After a battle with the Romulans, a strange energy field causes the Enterprise computer to gain an artificial intelligence, leading it to play practical jokes on the crew. However, the humor soon turns to danger.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Nurse Christine Chapel)
James Doohan (Lt. Cmdr. Montgomery Scott)

• This is the first appearance of the Rec Room, a pre-cursor to the holodeck. The computer traps Sulu, McCoy, and Uhura inside, also making this the first episode with a holodeck malfunction.