So says the voiceover of Anita "Needy" Lesnicki (Amanda Seyfried), a teenaged girl locked in an asylum. The film opens with a view of a hooded figure appearing and disappearing at a girl's window, before showing us Needy's room at the asylum. Commenting on her numerous mailings - which include religious figures and statuettes, Needy is reminded to head out to the recess yard. Credits roll as she changes into her orange correctional shirt and bunny slippers, eventually making her way past her fellow "Mental Olympians" to punch the tetherball from its pole.
We move to the cafeteria, as Needy is confronted by one of the nurses over her choice of a solitary Pop-tart meal. Continuing her voiceover, Needy responds true to her correctional file's namesake as the Kicker - planting her foot in the woman's face and sending her over a table. Subdued by the guards and thrown into a solitary cell, Needy cringes as the Muzak gets pumped in and she reflects back on the events that led her here...
As the flashback begins, we see the Devil's Kettle, Minnesota sign and Needy explains how the town was named for the waterfall - or rather the mysterious whirlpool that the cascading water seems to drain into. We see scientists tossing buckets of small orange balls into the pool as Needy tells how nothing seems to reappear once it falls in.
We are then cut over to a high school pep rally, with Jennifer Check (Megan Fox) leading the team in a flag routine. While Jennifer waves at bookwormy. glasses-wearing Needy, Needy waves back and recalls how they have been best friends "since the sandbox," despite a fellow schoolmate's interpretation of their mutual waving as a sign of a lesbian relationship.
Jennifer meets Needy at her locker, inviting her out to a local dive bar to see the indie band, “Low Shoulder” performing that night. Despite Needy protesting that she had promised to meet her boyfriend, Jennifer wins out and sends Needy home to dress for the event. As she changes outfits, boyfriend Chip (Johnny Simmons) bemoans how Needy always seems to follow Jennifer in everything she does. Convincing him that she and Jennifer are BFFs - even showing him her BFF necklace - Needy heads downstairs to meet her friend at the door. Chip watches the two girls playfully shove each other as all three head outside. Chip warns Jennifer to stop stealing his girlfriend, but the two girls are in the car and heading out.
In a voiceover, Needy complains about the seedy bar with its local yokels and sticker-covered toilets, as Jennifer runs into a former boyfriend Roman (Chris Pratt) and exchange-student Ahmet (from India). After an exchange where Jennifer tells how she hasn't been a virgin in a long time - with an emphasis on no longer being a "butt-virgin" as well - she drags Needy up to meet the members of the band. Lead singer Nikolai Wolf (Adam Brody) seems to take a shine to Jennifer, and she heads to the bar to use her "assets" to wrangle up a red, white, and blue drink. Needy heads over to play a pinball machine, but overhears Nikolai talking about Jennifer and her supposed virgin status. She gives him a piece of her mind and tries to convince Jennifer to leave with her. Unfortunately they get swept up towards the stage as the band begins a song called “Through the Trees.”
As Low Shoulder plays, the light and sound riggings create a spark that begins to rage out of control. Soon the entire bar is set aflame, sending panic throughout the crowd. Patrons get trampled, set afire, and crushed by falling debris; Needy leads Jennifer to the sticker toilet bathroom, pulling herself and then Jennifer through the window to safety. Outside, Nikolai finds the two girls and uses some alcoholic persuasion to get Jennifer into the band's van. Needy and Jennifer lock eyes as the door shuts, and Needy takes the car home to her empty house.
Upset by both the bar tragedy and the disappearance of her friend, Needy calls Chip and heads downstairs to investigate a noise at the front door. Exploring the house, she eventually comes face to face with a bloodied and disheveled Jennifer. Creepily smiling at her friend, Jennifer tears into the refrigerator and begins to devour the whole chicken inside. Suddenly she stops and begins to vomit up liquid black ooze all over Needy and the kitchen floor. The ooze takes on a sharp needle look before again puddling. Jennifer shoves Needy back into the hallway asking if she's frightened before disappearing into the night.
The next day Jennifer shows up to class looking perfectly normal. While their one-armed teacher Mr. Wroblewski (J.K. Simmons) comments on the tragedy of the bar fire and the loss of students and staff members, Needy seems to think Jennifer is not quite right. She gets into an argument with another student over the news report that Low Shoulder apparently played a heroic rescuing role in the fire; one of the football players breaks into tears while Jennifer just laughs.
We then cut to the athletic field, where our tearful jock is reflecting on the loss of his friend. Jennifer arrives and offers to help him in his grief, leading him into the nearby woods. As she unzips her jacket and drops his pants, he notices a large group of forest creatures assembling around them. When he asks what they are all doing, Jennifer replies, "Waiting." She then open her mouth to reveal a set of pointed teeth and begin to tear him to pieces. As he screams, we are shown Mr. Wroblewski in the school parking lot, encouraging the student to let his grief out.
While the audience gets a scene of Jennifer swimming in the lake, eventually to dress and wander back into the woods, Mr. Wroblewski stumbles upon the torn carcass - and deer drinking from it - leading to the news getting out across town. Chip calls Needy to meet her at his house to discuss the town's recent issues.
Days go by and Needy continues to hear about the rise of Low Shoulder in the media, commenting via voiceover how Devil's Kettle has gained national attention from the twin tragedies.
On the one-month anniversary of the bar burning, Mr. Wroblewski comments on the upcoming CD sales benefit by Low Shoulder, with their single Through the Trees to be used to raise disaster relief funding for the town. A haggard and unkempt Jennifer rudely comments about the event before the bell rings and the girls head to their lockers. They encounter emo student Colin Gray (Kyle Gallner), Needy's fellow writing class friend, whose attempts to ask Jennifer to the Rocky Horror Picture Show seem to end in failure. Upon hearing that Needy likes Colin's writings, Jennifer agrees to a date at her place and will text Colin the address.
The next scene cuts between Chip and Needy getting intimate, while Colin drives to an abandon section of town. Colin checks the address again on his cell before entering one of the derelict houses. As Colin explores, Chip and Needy are getting his condom on and getting beneath the sheets. Eventually Colin works his way upstairs to a candlelit attic, and is startled by Jennifer appearing behind him. As she starts to work her charms, her pupils become almost reptilian and Colin begins to back away. Jennifer then breaks his arm. The camera cuts back and forth to Needy and Chip and Jennifer and Colin; Needy starts to see visions of Jennifer's victims while the fanged silhouette of Jennifer tears into Colin.
Feeling something wrong, Needy bolts to her car and drives off. As she cranks off the radio, again playing Through the Trees, she swerves to avoid a monstrous-looking Jennifer. Looking back to see if she really hit her friend, Needy is startled by Jennifer leaping onto her windshield. Flooring it, Jennifer is thrown from the car and Needy races home, inspecting her home and looking for her mother before collapsing into tears on her couch.
As needy climbs into bed to reflect on what's been happening, she is shocked to find an underwear-clad Jennifer hiding in her bed. Attempting to seduce her BFF, Jennifer and Needy begin to kiss. Needy comes to her senses and shoves away from Jennifer. While Jennifer is ecstatic about being nearly unkillable - even stabbing herself to prove her point - Needy relents to hearing Jennifer out as to what has been going on.
We flash back to the night Jennifer left with Low Shoulder. Worried about being raped, she continues to lie about being a virgin in hopes of finding a means to escape the band. While she does make a break for it when the group stops, Nikolia and company carry her off into the woods near the whirlpool. Tying her up and gagging her, Nikolai reveals that they have turned to the occult for success. He has found a satanic ritual - online, of course - that will allow the group to break from indie status and hit the big time by summoning a demon. Repeatedly stabbing her with a Bowie knife and reciting the incantation in regards to Jennifer's body, Nikolai tosses the weapon into the whirlpool and the group departs.
Jennifer recounts how she stumbled away from the woods and encountered exchange student Ahmet on the same wooded road. Verifying that no one knows he survived bar fire, Jennifer pulls him into the woods and implies devouring him as her first victim. We are then returned to the scene of Jennifer encountering Needy in her kitchen from earlier in the film.
Upset at her friend's monstrous behavior, Needy demands Jennifer leave her home. Jennifer does so by dropping out of the upper bedroom window and disappearing.
We are then treated to another voiceover as Needy recalls Colin's funeral, her falling out with Jennifer and much of the school, and her disapproval of Low Shoulder offering to play their spring formal for free.
Though Chip tries to pull Needy out of her funk by revealing his purchasing of formal tickets, Needy confronts him about Jennifer's transformation and her research into the band's ritual. According to the books, the fact that Jennifer was not a virgin meant that the demon has become attached to Jennifer's soul. In order to stay strong, Jennifer must feed on human flesh; she has apparently been picking off the boys of the school as her meal ticket. Needy pleads with Chip to avoid the formal, calling it an "all-you-can-eat buffet" for Jennifer. Chip leaves, disappointed that Needy is trying to break up with him.
The night of the formal sees a voiceover as the girls and guys are getting primped. Chip is forced to get pictures taken with his younger sister, lying to his mother about picking up Needy and receiving pepper spray as protection against the town's boy-killer. Needy arrives at the prom, scanning for Jennifer. Mr. Wroblewski introduces Low Shoulder and they break into their hit song as the crowd goes wild. Not seeing Jennifer, Needy realizes that her former friend is hunting for Chip instead.
We cut to Chip walking to the dance and being confronted by Jennifer. She lies about Needy cheating on him with Colin, and attempts to seduce him. Jennifer leads Colin to an old mausoleum/pool as Needy races through the night to find her boyfriend. Chip follows Jennifer into the closed and overgrown building, as she teases him about going for a swim. When he rejects her advances, Jennifer shoves him into the muddied pool water.
Needy arrives to find Colin's corsage on the ground, and beelines to the pool. Tumbling into the window, she sees Jennifer chomping on Chip's neck, and Needy dives on top of her. As Jennifer surfaces to attack, Needy sprays Jennifer with Chip's pepper spray and attempts to pull Chip from the pool. Jennifer hovers out of the water, as the two girls come to a verbal argument over friendship. While Needy explains that she realized how terrible a friend Jennifer was to her, Chip is able to impale Jennifer on the sharp end of the pool skimmer. Though Jennifer slides out the skimmer, she's bleeding profusely and escapes through the window. Needy attempts to call for help on Chip's phone, but he dies in her arms of his injuries.
Knowing that she must now kill her best friend, we cut to the opening credits scene of the face appearing in a girl's window - only this time we now know that Jennifer in the girl in bed and Needy is crashing through to kill her. Jennifer hovers above the bed as the two girls grapple; Needy's stabbing attempts with a boxcutter are blocked until she reaches for Jennifer's matching BFF necklace. Ripping it from Jennifer's throat, the girls fall to the bed. Looking betrayed, Jennifer cannot understand what happened to their friendship as Needy jams the boxcutter into Jennifer's heart.
Hearing the tussle, Jennifer's mother comes upstairs and encounters the grisly scene. As she cradles her daughter's dead body, Needy rolls over, physically exhausted from the fight.
We are then taken back to Needy's solitary asylum cell. Here she reveals that during the fight, Jennifer had bitten her - and she now possesses some of the demon's power. Needy hovers to the window, kicking it out and walking through the asylum gates.
Needy walks along the wooded highway, eventually finding the knife used by Nikolai along the side of the road - with some orange balls. Picked up by a kindly elderly fellow, she tells her Good Samaritan that she is headed east - following a band. As the driver comments about the things kids do, Needy mentions how their next show will be their last...
During the end credits, we are shown Nikolai and the members of Low Shoulder enjoying the rocker lifestyle. From booze to M&Ms, they are enjoying the fruits of their success until a knock is heard at the door. The camera then cuts to a bloodied police scene in the hotel room, showing the deceased members of the band. We finish our tour with a shot of Nikolai, with his own knife jutting from his chest.
The last scene is a video monitor showing a hooded figure passing a group of screaming girls in the hotel. Needy looks up at the camera as the mob's screams of delight become ones of terror.
*CUT TO THE CHASE*
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Bookish Anita "Needy" Lesnicki (Amanda Seyfried) and cheerleader Jennifer Check (Megan Fox) are unlikely best friends. They meet up to see a band play at a local bar, only for an electrical fire to burn the place down around them. Jennifer is abducted by the band for a ritualistic sacrifice, only for her body to be revived as a flesh-eating demon.
Though Jennifer's return to school the next day seems normal, Needy realizes something is not quite right about her friend. Making matters worse is the sudden brutal deaths of male classmates all across town. Realizing the trail seems to point back at Jennifer, the girls see their friendship dissolve. Realizing that she must stop her friend at all costs, Needy arrives too late to save her boyfriend Chip (Johnny Simmons).
After a bloody battle in Jennifer's home, Needy is believed to have been the rampaging killer and is committed to an asylum. She reveals to the audience that a bite from Jennifer has given her the demon's power. Escaping from confinement, Needy tracks down the band and slaughters them in their hotel, exacting vengeance for her friend.
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