the story: Paris is convicted of murder, and sentenced to relive the crime repeatedly.
what it's all about: Somewhat predictably at this point, "Ex Post Facto" is one of those early episodes that negatively affected general perception of the series, on two counts. One is that it's similar to Next Generation's "A Matter of Perspective." (It's also similar to Deep Space Nine's "Hard Time," in some respects, but I doubt this is brought up much.) The other is that it fails to clarify the character of Tom Paris.
The first can be dismissed out of hand. Both episodes are ultimately defined by how the truth is revealed; in "Perspective" it's with the clever use of the holodeck, while in "Facto" it's with Tuvok's investigation, which actually serves to make it as much a Tuvok episode as a Paris episode. This one's not really worth talking about.
The second is patently ridiculous. Paris was introduced in "Caretaker" as a "bad boy" who'd gotten booted out of Starfleet and subsequently locked up, which is kind of a mash-up between the character Robert Duncan McNeill previously played in Next Generation's "The First Duty" and the backstory of Ensign Ro in the same series from the eponymous episode. But fans found it difficult to accept McNeill as a "bad boy," as he didn't seem the type. I'd argue that anyone who really needs convincing only needs see "Non Sequitur" in the second season to see where Paris might have ended up if he'd never been recruited by Janeway. Anyway, "Facto" also features him in the ladies man role Kirk and Riker previously filled, and that seems to make him not only redundant but a complete failure to convincingly pull off any aspect of his character.
But it actually fills both nicely. Riker might have ended up in a similar situation in "Perspective," but he didn't have anyone questioning his integrity, either before or after the episode, much less during. Kirk had numerous affairs, but it was considered part of his charm. Paris does it and pays a horrific price, right at the start of the series, and it's an important and necessary experience that helps define him. The problem with Paris wasn't so much that he was a "bad boy," but that when he got in trouble, he got in trouble. There was a running "joke" in Deep Space Nine (and Next Generation previously) that O'Brien always ended up in situations meant to torture him (such as "Hard Time"), but he was able to walk away from all of them (doubtless today he'd be portrayed with a permanent case of PTSD after just one of them) with little consequence. Paris debuted with plenty of consequence, and "Facto" affirms how these things tend to happen to him, and it's much the same reason why he seems like a Kirk or Riker style ladies man, because he doesn't think about the consequences of his actions (also explains why he ends up in a relationship with B'Elanna Torres). "Thirty Days" later in the series is another great illustration of this.
- franchise - Helps find a contrast in the archetype Paris shares with Kirk and Riker.
series- If events involved the crew in a more deliberate fashion it might seem more relevant.
- character - Explains Tom Paris.
- essential - See above statement.