The Menagerie gives us a tantalizing glimpse of what Star Trek might have been, had the first pilot, The Cage, been accepted by NBC; instead of the brash James T. Kirk, the Enterprise is commanded by an intense, almost grim (and definitely humorless) Christopher Pike, the first officer is a woman, Mr. Spock is still the science officer but is embarrassingly (by Vulcan standards) emotional and the ship's doctor is one half country doctor a la Leonard McCoy and one half Mentat. Had The Cage been given the green-light, Star Trek, as we know it, would have been quite different.
Christopher Pike displaying a very un-Kirk-like mien
As a means of showcasing the story of Captain Pike and his crew, The Menagerie adequately performs the task at hand. As an episode of Star Trek: The Original Series in its own right, it doesn't quite gel; most of the dialogue is exposition leading up to yet another segment of the story of Captain Pike's encounter with the Talosians and too much of the story jury-rigged around The Cage doesn't make any sense: Why were the Talosians willing to help Christopher Pike given the outcome of their first encounter with the captain? Why did Mr. Spock have to do what he did when the Talosians, who were evidently able to project their illusions as far as Starbase 11, could have, by their own machinations, brought Christopher Pike to Talos IV? Why did Christopher Pike keep signaling "No" in response to Spock's actions but then reverse himself when the Enterprise finally settled into orbit around Talos IV? All these problems in The Menagerie betray its origins as nothing more than a vehicle for presenting the footage filmed for The Cage.
Who watches the watchers? The crew of the USS Enterprise from The Menagerie watches the crew of the USS Enterprise from The Cage. Meanwhile, the Talosians (off-screen) are watching everyone. And we, the audience, are watching them all
On a positive note, The Menagerie further develops Mr. Spock's character as well as his relationship with Dr. McCoy; we are treated to Mr. Spock braving the death penalty so that his former commander can live out his days unfettered by his broken body and it is Dr. McCoy who vigorously defends Mr. Spock when Captain Kirk voices doubts about his honesty concerning recent events.