The City on the Edge of Forever

Ripples in time have provoked the curiosity of the Enterprise crew. One such ripple shakes the ship and gravely injures Sulu. McCoy brings him back from the brink with a powerful drug, but another ripple results in a tragic overdose, afflicting McCoy with dangerous paranoia! He beams down to the planet - the transporter was locked onto the source of the time disturbances. On the surface, he evades the landing party long enough to flee - into the past, where his psychosis results in him somehow erasing the Federation, and the Enterprise, save only the landing party. Their one chance is to use the same artifact - the self-styled Guardian of Forever - to arrive before McCoy and prevent him from changing their history.