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Mystery and serial dramas are rarely BFFs. As the binge-watching era has made season- and series-long plot lines commonplace, we've seen show after show struggle to maintain its narrative punch when there's no clear end date in sight. The network renewal—a cliffhanger for both creators and fans—has long been a pox on serial shows. For "LOST," for example, it meant each season stacked up more mysteries until arriving at a finale that couldn't possibly answer everything. "Twin Peaks," a saga that led to many of today's darker TV shows, spent its second season in an increasingly strange epilogue after being pushed to answer who killed Laura Palmer halfway through.

"Pretty Little Liars," which debuted its fifth season this past Tuesday, has achieved something few shows have: a single mystery stretched across multiple seasons without losing a drop of tension. It's one of the most sharply plotted shows on TV, a secret well-kept by its clamoring core audience of teen girls (and a handful of grown men, like yours truly). PLL's kept its suspense with a handful of careful tricks: shifting the suspicion through a rotating cast, and plenty of outside (and romantic) drama that's let the characters breathe beyond the tormenting texts of "A," the show's pivotal villain.

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"Pretty Little Liars" seemed to be heading into the home stretch with season four, with the revelation that Alison—the Laura Palmer of her time—was alive after all, and working to track down her rival. The finale found her reunited with our quartet of heroines and saw former "A" suspect Ezra Fitz shot down in cold blood. The chase seemed nearly over, and meanwhile, the Liars' high school horrors can't last forever: They're running out of time before graduation, college and a much more complicated series.

Last week's premiere found the show escalating, with A's identity more in doubt than ever and past enemies gathering new forces. Finishing off with season five seemed natural before that, but the show just announced its renewal for another two seasons. That's another 60-plus episodes of twists and turns—and while the show's been nearly perfect for its first 96, here are three challenges facing our Little Liars' future.

Actor Fatigue

It's a tale as old as teen dramas: Ratings explode, and a group of hot young stars, scoring magazine covers and film offers, suddenly hate their multi-year contracts. Seeing the cast exhaustion in the later seasons of shows such as "Gossip Girl" and "The O.C." has me worried that the "PLL" crew, already appearing in films such as "Spring Breakers" and "Consent," will get bored of sticking around Rosewood for another three years. That's not to mention seven years of playing teenagers. A shift to college worked for "Gossip Girl" because everyone just stayed in New York. If the timeline keeps stretching, that'll be a tougher change for the scholastically ambitious young women of "Pretty Little Liars."

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Social Media Overload

"Pretty Little Liars" is the most popular show on Twitter, but rather than let fans talk among themselves, the show has increasingly directed the conversation. Creator I. Marlene King occasionally takes questions and drops subtle hints, which is fun, but PLL's official account has been steering the mystery with hashtags and spoilers. That would be fine, if the show wasn't so purposefully ambiguous. After the show seemed to reveal Ezra was "A," PLL's social media feeds played along for weeks—until a show twist made his role more questionable. The same thing happened with the ultimate revelation of Allison's life-or-death status, which seemed Twitter-confirmed before the show proved it 100 percent.

It's one thing to leave us guessing, but another for the show's marketing arm to claim it knows better than we do. Let the show answer its own questions and viewers will keep talking.

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Plot Distractions

"Pretty Little Liars" has been a powerfully real show, its traumas coming from themes of friendship, honesty and identity. So when it added a supernatural element in the Liars' trips to Ravenswood, an eerie town a few hours down the road, it felt forced—as did the one-season spin-off of the same name, which won't be rising from the grave next year. With a fresh cast and story, "Ravenswood" could have been a way to ease the ratings pressure on "Pretty Little Liars" as the show wrapped up. Instead, the ABC Family cash cow is still shouldering the burden. What that kind of pressure might do to the show over seasons six and seven is scary.

A fantasized film noir episode in season four was another detour: a neat tribute, but indulgent in a show with flesh-and-blood stakes. Still, "Pretty Little Liars," more than most shows, has proven it knows what it's doing. How can it pull off three more years? That's a mystery I'll keep watching.

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