In the time of episodic television, the status quo was king. No matter how bad things got, you’d always know that by the end of the episode, things would be back to normal. The rare exceptions were two-parters, but even then, it almost never had a lasting effect on the characters or the world. In sci-fi, this was usually a little more justifiable; when Kirk and company take off, they leave the planet of the week behind, and all the problems along with it. Other shows that didn’t have a starship at their disposal had a much harder time escaping that fact, often to the show’s detriment. And even running away aboard the Enterprise didn’t excuse the lack of character development that, say, Kirk’s infamous trysts should have had on him.
But once in a while, an otherwise episodic TV series would make a bold step, and that changed the show (or sometimes the entire franchise) forever. It might be a profound character moment, or the side effect of a planned retool, or even something as simple as a marked change in tone, but for whatever reason, these episodes took that status quo and shook it up so much that it never was quite the same.
And surprise, surprise, this list includes SPOILERS for the tagged shows, so proceed at your own risk if you aren’t caught up.