Unlike its Bad Robot cousin 'Lost,' 'Fringe' began with a master plan, and (least as far as anyone knows) has stuck to it, which is to say that while there have been plenty of plot threads left dangling in its 80-odd episodes, the show’s writers have a persistent (and sometimes perverse) habit of dredging up stray details from unloved past episodes and making them central to the ongoing plot. The fourth season, which began with a “reboot” of the show’s timeline that obliterated much of what we’d come to know, served in part as a means of reintegrating episodes from the show’s first year, when it was largely a “freak of the week” procedural along the lines of "The X-Files," into what’s since become a densely interwoven mesh of history and counter-history.
The cases they investigate, from a serial killer who steals his victims’ pituitary glands to a bioterrorist whose targets choke to death on supersized influenza cells, begin to form a pattern -- called, simply enough, “The Pattern” -- suggesting that unknown parties are using the world as a testing ground for all manner of unscrupulous, amoral villains.
The idea that, for those of particular brilliance and sufficiently pliable morality, the world amounts to an enormous petri dish, came full circle with the fourth season’s two-part closer, “Brave New World,” in which it was revealed that Walter’s old lab partner, William Bell (Leonard Nimoy), was the mastermind behind the season-long plot to destroy our universe and replace it with another of his own construction. Since the episodes were shot with the "Fringe" renewal up in the air, “Brave New World” had to serve as a soft series finale as well as an end to the season, providing a sense of emotional resolution while leaving the show’s major mysteries unsolved. (Fox announced this week that the show will conclude with a 13-episode fifth season, giving the writers plenty of advance notice to structure a concluding story arc and taking "Fringe" to 100 episodes overall — the “magic number” that makes a show suitable for syndication.)
Here’s where the “Previously...” comes in. In the first season, we discovered that Olivia had been part of a group of children who served as test subjects for a drug called Cortexiphan, whose purpose was to unlock the full power of the human brain. (The study was run, naturally, by Walter Bishop and William Bell.) While some children couldn’t handle what the drug did to them, Olivia was the perfect guinea pig, blessed-slash-cursed with a range of abilities ranging from telekinesis to the ability to move between universes. Walter himself had successfully crossed over to a parallel universe and stolen the duplicate of his dead son, Peter, but the rift between worlds created a catastrophic instability that laid waste to “the other side.”
As a tangible record of Walter’s grief-stricken transgression, Peter was uniquely qualified to put it right, which he eventually did, using a massive steampunk device to create a stable bridge between universes. In the process, he resolved the paradox responsible for his existence, and so he vanished, eventually working his way back to a world in which he never existed. (Chances are, your brain hurts right about now, so find a nice patch of blank wall to stare at, wait a few minutes, and then jump back in.)