NOTE: This spoiler was submitted by Spectre.
In keeping with the found footage motif of the film, the film opens with a text scrawl with Warner Brothers thanking those that helped by giving footage in order for the film to be made and apologizing to those in the neighborhood.
The film starts with Costa (Oliver Cooper) being filmed by Dax (Dax Flame) going into his buddy's Thomas (Thomas Mann) house. He is talking to Dax about "getting his dick wet" when Thomas' mom shows up in the kitchen asking what he is talking about. Costa quickly deflects then go upstairs after hearing Thomas is in the shower. Costa rubs soap on Thomas' toothbrush and pulls the curtain back. Thomas is shocked and pulls it back. He later brushes his teeth, and tastes the soap calling Costa an asshole. Costa talks about Thomas birthday that night, saying they have to make it epic so they can get girls. Costa promises to take care of everything and "Make us look pimp." Thomas is hesitant to make everything too big.
Thomas' parents are going away for the weekend for their anniversary. His father lays down ground rules; he can have some people over but only a handful, no one goes into his office, he doesn't mess with the pool, and his Mercedes is off limits. Dax spies on Thomas' parents talking. His mom is worried about leaving, but his Dad saying they don't need to worry about Thomas because their kid is a "loser."
Thomas blows out his birthday candles and his Mom gives him the keys to the minivan as a gift. Costa mocks it, but Thomas says at least he has a car unlike him. They drive up to school. On the way, Thomas is starting to freak about the party, and Costa tells Thomas not to bitch out. They need the party to be big so they can have popularity for once.
We see why they are so worried about being popular. Thomas, Costa, their pal J.B (Jonathan Daniel Brown), and Dax are social pariahs, invisible to the popular kids. They just want to be noticed, especially Costa. "If you fuck this up for us, I swear to God I'll stab you," Costa says. Thomas relents but says only 50 people max. Just enough to be cool.
In class, Costa ignores Thomas' demands. He sends out a mass text about the party to everyone in the class. The news of the party spreads like wildfire.
Costa and Dax see Thomas heading to talk with his gal pal Kirby (Kirby Bliss Blanton) on the bleachers. Thomas mentions the party and Kirby says she might attend. Kirby mentions that she got him a present for his birthday then heads down the stairs. Despite his warning for her not to trip, she does and falls to the ground, saying she's okay.
Costa goads Thomas into asking Alexis, the hottest girl in school to the party. Thomas seems to charm her.
Thomas, JB, Costa, and Dax head to the supermarket to buy supplies for the party and Costa shoplifts some condoms and small bottles of liquor. At the checkout they see Miles (Miles Teller) who is apparently a young sports phenom. They mention the party only to find out that is where he is attending. Miles promises to bring a bunch of liquor and girls with him.
The group heads out to buy drugs. Costa says that he does not have sex with a girl with big titties tonight he will drown himself. JB just wants a blow job or to finger a chick, showing the guys numerous ways to do it.
Thomas, Costa, and JB head to the house where Costa's dealer T-Rick (Rick Sharpio) lives. He invites them inside and they buy some pot. In there though, Costa hands Dax a garden gnome to steal, thinking the party needs a mascot. As they go to leave with their weed. T-Rick runs out PISSED and demands his gnome back, breaking the windshield a bit with his fist. T-Rick grabs onto the hood but Thomas shakes him off. The group peel out in the minivan, and for a moment think they are safe when T-Rick bashes them from the side of the car down the street. The group heads off, Costa calling him "a terminator." Costa promises Thomas he can fix the windshield as he has a cousin who owns a body shop.
The group gets home to find out Costa hired two 12 year olds Everett (Brady Hender) and Tyler (Nick Nervies) to work security for their party. Thomas is shocked by it, but Costa says they are bad asses.
Costa and JB go around the neighborhood, telling everyone that a party will occur and that they will try their best to keep it down.
