The Enterprise Disappears (The City on the Edge of Forever)

On stardate 3134.0, the Federation Starship U.S.S. Enterprise investigates temporal disturbances centered on a nearby planet. During the investigation, Sulu is caught in a console explosion and suffers a heart flutter. Doctor McCoy is summoned to his aid and decides on a cordrazine shot to awaken him. Moments later, McCoy accidentally injects himself with an overdose of serum causing him to become violently paranoid. Delusional, McCoy flees from the bridge and beams down to the planet.

Captain Kirk forms a landing party made up of two security guards, himself, Spock, Scotty, and Uhura. Once on the planet, Spock finds that the source of the time distortions is an ancient ring of a glowing, stone-like material. The ring speaks and identifies itself as the "Guardian of Forever", explaining that it is a doorway to any time and place — with periods of history displayed in the opening. The team soon locates McCoy, however he runs away and leaps through the portal before anyone can stop him. Suddenly the landing party loses contact with the Enterprise. The Guardian informs the landing party that history has just been altered and as a result, the Enterprise has disappeared.