McCoy jumps into the time machine, and instantly, time is changed back in Earth's past which affects all time and space instantly (except for people in the zone of time protection by the Guardian for some reason; "because the plot says so"). Kirk and Spock jump into the time machine, and instantly, time is changed again back in Earth's past which affects all time and space instantly, again. The Guardian then returns the time travelers in reverse order of entering the machine <first in, last out; why? "because the plot says so">.
Time travel per "because the plot says so."
(McCoy leaps through the doughnut and the pictures stop. He's gone.)
KIRK: Where is he?
GUARDIAN: He has passed into what was.
UHURA: Captain, I've lost contact with the ship. I was talking to them. Suddenly, it went dead. No static, just nothing.
KIRK: Kirk to Enterprise. Scotty.
SCOTT: Nothing wrong with the communicator, sir.
GUARDIAN: Your vessel, your beginning, all that you knew is gone.
KIRK: McCoy has somehow changed history.
...
(Kirk and Spock leap back out of the Guardian.)
SCOTT: What happened, sir? You only left a moment ago.
(McCoy returns.) <McCoy was first in and last out.>
SPOCK: We were successful.
GUARDIAN: Time has resumed its shape. All is as it was before. Many such journeys are possible. Let me be your gateway.
UHURA: Captain, the Enterprise is up there. They're asking if we want to beam up.
KIRK: Where is he?
GUARDIAN: He has passed into what was.
UHURA: Captain, I've lost contact with the ship. I was talking to them. Suddenly, it went dead. No static, just nothing.
KIRK: Kirk to Enterprise. Scotty.
SCOTT: Nothing wrong with the communicator, sir.
GUARDIAN: Your vessel, your beginning, all that you knew is gone.
KIRK: McCoy has somehow changed history.
...
(Kirk and Spock leap back out of the Guardian.)
SCOTT: What happened, sir? You only left a moment ago.
(McCoy returns.) <McCoy was first in and last out.>
SPOCK: We were successful.
GUARDIAN: Time has resumed its shape. All is as it was before. Many such journeys are possible. Let me be your gateway.
UHURA: Captain, the Enterprise is up there. They're asking if we want to beam up.
Time travel per "because the plot says so."
