It took the "Fringe" creative team a very long time to figure their own show out, but they have - by embracing what the show is rather than what they wish it could be.

When "Fringe" was introduced two and a half years ago, its producers seemed afraid of the fact that they were making a science fiction show with an ongoing storyline. They wanted the series to be accessible to a wide audience, and both sci-fi and serialization have become a network TV kiss of death, particularly on FOX. So they told some stories that only kinda-sorta dealt with the kind of freaky fringe science that heroine Olivia Dunham (Anna Torv) was assigned to investigate. And they presented a lot of flat standalone episodes that were only watchable because of the kooky, compelling performance by John Noble as cracked scientist Walter Bishop.

But as the show went along, the writers - perhaps accepting that this wasn't going to be a mass appeal hit, and that they were alienating their core audience in trying to hook viewers who simply wouldn't watch this kind of show, no matter how much they're spoon-fed - slowly began to embrace the sci-fi, and the serialization. And this season, they've jumped hip deep into those murky, entertaining waters with an ongoing, complicated, enormously entertaining story arc that's seen Olivia trade places with her counterpart from a parallel universe. Episodes have alternated between our Olivia trapped over there (and at some points brainwashed into thinking she was her counterpart) and ones with the other Olivia (quickly dubbed "Fauxlivia" by fans) working undercover on our side and causing problems for Walter and his son Peter (Joshua Jackson).

The story was a masterstroke, not only giving the show some major narrative thrust from week to week, but making the usually problematic Monster of the Week episodes much more interesting, whether in the parallel universe (which was fun for all the changes from our own) or in the normal one (where Fauxlivia was always working an angle in the background).

But all good things come to an end, and this week's episode (Thursday at 9 p.m.) marks the beginning of this particular end. Our Olivia is trying frantically to get home, while Peter now knows that the Olivia he's been sleeping with isn't the right one, and all sorts of fireworks go off on both sides.

It's another strong episode of what's been a terrific season, but as we approach what would seem to be the close of this particular arc (or at least this phase of it), I wonder how the show will keep this momentum going.

Assuming Olivia and Fauxlivia wind up restored to their proper universes and intact, will the Monster of the Week shows be as entertaining? Will we even bother with doing episodes set in Fauxlivia's world  - and all its amusing little differences, like a "Back to the Future" trilogy where Eric Stoltz never got replaced by Michael J. Fox - anymore? Once she's not routinely playing another Olivia - or playing the regular Olivia pretending to be the other one - will Torv go back to being the uncompelling cipher she was too often in the show's first two seasons?

My biggest concern is about Peter Bishop. Once upon a time, he was a crucial part of the show: the bridge between his crazy old man and Olivia, a versatile hustler with abundant book and street smarts who was always saving the day through some unexpected skill, and a wild card in the show's crime procedural stories. But FBI agent Astrid Farnsworth (Jasika Nicole) has assumed Peter's role as the one who manages Walter's eccentricities, Peter mainly acts like just another G-man, and the writers appear to have sold out his vast intelligence by not letting him be even the slightest bit suspicious of Fauxlivia. I like Joshua Jackson, but watching the episodes set in the Peter-less other universe, I realized I didn't miss him at all.

The "Fringe" writers had to have a plan for how the show would work when/if Olivia came home, and I'm hoping they have something good in mind for Peter beyond being a pawn in the war between the other universe's Walter and ours, who stole Peter from the parallel world as a child to save his life.

For now, though, I'm enjoying the show far more than I ever would have expected in its early days, and I continue to hope that FOX's plan to move it to Fridays early next year won't be its death knell, the way it has for so many other post-"X-Files" sci-fi series on the network.

Alan Sepinwall may be reached at sepinwall@hitfix.com