual interest (other than Marianna Hill's bottom peeking out from under her uniform) and the story doesn't include any turn of events too surprising, making Star Trek's first episode with a Shakespearian title a somewhat average offering.
Remastered Version: As with "What Are Little Girls Made of?" this episode largely takes place on a planet underground, leaving little for CBS digital to do. Their work, however, is quite an improvement over the original - which to cut costs reuses shots of the Enterprise in orbit from "The Enemy Within" and reuses a matte painting from "Where No Man Has Gone Before". (In the days before reruns, video cassettes, and more modern viewing technology, this was fine. Today, however, many fans will recognize the penal colony's exterior as the Delta Vega lithium cracking Happily, the creators of the new effects choose not to honor the original with similar designs and instead give us a completely new looking planet (with a gorgeous, if improbable, ring) and a new (more basic) design for the colony's exterior.