What Happened That You Have to Know About:
Hard to say. It's a Smallville episode and I don't usually watch Smallville. The three founders of the Legion come back to the present to stop the Persuader from killing Clark Kent on the day that he's supposed to defeat Brainiac for good. They do so, but then they have to stick around to help Clark deal with Brainiac, who has possessed Chloe. The Legionnaires think that killing her is the only solution, but Clark insists they find another way, and they do.
Stipulated, part one: It's nice to see the Legion on TV and in live action.
Stipulated, part two: It's amusing to recognize all the bits of Legion lore and other shoutouts that Geoff Johns included in the script.
But neither of those things makes this a good TV show.
Now that that's out of the way:
It wasn't bad; I enjoyed it. The whole history of Brainiac and Doomsday on this show didn't quite leave me behind, and the characters weren't as mopey as I was worried they were going to be. I'm not sure I'm going to watch Smallville again on the strength of this episode, but it wasn't an unpleasant experience.
Lightning Lad's characterization has kind of been skittering around across the last few Legion versions. It was the SW6 and reboot Garths that first took him away from the stalwart figure of the original Legion, possibly inspired by the 5YL notion that that was really Proty II acting like that. Then animated Garth (my favourite, after the original) was a lot like the original, but harder to get along with, threeboot Garth was kind of a space case, and retroboot Garth is kind of a thug. Smallville Garth is probably closest to the threeboot version.
I guess the most notable part of the episode was the conflict about what to do about Chloe. The Legionnaires advocated killing her, and Clark was having none of it. I've already seen a couple of responses to the episode by people who really didn't care for that aspect of the Legion's portrayal, but I liked it okay. First, let's give some credit to the Legion cartoon for being the first to develop the whole idea that the Legion is where Clark Kent learned to use his powers and become an effective superhero. There's another side to that, though, which is that the Legion learned something from Clark too. The cartoon only touched lightly on that, but in Smallville, we got it all spelled out for us. The Legion learns about... I'm not sure what to call it... goodness? Heroism? Nobility? I don't know. It's not like the Legion are lacking in these qualities, but it's Kal-El's example that helps them develop them fully. I thought it was entirely appropriate. There's certainly no shame in learning something from Superman.
The episode left a big door open for future Legion content on this show. I wonder if they're going to use it.
You know what would have been cool? It would have been cool if, after the episode, there was a commercial saying something like, if you liked meeting the Legion of Super-Heroes in this episode of Smallville, you can read about them every month in a comic book published by DC Comics, available in fine comic-book stores near you. Of course, that would require that a) DC and Time-Warner be interested in expanding their audiences, and b) DC actually have a Legion series with any kind of future. Oh well.