How did people survive before cell phones? Especially thriller filmmakers, who lean on the device nowadays like no other as an immediate indication of salvation or doom. In the scare-fest “The Strangers,” there’s a lot of mayhem, but cell phones, which tend to run out of batteries just as someone is about to break through the door, are the first victims: cut from their power cords, stolen, tossed in the fire. Reaching for a cellular to call for help only to find it inoperable is a zinger “The Strangers” never tires of.
Without their cellies, the main characters — let’s call them hot guy (Scott Speedman) and hot girl (Liv Tyler) — are stranded in another decade. Not just hot guy’s dad’s house, decked out in record players and doilies, but a place where writer-director (Bryan Bertino) is free to torment them with an amalgam of “The Texas Chainsaw Massacre,” the Manson Family and AM-radio country. Guy and girl have just gotten back from a friend’s wedding and are staying at the house for the night, when a strange knocking on the door becomes a full-on assault of the property and them.
And that’s it for plot. At first the frights are all aimed at the imagination, a lot of spooky noises and weird goings-on. This build-up is scary in the guess-when-you’re-going-to-jump way of the usual thriller, but handled with enough agile pacing by Bertino to keep the proceedings from degenerating into tedium. These early moments have a truly creepy vibe that should be applauded. When the noises reveal themselves, however, the movie turns into an extended chase scene, perhaps best described as “Not Exactly the Most Dangerous Game.”
From here “The Strangers” springs all the traps of yore  — boyfriend disappears when he’s most needed, girl falls and disables herself just when she has a chance to get away, no one can seem to outsmart a bunch of people wearing sight-obstructing masks — but not all the payoff. A movie like this needs a third-act twist to rehook the audience once (it thinks) it knows Team A and Team B, but Bertino evidently forgot to include one. “Why are they doing this?” girl sobs to boy, who can only convey his creator’s wishes: “I don’t want you to think about that right now.” (R) 107 min.  S