Most quality shows, including my favorites Battlestar Galactica and Lost, occasionally have what's referred to as 'filler' episodes. Though 'filler' typically refers to an episode that does little to advance the plot, they are great for character development or to tell a self-contained story that can be just as compelling as a season's plot.
In the case of science-fiction shows, though it's technically a 'filler' episode, the "mirror universe" episode is a usually a fan favorite, where we get to familiar characters behaving most unfamiliarly. This past week's episode of Heroes, String Theory, explores a variation of the mirror universe by going 5 years into a dark future where the heroes have failed to 'save the world'.
I don't have to explain that I was looking forward to this episode, and it delivered everything that mirror episodes traditionally deliver: character allegiance flips, vague references to a past that hasn't happened, and new twists on old tricks. Yet, aside from containing one whopper of a cool twist towards the end, the episode was revealed to not be filler, but essential to the current plot, and ended up explaining more of what is going on than any other episode to date.
We understand how Isaac's knowledge will enable him to be a hero, why Peter had to save the cheerleader, and maybe most importantly, Hiro learns how to stop Sylar. Again. Or maybe for the first time. See, the coolest thing about the future episode is that it is actually the 'real' universe; the present is the alternate timeline the Hiro has created to change things. I haven't been so impressed by an episode since... well I don't know. I'm hard pressed to compare it.
Only three episodes left.