Dr. McCoy changes the past

As the Enterprise investigates ripples in time which are resulting in spatial disturbances, Sulu has a heart flutter after his control panel short circuits. Bones gives Sulu cordrazine, a mind-blowing drug at high dosages but a valuable cardiac medicine in small dosages. Sulu immediately recovers, but McCoy accidentally injects himself with the remainder of cordrazine in the hypo when the Enterprise passes through a particularly strong space disturbance. McCoy freaks out and becomes extremely paranoid, knocking out the transporter chief and beaming down to the planet to escape the ship of "murderers."

The transporter had been locked in on the center of the time ripples, and when Kirk, Spock, and a landing party follow McCoy down, they discover 10,000 century-old ruins surrounding the annulus-shaped structure from which the time distortions are emanating. The structure talks with the landing party, identifying itself as the Guardian of Forever, and is apparently a time portal. Spock finds the crazed McCoy and nerve pinches him, but McCoy recovers and rushes through the portal before anyone can stop him.

Communication with the Enterprise immediately ceases, and the landing party concludes that McCoy's actions changed the past, affecting the present.