[This review of It's Always Sunny in Philadelphia contains SPOILERS.]

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A decade ago, even the most loyal and ardent supporters of It's Always Sunny in Philadelphia couldn't have predicted that the series would be finishing its 12th season in 2017. What's even harder to fathom is that the diabolical sitcom centering on the most despicable band of ne'er-do-wells on television would have managed to keep things fresh and oh-so funny after all that time.

One could argue that the show's episodic nature and the well-defined personalities of its main characters are the biggest reasons for its consistent quality and longevity. But after completing one of its best seasons in recent years with an ending that even the most diehard Sunny fan would never have seen coming, we just might be left with a different show as It's Always Sunny in Philadelphia heads into seasons 13 and 14.

Even though season 12 saw the show continue its trend toward more original concept-driven episodes ('The Gang Turns Black,' 'Making Dennis Reynolds a Murderer') and episodes that focused on specific characters and continuity ('A Cricket's Tale,' 'Hero or Hate Crime'), the most memorable episode in the lot will most certainly be tonight's game-changing finale, 'Dennis' Double Life,' an episode that begins with a big reveal. It turns out that Dennis (Glenn Howerton) is a father to a young boy named Brian Jr., and has spent the past couple of years lying about his identity to Brian's mother (Christine Woods), a Midwestern woman he met on the Gang's beer-fueled trip across the country in season 10's 'The Gang Beats Boggs.'

Of course, the real shocker comes at the end of the episode. After going to extreme lengths to avoid the responsibility of fatherhood -- including pretending to be Mac's (Rob McElhenney) partner and faking his own death (in hilariously desperate fashion) -- Dennis has a real moment of connection with his son and realizes it's time to finally grow up, do the right thing and raise his child. This epiphany causes Dennis to rethink everything and literally walk out on the Gang, potentially for good.

For Sunny fans who reasonably expected that Dennis would weasel his way out of this situation and restore normalcy to the Gang's world, this move legitimately threatens to shatter the beloved Sunny mold of the episodic reset. After all, this is a show that's driven much of its humor from merely teasing character growth and maturity only to have its characters pull back their veils to reveal their deplorable selves time and time again. This practice -- while predictable -- has also made the comedy comfortable and accessible for those loyal fans. So, in this case, the idea of Dennis changing from a creepy and sociopathic philanderer to a respectable family man is actually oddly scary.

But perhaps even more unsettling for longtime fans is the looming question of Glenn Howerton's return as Dennis for season 13 after reports have confirmed Howerton's casting in an NBC sitcom with standup comic Patton Oswalt. The potential conflict could end Howerton's run on the FXX sitcom he helped build from the ground up, or it could mean a significant delay in production on the next season of Sunny. Either scenario would be bad news for Sunny fans, but there's no question most would prefer the latter.

Dennis' shocking turn at the end of the season 12 finale will undoubtedly create some anxiety amongst Sunny supporters, but the revelation doesn't necessarily take away from the episode's effectiveness nor the success of the season as a whole. Not only is 'Dennis' Double Life' one of the funniest standalone episodes of the season -- with its running gag about Dee's (Kaitlin Olson) "uncontrollable emotions" and Charlie (Charlie Day) consummating his obsession with the Waitress (Mary Elizabeth Ellis) only to discover the horror of actually being in a relationship with her -- it also proves that even the most established and habitual comedy can still shake things up.

There's probably a considerable portion of Sunny's audience that hopes the show is simply playing the long game with Dennis' exit, with a plan to reveal a yearlong prank and his return in season 13. We'd by lying to say we're not in that camp as well, and the show's creative team is certainly capable of such of stunt. However, it's also easy to imagine a new and exciting group dynamic emerging in Dennis' absence. One thing is for sure: No matter what form the show returns in, we'll be happy to see the Gang and Paddy's Pub back in business sooner rather than later.

Next: It’s Always Sunny In Philadelphia: 15 Worst Things The Gang Has Ever Done

It's Always Sunny in Philadelphia season 13 is expected to premiere on FXX in 2018.