The original unaired Trek pilot, "The Cage," becomes the subject of the series' only two-part episode when Spock commandeers the Enterprise to take Christopher Pike (Jeffrey Hunter)—the former captain of the Enterprise, who has been paralyzed in a recent accident—to the off-limits planet Talos IV. To reach there would invoke the only death penalty still on record.While en route, Spock undergoes a court martial held by Kirk and a Starfleet Commodore. Part of Spock's testimony is the story of Pike's original visit to Talos IV 13 years earlier. The testimony is broadcast from the Talosians themselves.
Although the episode was designed to save money, "The Cage" is nevertheless a Trek adventure worth unearthing. Although coming across as even more dated than an average TOS episode, it benefits from an ability to say something about a human inner struggle of mind over matter. The device used to tell this story is clever, if a little forced. Spock's theft of the Enterprise was nicely executed, and Kirk's decision to chase the Enterprise in a shuttle (hedging his bets that Spock would have to turn around to rescue Kirk before his shuttle runs out of fuel) also demonstrates ingenuity.
The shifting back and forth between "The Cage" footage and the new footage is sometimes awkward and is accompanied with rather weak explanations, but the suspense factor (What happened on Talos, and why are we going back there?) allows us to forgive all. The ending works pretty well too ... but I still wonder why Pike was so adamant on not going to Talos IV from the outset.