Thomas comes out and finds out Costa bought a bouncy castle for the party and got a few other inflatables. He also got a few signs for the pool that say "Naked Women Only." Thomas tells Costa he wants to keep people outside. No one should be in the house.
JB tries to work out on a treadmill before the party. Costa up the speed and makes him fall off it hard.
9 p.m. rolls around and it just Dax, Thomas, JB, Costa, and Kirby. Nobody has showed up and Thomas feels foolish. Costa tells him to calm down. JB comes down with a suit on. Kirby says he is totally getting laid tonight. They hear a bunch of car doors shut and look outside. People are showing up. A bunch of people, including Alexis.
The party starts to get hopping a montage ensues of people drinking, dancing and having fun. Somebody finds Thomas' dog Milo and ties him to a bunch of balloons, getting him high in the air. Thomas sees this and with Kirby's help, pulls him back down. They enter the house and put Milo in his dad's office. By this point, the "security" Everett and Tyler have been unable to keep people out of the house. Thomas tries to get people out but they just cheer him when he tries to say something. He lets it go, assuming nobody will mess up the house much.
Miles arrives with a bus full of hot women. JB is stunned speechless.
Roger, one of the parents Costa spoke to, arrives and tells them unless they shut the party down, he will call the cops. Everett uses a taser on him and Roger punches him. Roger says he will definitely call the police now, but Costa says he will call them on Roger for hitting Everett. When Roger says Dax filmed him getting tasered, Dax shrugs and says he only got the bit where he hit Everett. Roger storms off pissed.
Costa talks to Everett, and asks him to stake out Roger's house to make sure he doesn't call the police, telling he is the man for taking that punch.
Back in the backyard, Costa jumps into the pool in his underwear and causes Thomas to fall in as well. A girl tries to get in and Costa points out the naked women only sign. To his utter surprise, she complies taking off her top. Every girl who gets in goes topless. Thomas and Costa are shocked by the sea of flesh around them.
JB tries to talk to a chick while she shotguns a beer. JB tries to do the same but it just explodes all over him.
Thomas, JB, and Costa are upstairs in the master bedroom. JB is looking for condoms since he is working on a girl. "The only thing you are working on is diabetes you fat fuck," Costa says. JB finds a dildo and jokes that Thomas' dad is a freak. Thomas gets a text from Alexis saying she wants to do birthday body shots with him. Costa replies he will right down.
Thomas finds Alexis and she wipes some lime and salt on his neck then makes him put the lime wedge between his teeth. She licks his neck, does the shot of tequila, then grabs the lime with a kiss, giving Thomas a chance to make out with her. Just when it is his turn to do the same, Costa tells him his parents are calling him. Thomas races to the study to take the call. He runs into his Dad's office and takes it on speaker. His dad berates him for missing calls on his cell, but Thomas lies and says he was in the hot tub. His Dad warns him again not to get too crazy and Thomas says he is actually going to bed in a minute. His dad hangs up and Thomas sighs relief, looking out to the mass party outside.
Costa is radioed by Everett that Roger is arguing with his wife about calling the cops. Everett actually broke into the house. Costa is shocked that the kid is taking his job a little too seriously.
Kirby is playing beer pong when an older guy shows up, claiming he just wants to party. He plays with Kirby and does a shot or two.
Thomas is dancing when Kirby finds him and pulls him away upstairs to his bedroom. She gives him her present; a photo of them when they were younger before Costa moved into town. Thomas suddenly kisses her and says he made it awkward. Kirby says it is okay. They proceed to make out on his bed.
Everett returns and knows the cops are coming. He ushers everyone to the back and tells them to stay quiet. The cops arrive and Thomas and Costa downplay the party. The cops want to look around but Costa uses the law to his advantage saying they cannot search the premises without just cause. The cops back down, tell them to keep it quiet and that they don't want them to come back there. They see the cops pull off. Roger gives them the finger and Costa gives it to him right back. They return to the backyard where everyone has stayed crouched and silent until the cops left. "TO THE BREAK OF DAWN!!!" Costa screams. Everyone cheers and the party resumes once more.
