It’s kind of hard to call the time travel method in a story featuring time travel a deus ex machina.
What’s lazy about it? Everything about it was carefully set up in the beginning. The rest of the story sticks closely to that.
Using it is not deus ex machina which – since you are unclear on the concept – is bringing in something that hasn’t been mentioned before to resolve everything at the last minute. That doesn’t even begin to apply here.
That phrase does not mean what you think it means.
Typing define deus ex machina into Google gives you
My favorite example is in “Adaptation” which I won’t expand on since it would be a spoiler. By that point in the movie I had figured out what was going on enough so I saw it coming.
A better one (perhaps) would have been the climax to “The Trouble with Tribbles” if they hadn’t established in the bar that tribbles don’t like Klingons.
No, that is Chekov’s Phaser. Er, Chekov’s Gun.
Chekhov’s gun.
Thanks. I never got past the Tsar/Czar thing. 
An interesting question: In what way was Rodent relevant to the story? I’m assuming he was in some way inspired by Trooper, but his existence and death too is pretty much tangental (except for the destruction of McCoy’s phaser, which may or may not have been important).
Regardless of the above, the encounter in the alleyway was a great scene, and I still get goose bumps as I watch it unfold.
Another point: For Spock, rather than Kirk, to have ensured Edith’s death goes far beyond the non-drama of Kirk freezing in indecision. It suddenly shifts the focus of the story from Kirk to Spock. Throughout the story, the romance between Kirk and Edith was central, and Spock’s function was to reveal the truth about what had to happen. At the very end, it’s Kirk who’s left out of the equation and Spock who’s the dominant factor. Since the core of the story up to that point had been Kirk, this is very bad writing, and it’s clear why the ending had to be changed.
The most obvious Deus Ex Machina in * Star Trek* was Spock’s third eyelid that he had forgotten he had.
I counter with Spock’s green blood in The Man Trap!
Ladies and Gentlemen, we have a tie.
Although, the ultimate is Spock’s katra.
Perhaps for Trek we should use the term, Vulcan ex Machina.
His katra was at the start of The Search for Spock, and involved pulling in that “Remember” line from The Wrath of Khan that they put in specifically for the next movie.
It would only be a deus ex machina if they’d brought Spock back in the last minutes of TWOK.
I don’t agree about the green blood in “The Man Trap,” either. For one thing, it didn’t resolve the plot. For another, Spock was established as an alien, and that’s enough to think his physiology might be different enough to survive.
The Vulcan third eyelid, however, is 100% a deus ex machina. I could see some people quibbling about it resolving the plot, but I say it does, because it meant that blinding him was never really a possibility.
Neither was his being killed by the Salt Vampire. He survived to shock McCoy into firing his phaser, which resolved the situation in Man Trap.
It was the first episode aired (and only the sixth one filmed), so no one had never heard of Spock having green blood before. It was pulled out of thin air.
The third eyelid is worse because Spock forgot he had it. :rolleyes: He’s a science officer. One would think he knew his own basic physiology.
Actually, Spock’s green blood was mentioned very early on, when McCoy gives him a physical. It was probably in “Where No Man Has Gone Before,” which was filmed before “The Man Trap,” but aired later. So it’s the fault of NBC, not in the writing.
Nit pick:
Paul Fix plays the Doctor in *Where No Man has Gone Before.
*
You’re right; I think it was in “The Naked Time,” when everybody was getting physicals. “Green ice water,” I think was what McCoy called it.
Yes; but as he noted, Vulcans tend to ignore it. 
From Merriam Webster:
This is as apt a description of the Guardian of Forever as any I can think of. I think Kirk and Spock could’ve determined the nature of the problem by means that were a little more science-y than the God of Cheap Exposition, but if you’re okay with it, you’re okay with it. I guess one forgives a lot when presented with peak Joan Collins.
The Guardian can’t be a DEM because it wasn’t* introduced suddenly and unexpectedly and provides a contrived solution to an apparently insoluble difficulty*. It was a device** that set the whole plot into motion**. It also never showed up again anywhere in the series, so it isn’t a DEM in the TV continuity either. (Yes, I know it shows up again in books. Irrelevant to this discussion.) It may be a McGuffin, but a DEM it ain’t.