The Enterprise encounters a record-marker left behind by the S.S. Valiant 200 years ago as it tried to go through an energy barrier at the rim of the galaxy. The Enterprise sets course to the rim of the galaxy, where the energy barrier damages the Enterprise's engines and causes a strange behavioral change in Kirk's close friend and Enterprise First Officer, Gary Mitchell, who has gained incredible extrasensory powers and god-like abilities.

Gary Lockwood (Lt. Cmdr. Gary Mitchell)
Sally Kellerman (Dr. Elizabeth Dehner)
George Takei (Lt. Hikaru Sulu)
James Doohan (Lt. Cmdr. Montgomery Scott)
Lloyd Haynes (Lt. Lloyd Alden)
Andrea Dromm (Yeoman Smith)
Paul Carr (Lt. Lee Kelso)
Paul Fix (Dr. Mark Piper)

• This episode was the second pilot, the first episode produced (not counting the un-aired pilot). It was created nearly a year before it aired and there are numerous differences between this episode and the others, including the ship model, the uniforms, and Spock's ears and eyebrows. For some reason, it was the 3rd episode aired on NBC.
  
• Gary Mitchell and Elizabeth Dehner were chosen because they had the highest ESP ratings on the ship.
  
• This is the first appearance of Chief Engineer Montgomery Scott.
  
• Sulu is the head of the Astro-Sciences Department in this episode, and wears a blue uniform.
  
• Mitchell kills helmsman Lt. Lee Kelso. (He is the first crewman to die if you go by production order.)
  
• This episode displays some incredibly sexist behavior. Mitchell, the ship's First Officer, makes a pass at Dr. Dehner and when she rebukes him, he calls her a "walking freezer unit," which he and Kelso find funny. I guess this behavior was acceptable in the 60's, but it seems very "un-Starfleet".
  
• The Enterprise is at the rim of the Galaxy in this episode. That would be approximately 25,000 light years from Earth. Later information about the capabilites of the starships would render this trip impossible — it would take 25 years for the Enterprise to travel this distance, and another 25 to return.
  
• When Mitchell creates Kirk's tombstone, it is labeled JAMES R. KIRK. It will later be established that Kirk's middle initial is T. (Apparently, Gary does not know Kirk's middle name.)