By this point the party is really get out of hand. A skateboarder doing tricks goes too far and crashes through a window. Costa promises he can fix that too. Miles takes T-Rick's gnome and destroys it with a bat. It explodes, revealing it was hiding Ecstasy tables. The party goers proceed to take them.
Thomas goes into the bathroom and tries to make a tape for his parents explaining what has happened. He just wanted to be cool for one night, he just wanted girls to notice him, want him. Costa tries to comfort him but lets it slip he put the party on Craigslist and contacted other sources to make the party big. He just wanted Thomas to have a good night. JB and Costa try to keep Thomas from being depressed. JB puts on "Bump and Grind" by R. Kelly saying it is his favorite song. Costa gives him a tablet of Ecstasy and Thomas takes it.
Costa goes back outside and makes everyone sing Happy Birthday for Thomas.
A dwarf is in the kitchen causing trouble. A few guys throw him in the oven.
Thomas is talking to Kirby who is in the pool. Thomas notes she is not naked since she has her shirt still on (though to be fair it is a white see through). Kirby tells him she is having a good time and Thomas says it has been one hell of a party so far.
Costa hears about the dwarf in the oven and races to the kitchen to free him. Costa apologizes but the dwarf is beyond enraged and proceeds to punch Costa, JB and several other guys in the nuts. Thomas finds them and ribs them for it. The three sit lay down to decompress for a minute. Costa says the night has been awesome and apologizes to JB for all the times he called him names. Thomas admits he hooked up with Kirby. Costa says that it is cool but Kirby is practically one of them and she has always been there. Tonight is about being the girls they normally wouldn't have had a shot with. The night has to be a "game changer."
Another party/dance montage ensues. JB actually hooks up with a hot girl, and Thomas and Costa are nearby cheering him on.
Thomas is pulled aside by Alexis and they go up to his room. Everett is the bathroom filming. Alexis strips to her bra and panties and they proceed to make out. Alexis takes off her bra and they are proceeding to sex when the door opens. Kirby comes in and is shocked that Thomas would be with another girl after what happened earlier. Thomas tries to explain himself but Kirby runs off. Alexis kind of realizes what happened and quickly dresses, disgusted that he would do that. Thomas races after Kirby and tries to apologize but she is having none of it and leaves. So does Alexis, mad at what Thomas did.
Thomas, high, drunk, and somewhat sad by the turn of events stands on the roof and watches the madness below him. Costa and JB come up and join him. They hear a news chopper who surveys the party. Thomas looks up at the chopper and gives it the middle finger. The crowd sees the three and cheers Thomas' name. "Is this big enough to be cool?" Thomas asks Costa. Costa smiles. " A game changer," he says.
Thomas jumps off the roof into the bouncy castle that deflates some to the cheers of the crowd. JB also jumps off with the camera getting cheers too. He winds up breaking a finger though so they make a makeshift cast for him.
Everett meanwhile is in a parked car, scared out of his mind on how the party has gotten out of control. "God help us," he says.
Thomas and Costa see a news report about the party and hear a car start. The car is Thomas' dad's Mercedes that drives into the pool to his absolute horror. The driver it turns out was the dwarf. When Costa asks why he would do such a thing, the dwarf just says so what and screw the Mercedes. He punches another guy in the nuts then leaves.
Costa looks at the car in the pool in shock. "I can't fix this. I can't fix ANY OF THIS. Thomas I'm so sorry." Thomas says nothing, wondering how this could get worse.
The police with riot squads and SWAT show up trying to stop the party though they conclude they can only let it burn out before they can move in. The kids continue to party, defiant. Costa even gets revenge against a popular jock who treated them disrespectfully earlier by shaving his head and writing all over his face.
