The movie begins with Chapin (Britt Robertson) sneaking out of her house to go to Eve's (Alexia Fast). Eve is having a birthday party, and is waiting for her father to call. The doorbell rings, and it's Liza (Scout Taylor-Compton). Liza tells Eve that her mother made her come, and asks Eve if she wants her to leave. Eve refuses, saying that she'll get in trouble if she makes Liza leave. They agree to be civil to one another.
Liza joins Cecily (Janel Parrish), Sarah (Emily Tennant) and Nina (Carly McKillip) at the party. They're essentially Stepford Wives. Liza sits by herself, feeding pizza to a rat she keeps in her bag; her only friend.
Chapin shows up, an exact foil to Nina, Cecily and Sarah. They go upstairs, but not before Chapin corners Liza. She tells her that she's not gonna ruin Eve's birthday, so she's gonna be civil to Liza, but they're not gonna be 'down' until Liza apologizes. Liza refuses.
Flashback to 4 days earlier. Chapin and Eve are talking in the hall when Whisper, a friend of Chapin's that Eve likes, and his friends walk up. He pulls the whole 'points-to-chest-person-looks-down-flicks-them-in-the-chin' thing with Chapin, obviously flirting with her, and ignoring Eve, who is pretty much just staring at him longingly. Eve is then shown with her soccer team, running warm-ups. As they run across what's known as The Jogger Bridge, every girl kisses her hand and sets it on the railing. Kayla (Valerie Tian) tells Eve to do so also, to 'pay her respects'. When Eve asks why they do that, Kayla tells her it's in memory of Stacey St. Clair (Julia Maxwell), a girl from Sacred Heart, a nearby Catholic school, that jumped off the bridge 5 months ago.
In the present, Chapin is bitching about how they're not doing anything fun on Eve's birthday. Everyone gives lame suggestions for things to do, such as sudoku, pedicures, and reading each other's blogs. Chapin hesitantly suggests Triple Dog.
Back 4 days ago, Eve asks Kayla and Toni (Laura Carswell), 2 girls on her soccer team about Stacy St. Clair. Toni tells Eve what she heard; that Stacy was at a party where she was given an acid hit. She was tripping and seeing butterflies on her fingers. One flew away, and she jumped off the bridge chasing it.
Back in the present, Chapin is explaining the rules of Triple Dog; everyone dumps all their money and their most favorite possession they have with them into 'the pot'.
The rules are as follows: 1. Everyone gets and gives a dare.
2. You get to pick who dares you.
3. You have to do your dare, or else you get your head shaved.
4. If you can't complete your dare, you can challenge whoever dared you to complete it. If they do, you get shaved.
5. With the exception of daring someone to shave their head, any dare goes.
6. The whole night will be recorded.
At the end, they'll vote for who did their dare best, and whoever wins gets everything.
Eve picks Sarah to perform her dare first. Sarah picks Liza to dare her. Liza dares her to streak around the block. Sarah is hesitant to do so, as she is Catholic and very conservative. For fear of getting shaved, she does it, however, she is seen by one of Eve's neighbors and a car full of boys, who take pictures and videos. This is mostly due to Chapin following after her on her skateboard, yelling out about 'Sarah Algreen's virgin vag'.
Flashback to 3 days ago. Chapin is sitting in Study Hall playing Mad Libs with Whisper, Clarke (Jeff Ballard) and Stephan (Richard Harmon), who is throwing gummy bears at Liza, who is sitting alone at a nearby table. Liza gives Chapin a dirty look, which annoys her. Liza tells her to tell everyone why she got kicked out of Sacred Heart, and Chapin walks over to her and punches her in the face twice. Whisper has to literally wrestle her away from Liza. Liza is shown talking to Principal Scalco (Brian Markinson), who asks her if she's still carrying around that rat. Liza says no, but he gives her a piece of doughnut to give to the rat, as he knows she's lying. He asks her if she has friends, to which she lies again, saying yes. He tells her to stay away from Chapin. Liza leaves and Chapin walks in. She makes a couple smartass comments, and asks for her detention. Scalco tells her that they have a zero-tolerance assault policy, and suspends her for three days.
At her locker, Liza is checking out her bruise when Todd (Norman Gerard Funk), Eve's brother, and a few of his friends comment on her, not caring that she can hear them. Joe (Sefton Fincham) asks Todd if he would 'Superman that ho' for $20, and Todd says for half a million, he'd think about it.
