Before Kirk returns from being with Edith Keeler, Spock's newly constructed mnemonic memory circuit reveals some disturbing and conflicting information.
They discover that Keeler is the link, either dying in a traffic accident or meeting with the U.S. President. But they are unable to determine which event is the one caused by McCoy's presence or the nature of Keeler's fate.
Still in the grips of the cordrazine, McCoy is in full "murderers! assassins!" mode.
The breakdown of Spock's mnemonic memory circuit allows Kirk and Keeler more time together. Keeler once again reveals how uncannily ideal of a savior she is for Kirk and Spock as she tells Kirk of her certainty man will make it to the Moon. And predicting her possible fate as the founder of the peace movement, she expresses her desire that "one day they'll take all the money that they spend on war" and spend it on peace.
Kirk: The very same.
After his arrival, McCoy accosts a man on the street and then falls down unconscious. The man then accidentally vaporizes himself using McCoy's phaser.
Oh how I wanted that phaser!
Some young boy's dreams come true.
Drawn to it by the smell of freshly brewed, morning coffee, Bones finds and is given shelter at Edith's mission.
While Keeler is focusing on nursing McCoy back to health, Spock has repaired the mnemonic memory circuit.After hours of careful work using primitive vacuum tube circuits, Spock discovers that McCoy, if not stopped, will prevent Keeler's death. Keeler will then found a peace movement which will delay U.S. entry into World War II and allow Germany time to develop the atom bomb and conquer the world.
Spock: Jim, Edith Keeler must die.
McCoy recovers from his cordrazine trip, and tells Edith he is chief medical officer on the U.S.S. Enterprise. At first he does not believe he is in 1930 America, but soon realizes that indeed he is.
McCoy: It looks like old Earth, around 1929.
Edith Keeler: Would you care to try for '30?
McCoy: I'm a surgeon, not a psychiatrist.
Later, Spock inadvertently observes Kirk and Keeler returning home from a date. He watches as Keeler nearly falls down the stairs and breaks her neck, if not for Kirk's stopping her fall. While they think she may die in a traffic accident of some kind, Spock reminds Kirk that they aren't that sure of anything.
Before she steps out for another hot date with Kirk, Keeler checks in on McCoy, who she finds to be much better.
McCoy: A who movie?
By the way, while doing research for this post, I discovered that the only Clark Gable movies to have been made up to 1930 had him cast in largely uncredited, extra or bit roles. I guess Harlan Ellison could have used IMDb.
On his way to see a Clark Gable movie with Edith, Kirk learns from Edith that Dr. McCoy is in town and then immediately sees Bones across the street.
Edith Keeler: You know, Dr. McCoy said the same thing!
McCoy: You deliberately stopped me, Jim. I could have saved her. Do you know what you just did!?
Spock: He knows, Dr. He knows.
On a side note, I bought this ornament in a Hallmark store in La Grange, Texas, two Christmas seasons ago. When you press a button on it, there's a line of Guardian dialogue and the center of the Guardian flashes with blue light. It's a real testament to Star Trek (and this episode) that something like that would be made some forty years later!
Uhura: Captain, the Enterprise is up there. They're asking if we want to beam up.
Communications with the Enterprise are restored, and when The Guardian asks if anyone else desires to make a journey in time (Ha!), Kirk responds: "Let's get the hell out of here."
The Guardian: Time has resumed it's shape. All is as it was before...
Read more about it at the Wikipedia entry for "The City on the Edge of Forever".