I very rarely write reviews here, but this episode deserves it. I watched it a year ago and it I still remember how impressive it was. And how much better and surprising it is, compared to the rest of the otherwise ok series. I'm shocked it isn't rated number one, but I guess it's because the audience this show is aimed at is more immature, so they value the action episodes more.
This one has a very clever concept and an amazing twist at the end. It left me crying, just as much as the dog episode of Futurama. It's so impactful and thought out. It concentrates on the s1 characters that are the core of the story, it is less ridiculous than normal (but still funny) and it's much smarter. I don't want to comment on the story, because I don't want to reveal any spoilers, but I would definitely recommend it in a top 5 time-traveling episodes list of any show.