"Dagger of the Mind" is another episode that sets some precedents for later stories in the franchise. "Meld" from Voyager and "Chain of Command, Part 2" from The Next Generation come to mind. So you know there's some good material here.
For instance, an insane individual who must be figured out. Torture that pushes a character to their limits. That sort of thing. Good stuff. (Yes, for some reason, there are numerous examples of torture in the franchise that produced some really excellent material. Years before the War on Terror. Star Trek addressed the War on Terror so often well before it actually started you'd think...I don't know what you'd think, but it's certainly interesting.)
Is this also the debut of the Vulcan mind meld? Yes it is!
It's actually a pretty good episode, one that may not seem like especially essential viewing for the series, producing no real lasting effects other than that debut I just mentioned (but then, I don't think any fan particularly cares about how that occurred, per say, except that there are more notable moments with the mind meld elsewhere; most of the Star Trek concept was completely in flux in the early years, such as the terms for Starfleet and the Federation, which is taken for granted in other incarnations), no real classic moments or imagery; but perhaps one that takes on much greater significance for being a pretty good episode, generally, and its effects later in the franchise. For that, I'd just recommend watching it, even if you're not seeking to be a completist either for this series or the whole franchise. It's a minor standout.
Also, is the title of the episode one of an increasing number of Shakespeare references that would develop as the series and franchise continued? Yes, yes it is. But, on the chance that someone reading this at some point will be performing this particular play soon after, I will not mention its name.
It might also be notable that although she doesn't appear in this episode, "Dagger" is also crucial to the history of Yeoman Rand and actress Grace Lee Whitney in the franchise. I've been noting in these episode thoughts how often Rand seemed to be so important in the earliest episodes, and how that obviously changed at some point. Well, this is that point. There were a number of reasons why Rand was written out of the series. If you read the Memory Alpha summaries I always include in these things, you'll learn more about that, but in this instance you'll also want to read their biography of Whitney herself to learn even more.
franchise * series * essential * character
Memory Alpha summary