The very first Star Trek episode made (not counting the pilot episode, The Cage), although not the first aired. It differs from subsequent episodes in that there is no ``Space, the final frontier'' voice-over during the theme song at the beginning.
Near the edge of the galaxy, the Enterprise discovers a 200-year-old ship recorder from the SS Valiant. Shortly thereafter, the Enterprise passes through an unknown phenomenon which causes major damage and knocks out navigator Gary Mitchell and Dr. Elizabeth Dehner (both of whom have high ESP ratings). When Gary recovers, he begins to acquire telepathic and telekinetic powers. Kirk, alarmed at the prospect of having his ship taken over by an increasingly powerful and tyranical Mitchell, is convinced by Spock to maroon Mitchell at the lithium cracking plant of Delta Vega. Dr. Piper has no explanation for what is happening. Gary kills Lee Kelso, and escapes from his imprisonment. Kirk follows him and is able to destroy him with the help of Dr. Dehner, who is also beginning to acquire the power, but in the process, kills herself.