The film starts with best friends Amy (Kaitlyn Dever) and Molly (Beanie Feldstein) meeting up and heading to their last day at school. Both were straight A students throughout their whole high school career, and some of their classmates look down on them for being so nerdy. Even Principal Brown (Jason Sudeikis) would rather do anything else than to talk to Molly, who is class president. They do, however, have a friend in their teacher, Ms. Fine (Jessica Williams), who gives the girls her phone number if they ever need anything.During lunch, Amy observes a girl she has a crush on named Ryan (Victoria Ruesga). Molly encourages Amy to go talk to her, but when she does, she says something awkward and walks away bashfully.In the unisex bathroom, Molly overhears classmates Theo (Eduardo Franco), Tanner (Nico Haraga), and Triple A (Molly Gordon) talking trash about her. She steps out of the stall and asserts with confidence that she will have a bright future since she is going to Yale, while Amy is going to spend a summer abroad in Botswana to help the women of that region make tampons. It turns out Triple A also got into Yale, in spite of her promiscuous reputation. Tanner also got into a good school, and Theo, who was held back a few years, was recruited to work for Google. Molly starts to lose her mind, and she goes around asking the other party-happy kids what schools they got into, like Georgetown and Harvard. After school, Amy notices Molly is pissed, and she vents that she can't believe that their hard-partying peers got into the same good schools they did while they spent their time in high school only hitting the books. They know another student named Nick (Mason Gooding) is throwing a party that evening at his aunt's house before graduation the next day, and Molly wants to go so that they can say they had fun while they were in school. Although reluctant, Amy agrees to join her friend.The girls put on outfits and tell Amy's parents (Lisa Kudrow and Will Forte) that they are sleeping over at Amy's. After they leave the house, they call their wealthy classmate Jared (Skyler Gisondo) for a Lyft ride to the party. He instead takes them to a party on a yacht that he threw, which he claims to be better than Nick's party, but nobody is there except for the staff and their loony classmate Gigi (Billie Lourd). She gives the girls strawberries and invites them to party with her. While Molly tries to find a way to get to Nick's, Amy goes to the deck to talk to Gigi, who is smoking a joint. After being told by a staff member that she can't smoke, Gigi goes crazy and breaks a bottle before trying to jump off the ship with both girls, but only she ends up jumping in the water. Molly tells Jared that he shouldn't try to buy peoples' affection, and they call for another ride after she gets the address for a party from another classmate named Alan (Austin Crute).Another Lyft shows up, and it's Principal Brown doing his second job. In the car, the girls discuss the potential of Amy hooking up with Ryan, but she wouldn't know what to do since she is inexperienced. Molly pulls up a porn video on her phone as it is charging in the car. They listen to it with headphones, but Brown assumes they are listening to music and puts it on the stereo. He instead hears the squishing from the porno, and he leaves the girls elsewhere.They end up at the home of fellow student George (Noah Galvin), who is throwing a murder mystery party with Alan. He assigns Amy and Molly roles before they run into Gigi again after she SOMEHOW made it to the party from the yacht before the girls did. Gigi tells them that the strawberries were laced with a type of drug similar to ayahuasca. Moments later, the girls begin to experience a trip in which they see themselves as dolls dressed as the roles George gave them. Molly critiques her doll body's anatomy while Amy seems to dig it. The two of them try and compose themselves before they leave the house. On their way out, George suggests that Molly only wants to go to Nick's party because she has a crush on him. Molly admits this to Amy.While they come down from their trip, the girls see a video of Theo at the party karate-chopping a stack of pizza boxes. They decide to find the pizza place and find out the address from a delivery guy. They hop into the car of one driver (Mike O'Brien) and make it look like they are going to burglarize him by using their hair as masks. The driver calls them out on how stupid their plan is since he has a gun in his glove compartment and because he could be a serial killer or rapist. Molly asks for the address to the party, and he gives it to them before ordering them to get out of his car. Before they can call for another ride, Amy realizes she left her phone in the driver's car, while Molly's phone is at 2%. She uses it to call for a ride just before the phone dies.The girls are picked up by Ms. Fine, who proceeds to drive them to the party. She commends them on taking the night to have fun, as she didn't party in high school and overcompensated for it in her 20's, which she warns the girls not to do. Ms. Fine also gives them new clothes to change into.Upon arriving at the party, the girls split up to try and find their respective crushes. Molly plays beer pong with Nick and chats with him flirtatiously, while Amy finds Ryan and hangs out with her. They do karaoke for a while before Ryan invites Amy to go swimming. When she does, she finds Ryan and Nick kissing in the pool. Dismayed, she gets Molly and tries to leave, but Molly doesn't want to. Amy gets more upset and starts to argue with Molly in front of everyone, calling her out for always forcing her to do what she wants to do, while Molly hits back that Amy is a coward. Amy then reveals that she isn't just spending the summer abroad in Botswana, but she is taking a gap year to stay there, which Molly is only just finding out about. This hurts her as it throws off their schedules of graduating together and progressing as friends, so they angrily leave each other.Amy retreats to the bathroom to cry, where she finds mean girl Hope (Diana Silvers). As Hope says rotten things to her, Amy stands up to her and says she is only going to peak in high school. She then kisses Hope, leading to the two of them making out. They are about to have sex on the floor, but Amy messes up and eventually vomits on Hope. Meanwhile, Molly sees Nick with Ryan, finally realizing why Amy wanted to leave. She ends up finding Jared, who admits that he likes Molly a lot for how smart she is and how she doesn't have to try and prove anything to anyone else, while also telling her that Gigi is sad deep down, which is why she acts the way she does. Molly later walks outside as the cops arrive, forcing everyone to run out. