We open in the water of a nearby lake. A young blond woman walks into focus. She is alone as she walks around in the water. She suddenly feels like she is not alone. She is not. Her boyfriend had been swimming beneath her and grabs her up onto his shoulders. They proceed to make out and despite her protests, takes her top off and throws it away. She tells him to get it, but he will not, saying he is getting some food. He leaves to go back to their camper, and the girl grabs her top and puts it back on. Suddenly, she is jerked around in the water. Something is attacking her. She screams for help from anyone, but no one can hear her. She is dragged underwater, and the water goes red. She is dead. Her boyfriend didn't hear a thing.
Cut to Tulane University in the dorms. Gordon (Joel David Moore) is playing Halo with a couple of guys online, kicking their asses, though having some problems with his teammates. He yells at his friend and roommate Nick (Dustin Milligan) to play, but Nick tells him he needs to finish his med schools applications. Gordon forces him to play just as Malik (Sinqua Walls) bursts into the room. Malik seems to pissed at Nick, explaining he needed at least a C in the class he was taking in order to keep his free in college (Nick tutored him). Nick is scared that Malik will hurt him as Malik tells him to come closer. Malik breaks into a smile, and shows he got a B+ in the course. Malik tells Nick and Gordon they are going to a lake house for the weekend to celebrate. Nick says he can't due to paperwork and tests, but Malik shuts him up since the house they are going to is Sara's (Sara Paxton). Nick smiles slightly (he has a crush on her). He decides to go.
Sara is at the gym when she notices she is late. She leaves to get ready.
Sara calls her friend Beth (Katherine McPhee) and tell her to get ready. Beth is at a tattoo parlor getting a new piece.
Another one of their friends Blake (Chris Zylka) is a nude model for an art class. He sees a cute girl checking him out and flexes at her a bit.
Malik is talking to Nick and Gordon about his girlfriend Maya (Alyssa Diaz). He plans to propose to her this weekend. Gordon calls him crazy as he is about to go pro and have girls around him. Plus Gordon will need help with the excess girls, something Nick agrees with. Malik tells them Maya was with him since he was poor and she deserves him at his best now he will be rich. The seven friends get into Sara's SUV and drive towards their destination.
After what seems a day of driving, the group wind up at a local bait shop/convenience store. Sara warns the group there is no hard line and cell phones won't work at the house that they should make their calls now. Sara and Beth go into the bathroom. Beth mentions how cute Nick is and this would be a perfect time to "take his innocence." (Beth likes the boys). Sara scoffs at that, and mentions she thought Beth was after Blake. Beth says no; he sprays tans everywhere, including his genitals. When they had sex she had tanning marks all down there. It's clear that Beth knows Nick likes Sara and possibly likes him and is messing with her.
Meanwhile, Gordon is trying to give a pep talk to Nick about going after Sara. Gordon tells him to stop being such a pussy and get with her before someone else does. They go to the checkout where unbeknownst to them, the clerk has a video feed of the girls bathroom, and have been watching them the entire time. Gordon is freaked out by the clerk so he leaves while Nick pays.
Malik and Maya are outside when a truck pulls up. A man, Red (Joshua Leonard) gets out. He sees Malik and proceeds to make racist comments to him. When Maya objects to that, Red makes sexual comments about her. Malik gets pissed and wants to kick his ass. Red's friend Dennis (Chris Carmack) gets in the middle and Malik threatens to bust his face. Dennis takes off his glasses and shows him a nasty scar (important later). They are just about to go out it, when Sara recognizes Dennis. That diffuses the whole situation. Sara and Dennis have an awkward reunion and Sara says it was nice to see him. The group get back into the SUV.
They head to the docks to Sara's boat (the house is on a island). They take off and drink beers on the way. Nick tries to talk to Sara but just makes an ass of himself. Suddenly, they see a police boat behind them. The group is worried, but Sarah guns the engine and drives to the house. The sheriff Sabin (Donal Logue) comes up and Sara remarks he lost again (they use to race their boats). Sabin notes she hasn't been up in three years. Sabin tells her he'll be up tomorrow and for the kids to have fun and be safe.
A montage ensues where we see the group have drinks, suntan, and have fun.
Malik, Nick, Maya are on the boat about to do some wake boarding. Nick asks Malik about Sara and he notes that he never saw Sara with a guy, even for a casual hookup. She just spent time with her dog, Sherman.
Malik is doing some tricks and impressing the group. We see that something is following them in the water. It begins to tap Malik's wake board hard. He is extremely worried and tells them to go faster but they don't realize he is in trouble. Something finally taps him hard enough for him to wipe out. The group turn around but don't find him.
Meanwhile, Gordon and Beth are playing beer pong on a raft in the pool. They watch as Blake comes out and sprays tans his privates in front of them. Gordon says she should stop messing with him and get with a real man (him). They banter some as they play.
Nick finally sees Malik swimming for shore. He swims right up to Sara who is sunbathing. He is missing his arm.
