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.