"Star Trek: Discovery" is set ten years before what is now referred to as "Star Trek: The Original Series." However, one episode of "The Original Series" actually occurs before "Discovery." In many ways, it is the beginning of the entire "Star Trek" franchise. While most viewers back in the late '60s probably remember it as the story told in the exciting two-parter "The Menagerie," which reuses the footage captured for the original "Star Trek" pilot called "The Cage."
We have to begin here because these events play a crucial role in "Star Trek: Discovery" Season 2. "The Cage" sees the Enterprise crew, led by Captain Christopher Pike, visiting Talos IV, encountering a strange alien race. These are the Talosians, telepaths who experience life by manipulating beings they keep in their menagerie. By forcing Captain Pike to endure multiple illusions of their creations, they hope to understand love, anger, fear, excitement, and other emotions they forfeited by choosing to further develop their mental abilities.
Years later, after Captain Pike temporarily assumes command of the USS Discovery, he is forced to revisit Talos IV to cure Spock of his current mental disorder. Not only is it a chance to save Spock's life, but it also allows Pike to seek out closure regarding his past experiences on the planet.
