Anyone who knows anything about ST:TOS knows that fans consistently give the 'best episode' vote to the Hugo Award winning "The City on the Edge of Forever", by Harlan Ellison. Anyone who knows even a little more knows that Harlan Ellison was ... unhappy ... with the episode as aired, and that he and Gene Roddenberry ... disagreed ... over some of the historical details.
In this book, Ellison puts his side of the story, forcefully, pungently, and backed up by documentary evidence. He also includes the script as he originally wrote it, which was the version that won the Writers Guild of America Award for best teleplay, to let the reader see for themselves what changed.
I see three major changes that mangle the logic and depth of the story. Firstly, eliding the character of Trooper affects the depth. Secondly, in the original, the character who changed the timeline by saving Edith Keeler was not a temporarily demented McCoy, but a rotten-apple drug-dealing Starfleet officer. (No, not Scotty.) So the act that unintentionally dooms the world, the act that has to be prevented, is ironically a single out-of-character selfless moment by an otherwise evil person. Thirdly, in the original, Kirk fails to stop that act, because of his great love for Edith Keeler, and it is Spock who has to step in and save the universe.
It seems that two of these changes were made because no Starfleet officer could possibly be evil, and because Kirk could not possibly fail to be perfect. If that were all, the only problem would be the viewers' loss, that the episode was reworked into something so much less than it could have been. But, reading Ellison's story, it seems that there was an attempt to rewrite history, too, and snatch credit where credit wasn't due. This is shocking stuff, but essential reading for anyone who wants to know how things really are, not how things are retrofitted by deifiers.