But in each time-period there is a focal point, the Guardians warn them. Something or someone that is indispensable to the normal flow of time. Something that may be completely innocent or unimportant otherwise, but acts as a catalyst, and if tampered with, will change time permanently.

They say that Beckwith will try to reach this focal point, and in some way alter it, so that time stays forever altered. Kirk wants to know how Beckwith knows what the focal point is. The Guardians assure him Beckwith doesn’t know, but that because of the stresses and fluxes of the time-flow, Beckwith will be inexorably drawn to this focal point, and will alter it, even without knowing he is doing so.

“Then how can we stop him?” Kirk asks.

The Guardians tell him the machines in their city, machines which probe and delve into the time-flow continuously, have isolated the focus point in the era Beckwith has invaded. They tell Kirk and Spock it is a girl, EDITH KOESTLER, who is scheduled to be run down and killed by a moving van at a specific time. Beckwith’s subliminal drive will take him to the path of this occurrence, and he will somehow prevent its happening thus fixing time permanently into the altered shape it has now assumed.

They must capture Beckwith, and bring him back into the present, the now, before he can meet Edith Koestler, and alter her fate.

They go into the pillar of light a week earlier than Beckwith, and when they appear in the past, they find themselves in Chicago of 1930, on Old Earth.

It is, literally, the city on the edge of Forever…

Linked to a tall-spire city on a frozen mountain peak across the stars and hundreds of years in the future, another city on the opposite edge of Forever, by the tenuous thread of life called Kirk and Spock and Beckwith.

But now that they are here, in the past, they must learn to make their way, at least until Beckwith comes through the time machine. Yet imagine the circumstances—they are men out of time, out of joint with the world around them. They have trouble understanding the language (after all, even English alters to indistinguishability in three hundred years), they have no skills that can be put to use in this “regressive” age. Their clothes are peculiar. They have no place to live, no money (and don’t even understand the medium of exchange or have a way of earning money) and most obvious of all—

Spock is an obvious extraterrestrial.