Back in the present, Eve picks Liza to go next, and Liza picks Nina to dare her. Nina dares her to stay in Todd's closet until midnight, wearing Nina's Cheerleading outfit. Eve and Chapin lure Todd and his friends out so Liza can infiltrate his closet. Eve asks Todd if he wants to talk to heir father when he calls, and Todd tells her he's not gonna call. The girls give Liza Sarah's number, and demand a photo every half hour so they know she's still in there.
Eve picks Nina to go next. Nina picks Eve to dare her. The group drives to a local Western restaurant, where Mallory Silvers (Aubrey Mozino) works. Mallory has short hair, is on the soccer team with Eve, and was kicked out of Sacred Heart along with Chapin and a few others. She is in an embarrassing uniform, which Chapin takes pictures of and blackmails her into helping with Nina's dare by putting Nina's name next on the karaoke list. Nina gets on stage, and fakes a seizure, complete with foaming at the mouth (with the help of Chapin, who gave her Alka-Seltzer to chew on). Mallory's boss (Raugi Yi) tells Mallory to call an ambulance, and yells at her when she refuses. As the girls leave, Mallory pulls Eve aside, asking if she's brought up Stacy St. Clair to Chapin yet. Eve just walks away. The next time Liza sends them a picture, Chapin takes Sarah's phone and calls Liza, forcing her to answer and Todd and his friends to find her. Todd tries to pull her out of his closet, and she bites him. He goes to get his mom to get her out, and she calls him a baby, embarrassing him and blackmailing him with the thought of his mother finding his porn stash, into letting her stay until midnight. His friends leave for a party, but he stays, because he doesn't want to leave Liza alone in his closet.
The girls go find Whisper, who tells them that he and his friends are crashing a party at midnight. Chapin says they might go too, after their game. She tells Whisper that they're playing Triple Dog, and he tells her to be careful.
Flashback to 2 days earlier. Liza and Eve's moms see each other a the grocery store. Eve finds Liza and asks what she said to Chapin to get her suspended. Liza tells her that she told Chapin to tell everyone why she got expelled. Eve asks why, and Liza tells her that Stacy and Chapin got in a fight, and that Chapin pushed Stacy off the bridge in the heat of the moment.
Back in the present, Cecily is up next, and Chapin dares her to pee on Principal Scalco's (who she thinks is sexy) stoop. She drinks a bunch of water to prepare. Cecily gets on his stoop, pulls down her underwear and starts peeing, going unnoticed. Chapin however, isn't gonna let Cecily off the hook that easily, and jumps on Scalco's car, setting off his alarm. He opens the door to Cecily pissing on his Welcome mat, and she runs off.
Back in Todd's room, Liza starts crying, realizing that she's having the worst night of her life. Todd tries to sympathize, even spraying air freshener in his closet when she tells him that it smells. She sees him failing at Rock Band, and schools him at Expert mode. He watches her play, noticing how she moves, and realizing that he thinks she's hot. He tries to hit on her, saying that she's not ugly, he's sorry for insulting her a few days earlier, and that if she left the rat at home, she may even get a date. She sees that it's midnight, and comes out of Todd's closet. She says she should go find everyone else, to which Todd replies, "Or we could bone." When Liza walks away to change, he asks if he can watch.
The girls pull off at a park, and while Sarah comforts Cecily, who's afraid of getting kicked out of Student Council, Nina watches the video, and Chapin and Eve talk. Eve asks Chapin if she likes Whisper, to which Chapin replies that that's a shitty thing to say, because she would never go after someone her friend liked. It's Chapin's turn for a dare, and Cecily dares her to steal a Penthouse from a gas station, even though the porn is behind the counter, and there's a security guard on duty. Chapin is reluctant, and Cecily offers to do it herself, and Sarah explains that Cecily's father owns the station.
Todd drops Liza off at the gas station, and tells Eve that their father still hasn't called. He says good-bye to Liza, and they share a 'look' (it's my interpretation that even if they didn't have sex, they at least hooked up in some way).
Chapin completes her dare, distracting the clerk and the guard by microwaving a metal can of baked beans and exploding the microwave. In addition to the Penthouse, she stole ointment for Liza (evidence that she slept with Todd), condoms for Sarah (to mock her), hair gel for Nina and a bottle of champagne for Eve. Cecily makes a comment about Chapin getting busted by the security camera, and Chapin produces the videotape from the camera from her vest.
Eve is the last dare-ee, and Sarah is the last dare-er. Chapin told her earlier what to dare Eve with, so Sarah dares Eve to go to third base with (give a blow job to) Whisper.
Flashback to the day before, when Mallory and Eve are at soccer practice together. Eve asks Mallory what happened with Stacy, and Mallory says that she doesn't talk about Stacy or Sacred Heart, but tells Eve to ask Chapin.
