No Star Trek fan needs to be told that Star Trek: Enterprise had some rough spots, but even though it was canceled after only four seasons, the series has its legion of fans. We're the ones who can overlook the not so good episodes much like we do for the other series in the Star Trek franchise because we know a series doesn't rise and fall on one or two episodes.
Series co-creator, Brannon Braga, in the oral history book "The Fifty-Year Mission: The Next 25 Years: From The Next Generation to J. J. Abrams" edited by Mark A. Altman and Edward Gross, via Slashfilm, points to one particular episode where he felt the series was in trouble, and it was only in the first season.
The episode was "Terra Nova," where the Enterprise encounters colonists on a planet who've been living underground to protect them from high radiation levels on the surface of the planet. Though it wasn't a bad episode overall, Braga wasn't at all happy with it.
""It was rough. I was pretty depressed, because I felt like the first episode after the pilot was good, and then the third was good, and then there was some episode called 'Terra Nova' and it was dreadful. It was a stinker, and I thought, 'we're in trouble.' It happened that quickly. I was hoping to do something more serialized and felt like I was in a situation where I'm starting to fall into some familiar rhythms."- Brannon Braga
To say that this episode was the beginning of the end is, to me, not at all accurate. Enterprise went on to air some incredible episodes like "Vox Sola," "Dear Doctor," "Regeneration," and "Proving Ground." And the fourth season of the series got even better.
The cancellation of Enterprise came about for a series of reasons and not just because there were a few bad apples in the bunch. If that were the case, Star Trek: The Next Generation would have been canceled after the first season.