This budget saving bridge-based bottle show is basically "The Corbomite Manuever" meets "The Doomsday Machine", but with a focus on the Kirk/Spock/McCoy relationship in place of the guest stars. Specifically, by highlighting Spock and McCoy's common scientific backgrounds and contrasting their specialties, writer Sabaroff brings out their competitiveness while maintaining undertow of hidden respect and friendship, laying the template for the relationship fans would want to see more of. Then there's the captain having to be the captain, forced to decide which of his friends' lives to risk to save the ship. It's all great drama in the short term, but there's also an enormous takeaway: an enriching of the big three's relationship that not only spills into all the future episodes and feature films, but also reverberates into past when old episodes are seen again.
So why is "The Immunity Syndrome" a second tier episode, never showing up on top ten lists and rarely mentioned as a favorite? Well, the truth is it gets off to a slow start that features a lot of pain and suffering (including Spock feeling a great disturbance in the Force, as if dozens of voices suddenly cried out in terror and were suddenly silenced), but little story. And while the episode gets by without any guest stars, it does lack the wild cards and outsider's perspective new characters can provide.
All the same, while "Immunity" might be a poor man's "Doomsday Machine", it's still one of TOS's better second season episodes and worth a look.
Remastered: The genius of the original episode is how it saved money while presenting dynamic new visuals, thanks to a simple colored liquid effect layered into the background. The end result has always been impressive, and the new version simply uses CGI to achieve the same idea. A "zone of darkness" gets a similar treatment, but there's a notable change in the timing of its first appearance. In the original episode, the intention is to have Mr. Spock hit a button to punch it up on the viewscreen. Just one problem: the show's sound editors forgot to take out the sound of Nimoy's finger thudding against the panel and substitute it with some kind of "boop" to sell the idea of a working button. So now CBS Digital comes along to do their upgraded versions, and you'd think the obvious fix here would be to simply add a "boop". (It wouldn't even have to be from the original sound effects library; no one would know the But the CBS team steadfastly refuses to change any audio, so they instead attack the problem a different way. The "zone of darkness" appears before Spock takes any action at all, rendering the button pushing moot and making it seem like he's just tapping the console for effect. This is all well and good, except for Uhura. In the new version, the effect is already up on the viewscreen when she asks Mr. Spock what he's looking for. When he says, "I would assume, that" and indicates the viewscreen, it looks like Uhura simply isn't paying attention.
Meanwhile, there's also an oddity with the shuttle bay. The original version uses both a live set (for the actor's to use) and a miniature set (to show the shuttle rotating in preparation for launch), but the floor markings for the two don't match. The remastered version uses CGI to replace the miniature set and has different floor markings, but they don't match the live set either! The real issue here is that both versions steal their "shuttle about to launch" shots from other episodes ("The Galileo Seven" for the original and "Journey to Babel" for the remastered). In the end, it doesn't really matter too much, and these sort of discrepancies can be part of the show's charm. Besides, the added benefit of using stock footage is that it frees up the effects team to work on something else, with the CBS Digital team using the opportunity here to create a whole new look for the Enterprise for when there are no stars to light it. (Truth be told, lighting for space sequences is usually a cheat. But in this case there's such an extreme circumstance, it makes sense to drop the Less noticeable, but just as impressive, CBS Digital fixes a quick shuttle shot (where Spock falls down) which has dark windows in the original because the camera is shaking, making it impossible for a 1968 effects crew to composite anything into them. Using some fancy rotoscope work, the CBS team adds the proper view outside.