Gaffer Tape said:

Whether he intended it or not, it was still basically the same plot, just done better. Hell, the V'Ger plot was once going to be used as the pilot for Star Trek: Phase II, if I recall correctly.

EDIT: What freaked me out about that episode was what happened with Uhura. The damn thing wiped her entire brain... yet they're able to re-educate her?! Were they able to re-educate all her memories and feelings and instincts? Or is the Uhura we see from that point on basically a whole new person?

There were a lot of ideas tossed around on TMP's strange journey between tv and feature film. There was a false god for an antagonist at one point, who actually makes the crew younger as a gift. (A prototype "Q" perhaps?) How they were going to make Shatner and company look like they did in 1966 I have no idea.

The "explanation" the ST Compendium offered was that only Uhura's factual memories were affected. Maybe Uhura had amnesia of the Jason Bourne variety? ;)

It's one of those things like the beaming people into their past selves solution in "Tomorrow is Yesterday". It's best not trying to find a logical explanation for how it works.