This is where things get really bad. Thomas' sees fireballs out front. A man with a flamethrower is torching his house and the neighborhood. Costa realizes it is T-Rick looking for his gnome. Thomas says they will give it back but Costa says it was full of Ecstasy tablets and the kids ate them all. Thomas tries to reason with T-Rick but nearly gets burned by him. T-Rick burns more things including cars (including Thomas' minivan), and a tree in the yard of Roger's. Roger says Thomas owes him a new tree but Everett and Tyler tackle him and beat him up instead. Meanwhile, police have no choice but use live ammo, piercing his gas tank. It explodes, catching T-Rick on fire. He runs into a nearby trailer at another house, where that too explodes.
The fire department has sent water helicopters to put out the blaze and police are using flash bangs to disperse the crowd. Thomas, Costa, JB, Thomas' dog Milo, and Dax run as well. Everett and Tyler are with them and plan revenge on Roger for calling the cops. Costa tells them to shut up and go home. Costa doesn't pay them seeing as the house was destroyed, calling them "the worse security ever." Costa mentions that they are so screwed with the damage they caused. Costa heads home, taking JB with him saying he will call a lawyer to see if he can stay out of jail. He does note that no matter what happens, they had "One hell of a night." Dax heads off as well, as Thomas sits in the school bleachers with his dog trying to figure out he can get out of the mess he is in.
Thomas is in the backyard with his father, apologizing profusely for what happened. His dad angrily berates him for the damage caused by the mess, saying it will wipe out his college fund. His dad says he never knew he had it in him. Thomas says sorry again. His dad, dropping his anger for a moment says no I meant I didn't know you had this in you (to throw a party and be a crazy, normal kid). His dad gives him a brief proud smile and asks how many people showed up. Thomas says 1000-1500 people. His dad's anger returns and he notes, "You're still fucked." as they watch the Mercedes be crane lifted out of the pool.
Thomas goes to school on Monday in his charred minivan that is missing doors. He tells Costa that he is being charged with 6 offenses, he is under house arrest while they live in a hotel until the house is fixed, and he shouldn't even be talking to them. Costa notes that he got a lawyer, and "even his Jew ass thinks we're screwed." Thomas asks about Dax's parents but Dax notes he lives alone. "You are really starting to scare me Dax," Costa says.
Thomas, JB, and Costa walk down the halls where everyone cheers them on for the crazy party. It gets to the point where a mass crowd cheer erupts for Thomas. The three smile, getting the popularity they craved. Thomas sees Kirby in the back of the crowd. She scoffs and leaves.
Thomas chases her down and apologizes for what he did. Kirby said he really hurt her. Thomas says he was sorry, saying he didn't know how to deal with it. He notes that he destroyed his house, bankrupted his parents, and put his future in question but the only thing he wants to fix is his relationship with her. Thomas says he wants to have his 18th birthday, but only with her. Kirby sees he is genuinely remorseful and forgives him. They kiss and Thomas asks if they are cool. "I never thought you were cool," Kirby jokes.
We see clips of the party via Jimmy Kimmel making fun of it and it is revealed that T-Rick survived his burns and was taken to the hospital. We are shown the kids fates. Thomas was found guilty on three charges that included disturbing the peace and inciting a riot, thoug he was also named in his yearbook as "Most likely to succeed." Costa's lawyer was good enough that he was found not guilty on any charges but he is now facing 3 claims of paternity. JB's parents convinced the court that he was a special need child and was not fit for trial. Unfortunately, he had to ride the short bus for the rest of High School. Dax is currently being investigated for the disappearance of his parents and he still lives alone (in one of the few time he is shown on film he looks like a Columbine reject).
The film ends with Costa being interviewed on TV. Costa takes offense that the party was considered as one of the greatest high school parties ever. Costa says it was THE greatest high school party ever and says the attractive newscaster can make it up to him by attending his next party. "Wear something tight fitting," Costa says.