"The Menagerie," in the off-chance you've actually seen "The Cage," the original pilot, probably has a ton of familiar material. That would be because it basically is "The Cage."
It's also the first two-part episode in franchise history. Although I will limit my thoughts to a single entry, because for all intents and purposes, there are very few distinguishing elements between the two installments. It's basically one long episode. Because it has one episode, stuck in the middle of another episode.
And it's probably the better for it. Easily one of the most clever things the series ever did, sort of like "Trials and Tribble-ations" from Deep Space Nine, well before the fact, splicing disparate stories together in the most literal way possible.
This is one of those TV experiences that asks you to believe memory can look exactly, well, like a TV experience. Mission recorders happened to capture every moment of the events from "The Cage" (calling the NSA!) so that they could later be played back at Spock's trial.
Spock's trial??? Yes. And that's another part of the cleverness on display. Spock is clearly in the scenes from "The Cage." So he's our link between these stories. So when everyone's trying to figure out what's going on with Captain Pike (memorably depicted in a catatonic state in a futuristic...boxy wheelchair device; not depicted by original actor Jeffrey Hunter, by the way), and clearly Spock has been aiding in his shenanigans, our Vulcan friend is strongly believed to have gone rogue.
There are other episodes in the franchise where characters seem to have gone rogue when they really haven't ("The Enterprise Incident" from this series, for instance, or "Clues" from The Next Generation), by the way, as well as others where a main character is put on trial ("The Drumhead" from Next Generation, "Rules of Engagement" from Deep Space Nine, "Judgment" from Enterprise, and of course Star Trek VI: The Undiscovered Country).
The last clever element I'll mention is that "Menagerie" is not just "Cage" repackaged, but also, incredibly, a sequel. It explains how one series of events leads to another, and actually provides resolution to the earlier ones. As such it is the earliest serialized storytelling of the franchise, although no one really knew that at the time, because "Cage" really didn't exist as far as most people were aware.
It also, besides, provides layers of history. Spock's prior service acknowledged and even depicted. Pike's career.
All around, good stuff, well worth acknowledging as a classic in franchise lore.
It's so good, in fact, that it won a Hugo.
franchise * series * essential * character
Memory Alpha summary 1 & 2