Nick and Maya return to the house in shock. Blake thinks they ran Malik over, but Nick denies it happened. They quickly put Malik's bad arm in a tourniquet to stop the bleeding. Nick jumps in the water. He is going to find Malik's arm (the idea being, him being a pre-med, he would know that a limb has a limited amount of time before it cannot be reattached). Getting out of the water, he notices something is chasing him. It is a SHARK. He barely gets out of the water with Sara's help. The shark chomps on a ladder instead.
The gang is severely freaked out about sharks in the water, but they have bigger problems. Malik is bleeding out. Nick decides to take the boat with Sara navigating. Malik will be with them and his arm in a cooler. They have two hours before the arm is useless to him. Maya tags along, swearing she will not leave him.
Malik is being held by Maya. Neither realize he is dripping blood into the water, which works like a bread crumb trail for the sharks.
The boat starts to rock from a shark hitting it. Maya is thrown overboard. They throw her a floatation ring just as they see a shark coming for her. They gun the engine to try and outrun it but it is no use. Maya is soon grabbed by the shark and disappears into the water. Having no choice they continue on, only for the shark to keep hitting their boat. That causes the throttle to stick and the steering go bad. They can't stop the boat. They grab up Malik and the cooler and jump off, just as the boat crashes into the pier nearby at Sara's home. It explodes, shocking Gordon, Beth, and Blake.
Now down to six, they set Malik down in a bed outside. Blake remembers a flare gun and races for it. He shoots it off to try and get a response. Sabin is in range, but is too busy listening to rock music to notice.
Nightfall. Too much time has passed to save Malik's arm. Now they are only concerned with keeping him alive. Beth is coming apart, and blames Sara because they came to her house. Beth slaps her and immediately regrets it, blaming it on the stress. Nick and Sara hear a gunshot. They race outside.
Blake is firing off his shotgun when he sees a boat nearby. It is Dennis and Red. Blake tries to tell him somebody is hurt and Red makes sexual comments about Maya. Blake tries to hit Red, but Red disarms him and points the shotgun at him, pretending to fire. Nick tells him to stop, since they don't know. Nick tells Red and Dennis that Maya is dead and Malik needs to go to a hospital.
Dennis says they need to get Malik to St. Francis. So they will take one of the group with them, and when they get in range of a signal they will call for a medic. Beth volunteers to go, and Gordon tags along, telling Nick Beth is on shaky ground and he won't leave her alone with the perverted Red. Red explains they have a sonar machine to keep sharks away from them and a stun gun weapon that could kill a shark dead with one strike, so they will be fine. Sara tells Dennis thanks, and that she owes him one. Dennis looks her and says "No, we're even Sara."
Blake sits at the bedside of Malik. Blake tells Malik he is going to be fine. Malik asks where Maya is. Blake tries to divert but Malik presses the issue. Blake has no choice and tells him she's gone. Malik breaks down in tears.
Beth and Gordon are in the boat. where the lecherous Red (who has his teeth filed down to look like shark teeth) asks how big pointing a flashlight at Beth's breasts. Beth is appalled but Red says he meant how big was the shark that attacked Malik. Beth says she doesn't know. She is disturbed by the whole conversation and grabs a knife when Red isn't looking. Red tells them that the hurricane season allowed the ocean sharks into the lakes and that one apparently decided to stay and make a home. Red seems overtly interested in the sharks. Dennis stops the boat unexpectedly. Gordon asks what the hell they are doing; they need to get help. Red takes the sonar device out of the water. He says they have 5 minutes for the entertainment to start.
Nick and Sara are on the porch talking. Nick asks why she hasn't been back in 3 years. Sara reveals she used to date Dennis when she was younger. But he was never going to leave town and she was going to college. So she broke up with him. A few days before she left, they went diving. Her tanks malfunctioned and when signaling for help, he ignored her. She somehow got to the surface and turned on the boat. He was right by the propeller when it started. She was the one that cut up his face, by accident. She took him to the hospital and that was the last time she was in town until this weekend. Nick tries to tell her everything will be okay but Sara says she wishes she could believe him.
Dennis pulls a gun and tells Gordon to get into the water. Gordon is resistant, so Dennis shoots him in the shoulder. That causes him to fall into the water. Now leaking blood, the sharks come to him. Gordon tries to swim away, grasping onto a tree growing out of the lake. He has only a moment before a shark jumps off the water and attacks him. He struggles in the water for a moment before he is pulled underneath for good. Bye Bye Gordon.
Beth is next. They are going to force her into a cage with a special small kind of sharks, nicknamed "cookie sharks." Beth tries to resist and stabs Red but it only pisses him off. Dennis pushes her into the cage and she is immediately attacked by the sharks. She tries to pull them off but their numbers are too great. Eventually, they bite her so much she dies. Red and Dennis film the entire thing.
Malik is at the shore with a spear. He is going to kill the shark that killed Maya. Nick tries to dissuade him but Malik is firm. "They took one of mine, so I'll take one of theirs." Malik goes into the water, bleeding which leads a shark to him. Nick sees it and warns Malik but it does little good. It still grabs onto Malik. But Malik doesn't go down easy, punching the shark with his remaining arm. Nick gets in, and holds the shark down as Malik stabs it to death with a spear.
