Orbiting an unknown planet that appears to be the source of strange, disruptive ripples in time, the Enterprise is violently buffeted by one of these ripples. The ship is so badly shaken up that the helm controls explode in Mr. Sulu's face, seriously injuring him. Dr. McCoy is called to the bridge, where he risks giving Sulu a small dosage of cordrazene, a miracle drug that can cure with just a few drops. Sulu is revived without any side effects, and McCoy starts to put the hypo-spray away until the ship is struck by another violent time ripple, causing McCoy to accidentally inject himself with a full dosage of cordrazene. Driven insane by the drug to the point where he thinks everybody is out to kill him, McCoy escapes captivity and knocks out the transporter chief. He beams himself off of the ship, and onto the mysterious planet below, where the transporter was automatically locked on.

Kirk and Spock lead a landing party down to the planet's surface in pursuit of the frenzied McCoy. They encounter The Guardian Of Forever, a sentient gateway through time and space, which has been the cause of the ripples in time that rocked the ship. The Guardian proves to be benevolent, if lacking some tact with regards to the ignorance of the Enterprise crew in advanced time/space technology (something which ticks off Spock to no end). It shows Kirk scenes from the past on earth, and the Captain quickly comes up with a plan to use the gateway to prevent the accident that caused McCoy to inject himself with the cordrazene. However, before they can even set the plan in motion, the frenzied McCoy slips from the land party's grasp and dives into the gateway, vanishing in a past century on earth.

The effects of McCoy's presence in the past is immediately felt when the Guardian informs Kirk that the Enterprise, along with everything else in their era, has been swept away. Kirk and his landing party are all alone, having been marooned on the planet. Kirk and Spock decide to use the Guardian to go back in the past a week before McCoy does, in the hopes of stopping him from changing the past. They wind up in America in the year 1930, working in a soup kitchen that is run by Edith Keeler, an idealistic young woman who, despite the grimness of the day, is full of hope for the world of tomorrow. Kirk finds himself falling in love with this charming woman, while Spock discovers a chilling fact. In a few years Edith Keeler will become famous as a peace advocate. Her influence will delay America's involvement in the Second World War to the point where the Axis Powers will win, thus changing all of earth history from that point on. Kirk realizes that in order to save the future, Edith Keeler, the woman he loves, will have to die.

Written by SF legend Harlan Ellison (who has famously denounced the filmed version as being a bastardization of his original script), "The City On The Edge Of Forever" is a stark, memorable drama that deals with making a sacrifice for the greater good. Joan Collins, better known to later generations as bitch queen Alexis on the nighttime soap Dynasty, gives a terrific and sympathetic performance as Edith Keeler. One can see why Kirk falls for this woman, and can easily understand the morose, haunted look on his face at the end of the episode. "The City On The Edge Of Forever" is a time travel story at its best.