Back in the present, the girls arrive at the party where Mallory, Whisper and his friends, and Todd and his friends are. Eve asks Chapin about Stacy St. Clair, and Chapin says that she didn't even know Stacy, and that she (and assumably Mallory, also) was expelled from Sacred Heart simply because she was at the party. Eve sees how Todd is looking at Liza and asks if he was mad when he found her. Liza tells her that he got over it. Todd intercepts Liza and leads her off to a private room.
Chapin and Whisper walk outside together and Chapin tells him what her dare was. She tells him she has no intention of ever shaving her head, and he tells her she'd still be hot. Chapin pretty much figures out that he's got a crush on her, and leaves, luring him into the arms of Eve, who dances with him for a while before leading him downstairs.
Sarah sits down and starts knitting, and Stephan sits beside her. He is amused with her knitting, and she starts to teach him how. He gives her a flask, and she drink liquor for the first time. Someone texts him a picture of her streaking, and she gets embarrassed, but gets over it. She tells him that streaking was liberating, that even though she's a 'good' girl that she wants to rage occasionally, too. She even says 'fuck' twice when Chapin takes her downstairs to videotape Eve's dare.
Eve and Whisper kiss a little, and she starts to go down on him, but he stops her, saying that even though she's cute, he's into Chapin. Eve had suspected this, and she storms off. The girls run after her, and Chapin sees Todd and Liza come out of a room, both of them with clothes and hair disheveled.
Chapin drags Whisper into a room, yelling at him for not hooking up withe Eve. She tells him about her being upset that her father hasn't called and that now, if he doesn't hook up with her, she has to get her head shaved. She asks him if she kisses him, if he'll hook up with Eve. She kisses him, but pushes him away when he tries to kiss her again.
She says about the game, "It's supposed to be fun, and then everything goes wrong," to which Whisper replies with, "Like with Stacy?" He tells Chapin that accidents happen, and what happened to Stacy wasn't Chapin's fault. She gets mad at him, saying that she shouldn't have even told him about it, and yelling at him to shut up, before storming out.
Eve finds Mallory. Mallory mentions Stacy St. Clair, and Chapin walks up. Mallory explains to the whole group (and half the party) that yes, Stacy did jump off the bridge... Because someone dared her to.
Flashback to the night of Stacy's death. Stacy, Mallory and Chapin, along with a few other girls are playing Triple Dog. Mallory's head is shaved, which explains why her hair is so short in the present. Stacy's up, and she picks Chapin to dare her. Chapin triple dog dares her to jump off The Jogger Bridge. Mallory warns them that the dare is stupid, but when Chapin mentions that there's $280 in the pot, Stacy agrees to do it. Stacy gets on the railing, and blows a kiss to the group before she jumps. She loses control in the current, and drowns.
In the present, Chapin says that she didn't know about the current, and that people jump off the bridge all the time and survive. Mallory says that Chapin had to be the center of attention, so she dared Stacy to do something that she couldn't. Chapin takes this personally, running off to the bridge. Everyone runs after her, including Mallory, Stephan and Whisper. Chapin gets to the bridge, climbing on the rail. Everyone is begging her not to jump, but she blows them a kiss, like Stacy had done, and jumps in the river.
Everyone runs to the river bank, but the only thing they see is her shoe. Most of the group run to the other side of the river, looking for her there. Eve and Liza stay, and Chapin walks up behind them, alive and unscathed. Eve yells at her for being stupid and selfish and attacks her, but Liza separates them. Chapin gets angry at Eve, saying that she needed to see for herself, because it was her fault. She tearfully confesses to Liza and Eve that she did dare Stacy to jump, and that the current was rough, but she didn't know how rough it was. She admits that Stacy's death was her fault, even though Liza and Eve tell her it was an accident. They all walk back up to the bridge arm in arm, and the group runs separate ways as police sirens approach.
The girls vote, with everyone but Cecily (and of course, Chapin) voting for Chapin. Cecily mentions that technically, they have to shave Eve's head, because she didn't complete her dare. Eve stops waiting for her father to call, instead taking initiative and calling him. He yells at her for calling so late. She literally screams in frustration, then agrees to have her head shaved. Chapin shaves her. Before Chapin heads home, she tells Eve that nobody in their right mind would leave a friend like her behind. It seems as if everyone is friends, even Chapin and Liza and Chapin and Cecily.
The next morning, Chapin returns to the bridge. She steps up to the side, kissing her hand and setting it on the railing before walking away.