Malik, Blake, Nick, and Sara look at the shark and wonder what the hell is going on. Nick notices a camera on the body and realizes to his horror that someone put the sharks in the lake. Malik is getting worse due to the second attack and Blake says they need to get him help. Using a wave runner, he plans to take Malik across the river.
Sabin comes to the house, and is apprised by the situation. He calls for help and tells Nick and Sara everything will be fine. Nick starts to feel funny and passes out.
Sara's dog Sherman finds spilled chicken soup from Sabin on the floor and licks at it.
They put him to bed; Sabin says the stress became too much for him. Sabin tells her to get Nick a cold cloth.
Sara finds her dog knocked out and Sabin's radio cackles. It is Red, telling him they killed Beth. Sara realizes her friends are dead and the sheriff is working with them. She pulls a knife and sneaks up behind to stab him but is disarmed by Dennis. She tries to run but is hit with a tranquilizer dart and passes out.
Meanwhile, Blake and Malik are trying to get away but Malik is still bleeding, leaving a trail. Malik sees a shark and realize it will kill them both. "I'm not taking you with me Blake." Malik says He jumps off and lets himself be killed so Blake can live. Blake doubles back as Malik is killed but can do nothing. He rides off to escape and sees no sharks. In front of him, a shark jumps out of the water and kills Blake. Malik's sacrifice was for nothing.
It is now about morning. Dennis and Red have placed Sara in a shark cage ready to feed her to something nice. Sara tries to plead with Dennis but he is not willing to listen. He is angry she carved up his face, but she reiterates it was an accident. She was so tramuatized by it she hasn't been able to be close to another guy in years. She came home in part to make right with him, to apologize for what went wrong. Sara tells him she did love him and they seem to have a moment where he will reconsider. However, she was using it as a distraction to grab his gun. Unfortunately she never has used one (and the safety was on) so he grabs it back. Dennis tells another accomplice to release a particularly dangerous shark into the water. Sara asks where Nick is. Dennis tells her Sabin is dealing with him. Red has stolen Sherman and when Dennis says he can't keep him, Red throws him overboard.
Nick has been tied to a chair by Sabin when he comes to. Nick wants to know why they are doing this. Sabin tells him why, explaining that 20 million people a year watch "Shark Week" and other related programming. There is a good percentage of those that would want to see the up close, real life shark attacks. Snuff films if you will. So Sabin, Red, and Dennis decided to trap sharks, rig cameras on them and sic them on innocent people on the lake in the pursuit of money. The plan goes beyond normal psychotic behavior into pure greed.
Nick tells him they are crazy but Sabin shows another side to the scheme; he is tired of being a small town sheriff looked down on by the "rich kids" like Nick (even though the group treated him with respect). Sabin attaches a winch cable to Nick's chair and cuts him to draw some blood. At the same time Nick knocks over a gas can. Sabin dunks him briefly before bringing him back up. Nick has a lighter from his pocket and proceeds to melt the zip tie that binds his hand. To buy time, Nick requests Sabin puts on some Guns and Roses. With Sabin distracted, Nick breaks free, and throws the lighter at the spilled gas. It causes a fireball that sets Sabin on fire. Nick jumps out of the way as Sabin falls into the water. Nick shuts the door to the opening and Sabin pleads for him to help him. Nick doesn't listen and Sabin is eaten. Nick races to the water and dives into to get Dennis' boat to save Sara.
Sara has been halfway submerged in the water. Dennis throws a bucket of blood at her to attract a shark. Red calls out. Nick is holding him hostage with a gun to his head. Nick tells him to pull Sara up. Red asks for help. Dennis pretends to comply but grabs a knife and throws it...killing Red. He was expendable. Nick and Dennis scuffle and Nick kicks him off the boat. Nick tries to pull the cage up but Dennis grabs onto it and detaches it. Nick dives in.
In the water, Sara cannot breathe so Nick gives her a kiss for oxygen. Dennis attack and the three scuffle. Sara grabs his head and bangs it into the cage, drawing blood. Dennis gets his clothing stuck on the cage as the shark gets closer. Dennis looks at Sara scared. She flips him off. The shark chomps on his head.
Meanwhile, Nick is above water getting air. He sees Sherman with Red's stun gun. He grabs it and goes back under.
The shark is beating the cage up trying to get to Sara, almost getting there. Just then, Nick attacks it with the stun gun. One shot blows a hole through it the size of a basketball. It is literally dead in the water. Nick grabs Sara. She is not breathing.
Nick gets her on the boat and performs CPR. After a few moments she finally comes to. She thanks him for saving her and kisses him. The camera pans away a distance from the boat, and a shark jumps out of the water, signifying their are still sharks alive in the lake.
NOTE:Stick around after the credits. Their is a ridiculous, self deprecating "Shark Bite" rap the cast does involving the plot of the film.
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