Dr. Leonard McCoy changes history

Stardate: Unknown. While the Federation starship USS Enterprise is investigating temporal disturbances from a nearby planet, Lt. Sulu is injured in an explosion. Chief Medical Officer Dr. McCoy gives him a shot of cordrazine and accidentally injects himself with an overdose when the ship is shaken. Delusional, he flees from the Bridge and to the Transporter Room, beaming himself down to the planet. Captain Kirk beams down with a landing party to look for McCoy and finds the source of the time distortions, an ancient glowing ring with an aperture the height of a human. The "Guardian of Forever" explains that he is a doorway to any time and place. While Spock is recording historic portal images, McCoy escapes through it, into the past. Suddenly the landing party loses contact with Enterprise and is informed that the past has been altered and the ship - along with the entire Federation - no longer exists.