As quickly as Sulu recovered, McCoy becomes paranoid and psychotic, yelling "murderers!" and "assassins!" as he overcomes bridge security to escape to the transporter room, where he knocks out the transporter chief. Forget about the Vulcan nerve pinch, don't mess with McCoy when he's had 200 drops of cordrazine pumped into him:
The landing party discovers 10,000 century-old ruins surrounding an annulus-shaped structure from which the time distortions are emanating.
The Guardian has waited for 10,000 centuries to be asked a question, and it takes every opportunity to insult Spock's intelligence in answering it. You gotta love it:
As the crew stands transfixed, watching thousands of years of Earth history pass before their eyes, McCoy (who has been found and subdued by Spock's nerve pinch) recovers and rushes through the portal before anyone can stop him.
Communication with the Enterprise immediately ceases. The Guardian tells them "your vessel, your beginning, all that you knew is gone." The landing party concludes that McCoy's actions changed the past, affecting the present.
Captain Kirk: Earth's not there...at least not the Earth we know. We're totally alone.
To return the present to what it was, Spock and Kirk enter the time portal at a time shortly before McCoy did so that they may find McCoy and prevent him from changing history. Kirk tells the red shirts to wait for a time, and then to enter the portal themselves, so at least they'll be somewhere in time.
I don't know enough about her career to know where it took its turn that lead to her being typecast as the scheming bitch in roles like this or this, but Joan Collins was perfect for the role of Edith Keeler. I can't imagine anyone else in her place.
Kirk and Spock agree to do odd jobs for Keeler to obtain the funds necessary for Spock to construct a mnemonic memory circuit to read the information in the tricorder (Spock recorded much of what The Guardian showed them) and discover what historical events McCoy has changed.
Edith Keeler turns out to be a very progressive and forward thinking, highly enlightened person (lucky for them!):
Keeler compliments Kirk on his and Spock's cleaning abilities (they cleaned up the basement in which she discovered them), and offers more work, as well as a "flop" (an apartment) in the very building where she lives. And of course, just as every female (be it human or not) who came into contact with James T. Kirk experienced, she is powerless to resist his charm and good looks.
Meanwhile, Spock is hard at work on his mnemonic memory circuit, which he manages to construct, as he puts it, in "this zinc-plated, vacuum-tubed culture."
Construction is slow going, and Spock complains to the Captain about how he is having to use "equipment which is hardly very far ahead of stone knives and bearskin."
Edith Keeler discovers their theft, but is willing to overlook it (after Kirk tells her Spock really needed those tools), on one condition: that Kirk walks her home. She has more questions about who they are.
Edith Keeler: You know as well as I do how out of place you two are around here.
Spock: Where would you estimate we belong Miss Keeler?
Edith Keeler: You? At his side, as if you've always been there and always will...
Sometimes a man's got to do what a man's got to do. And Kirk is a man, man. In fact, it's safe to say he's a man's man, man.
COMING UP in part 2: