The Enterprise is sent to planet Psi 2000 to pick up a scientific team and to observe the dying planet in its final stages. A member of a landing party investigating the ruins on the collapsing planet contracts an unknown infection and returns it to the Enterprise, where it begins to spread rapidly among the crew. Dr. McCoy works to find an antidote before the crew goes out of their minds.

Stewart Moss (Lt. Joe Tormolen)
Majel Barrett (Nurse Christine Chapel)
Bruce Hyde (Lt. Kevin T. Riley)
DeForest Kelley (Dr. Leonard H. McCoy)
Grace Lee Whitney (Yeoman Janice Rand)
George Takei (Lt. Hikaru Sulu)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
William Knight (Amorous Crewman)
John Bellah ([Dr. Harrison)
Frank da Vinci (Lt. Brent)

• After Riley shuts down the engines, Scotty has to start them up in less than standard 30 minutes to prevent the Enterprise from crashing into Psi 2000. The new procedure that he and Spock creates starts the engines successfully, but it sends them backwards in time.
  
• This is the first appearance of Nurse Chapel, played by Gene Roddenberry's wife, Majel Barrett. Chapel has a crush on Spock. Majel Barrett previously played "Number One" in the unaired pilot, "The Cage". She would later play the voice of the Starfleet computers and Lwaxana Troi on Star Trek: The Next Generation.
  
• This is the first episode Spock uses the Vulcan Nerve Pinch, which he uses on Sulu.
  
• When Spock sends Riley to Sickbay, he orders Uhura to take the Navigator position.
  
• Star Trek: The Next Generation will pay homage to this episode in "The Naked Now".