Fringe resolved most of its ongoing mythology over the course of its fourth season, which ended in May, an appropriate move given that most thought it was going to be the show’s final season. However the show was given a last minute reprieve which will allow it to have a final 13 episode season, premiering on September 28, even though the central conflict of the show has pretty much come to a resolution.
The Fringe writing staff did give themselves an out though, in the form of Season 4 Episode 19, “Letters of Transit.” There is a tradition in Fringe of the nineteenth episode of a season being a sort of experimental episode that doesn’t fit into the main story arc of the show. In this case, the future dystopia set “Letters of Transit” functioned almost as a sort of Backdoor Pilot for a new, different version of Fringe, one concerning a future war between humanity and the suddenly malevolent “Observers” as opposed to a conflict between two alternative universes. The events of “Letters of Transit” weren’t referenced again until the final scene of Season 4 finale “Brave New World (Part 2),” a scene that was filmed only to air in case Fringe actually was renewed for a final season.
Now that Fringe has actually been renewed the writers still have a lot of heavy lifting to do. It’s a strange choice to make the “Observers” the new villains when in the past they’ve appeared to be at worst neutral (like some sort of “observers” if you will) and at times even sort of benevolent. According to a report on TV Line the action of Season 5 will pick up immediately after the events depicted in “Letters of Transit.” The short teaser trailer that’s been released is pretty much just a bunch of Observers walking menacingly towards the camera while a voice over plays that consists of various lines of dialogue that have been uttered about the Observers over the course of the series. Watch below: