The Enterprise Incident
The Romulans have developed a prototype of a cloaking device, and Starfleet creates an elaborate plan (as carried out by Kirk and Spock) to steal it, lest the balance of power tilt too far.
One of the better thrillers, even if it take a whole lot of suspension of disbelief to watch. For instance, how could the bridge crew not have suspected something up with Kirk? He's blatantly overplaying the "burnt-out captain" pose. (Viewers certainly aren't fooled.) Even presuming they believe him, it takes many coincidences for the plan to work. But work it does, and the details are not revealed till later on, creating suspense. The Romulan commander is one of the more noteworthy guest stars, equally defined by her duty to the Empire and longing for passion. Spock can apparently whoo any woman if his duties require it. The plan gets silly with the Romulan disguise for Kirk, but at last we know what they're after. It also seems underhanded to use the device, and Starfleet is definitely on the offensive. Yet that's one thing I've always liked in TOS: since Starfleet has a military component, they will be involved in military actions (like espionage), so claims of a truly peaceful galaxy ring hollow. Whether the writer recognized this is unknown, but they do explore it.
Spock Saves the Day: Spock romances the Romulan commander, gains her trust, "kills" Kirk, and distracts her while he beams aboard.
Oops: There is some confusion over this death grip. No such thing? Well, apart from tal s'hy'a, mentioned in Journey to Babel. But what Spock does here is a dramatic mind meld. That's more an oops on confused viewers' parts, but it seems a good place to clear it up.
Developments: Again, we can't really believe Spock would kill Kirk under circumstances such as these. But what is captivating is the subtle romance with the commander: for once, Spock seduces while staying totally in character. (Since it's mostly a deception, it doesn't appear in Relationships.) Spock's comment that they "shared something more permanent" than military secrets seems like an apology or admission of shared pleasure in the mission rather than a declaration of true love. It wouldn't be hard for Spock to run away with her. But he's Starfleet, and will stand by his captain.