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.

In this pilot episode, the Enterprise is lured to a planet by a repeating distress call. Capt. Pike and the crew find the aged survivors of a crashed spaceship. However, the crash and the survivors turn out to be illusions created by an advanced race of beings, with mysterious motives. They capture Capt. Pike and watch as they put him through false imagined adventures. But, the same woman keeps appearing in all these adventures with him. Can Capt. Pike learn what the beings' motives are, and can the Enterprise crew rescue their captain in time?