Edith Keeler
Joan Collins

Lt. Galloway
David L. Ross

Rodent
John Harmon

Voice of The
Guardian of Forever
Bartell LaRue

Having tracked some time-space distortions to an unexplored planet, the Enterprise enters orbit only to be shaken by severe turbulence. McCoy hurries to the bridge to assist the injured but more tremors cause him to lose his balance and fall, accidentally injecting himself with a massive dose of cordrazine. He becomes affected by severe paranoia, races to the transporter room and, after overpowering transporter operator Kyle, beams down to the surface of the planet.

Kirk immediately takes a landing party to pursue him, arriving in the ruins of a 10,000 century-old city. They split up to search for McCoy and their route takes Kirk and Spock to a mysterious arch-like rock that pulsates with power. Spock scans it and finds that it alone is responsible for the time-space distortions they have been experiencing. When Kirk asks it to identify itself it tells them that it is the Guardian of Forever and shows them scenes from Earth's history. Suddenly, McCoy runs out from among the ruins and is eventually caught and overpowered by the landing party.

Spock starts recording the scenes being offered by the Guardian on his tricorder and is taken by surprise when McCoy breaks free of his captors and dives through the Guardian's portal, disappearing into Earth's past. Uhura tries to contact the Enterprise but cannot raise them. The Guardian explains that McCoy has somehow managed to change history and that neither the Enterprise nor anything they knew still existed. They get the Guardian to replay the images it was showing and, using the tricorder to calculate the approximate moment, Kirk and Spock also leap through the Guardian's portal, with its promise that it will return them if they succeed in restoring whatever has been changed.

They land in a street that proves to be somewhere in New York during the 1930s. Stealing some clothes to cover their uniforms, they manage to escape from a pursuing policeman by ducking into a basement room. After changing, they are discovered there by a woman called Edith Keeler who runs the 21st Street Mission, a hostel for down and outs. She is befriended by Kirk and finds them a rented room and some jobs. Spock spends all their spare money on parts to build a rudimentary computer with which he plans to interface with the tricorder to find out the exact point at which McCoy manages to change history.

Edith catches them 'borrowing' some watchmaker's tools which Spock needs for the work and becomes suspicious of them but Kirk, who finds that he is falling in love with her, persuades her of their honesty and walks her home. Spock manages to get his 'stone knives and bear skins' computer to work and almost immediately sees a report of Edith Keeler's death. On Kirk's return he shows him a different version of history, six years on, where Edith Keeler is seen meeting President Roosevelt, at which point the makeshift contraption burns out. Spock tells Kirk what he first saw and explains to him that somehow McCoy manages to save her life and that, if she doesn't die, their history will remain forever changed.

They had arrived ahead of McCoy who was shortly to appear in another back street, also finding his way to Keeler's Mission. She sees the condition he is in and promptly puts him to bed in a back room and nurses him back to health. One evening, as Kirk is taking Edith to the cinema, she starts telling him about her new 'patient'. Realising who it is, Kirk runs back towards the Mission calling for Spock. McCoy hears him and emerges on to the street where they hug each other. Being extremely puzzled by this event, Edith steps into the road to speak with them, but doesn't notice an approaching truck.

Both Kirk and McCoy see it coming, but as McCoy moves to save her, Kirk grabs and restrains him. McCoy angrily turns to Kirk, asking him if he realises what he has done but is gently taken aside by Spock who says to him 'he knows, doctor . . .he knows'. Having completed their task they are all returned to their own time and place by the Guardian of Forever. The waiting landing party members are surprised to see them as, to them, only moments have passed since they went through the portal, but they find that all has been returned to the way it was.