It is a well known fact that when Gene Roddenberry first pitched Star Trek to NBC, the original pilot episode, The Cage, was rejected for being "too cerebral". When the series was given another chance, Roddenberry thought it would be fun to establish the events of the rejected episode as canon, and did so by writing The Menagerie, which has the unique distinction of being the sequel to what was still, at the time, an unaired episode.
This time, rather than exploring a new planet, Kirk and his crew are on Starbase 11, paying a visit to the former commander of the Enterprise, Christopher Pike (Sean Kenney), now horribly disfigured and paralyzed because of an accident. Pike joins his successor on the starship, where an unpleasant surprise awaits: Spock, who used to serve under Pike, has effectively hijacked the vessel and set the course for Talos IV, a planet which is off-limits (the punishment is death) since Pike and Spock's last visit there, 13 years earlier. Naturally, being a logical creature, Spock turns himself in and arranges a court-martial so that he can justify his actions.
There's no need to say more about the plot, since the rest will play out in Part 2. What really impresses is how Roddenberry creates the connection between The Cage and the rest of the Star Trek universe, by coming up with a particular type of flashback (to say more would be too much) that allows everyone, on screen and off, to see what could have been of Trek, had NBC not turned down the original project. In particular, it's fun to see Jeffrey Hunter (who was unable to return in The Menagerie) play Pike as a more serious captain than Kirk usually is and Nimoy's early days as Spock, whose personality hadn't been fully established yet: this is the only time in the entire series that everybody's favorite Vulcan spontaneously grins.
In short, not just a great "mystery" episode, but also a treat for those who can't be bothered to track down The Cage in its original form (it's available as part of the Season 3 box set).
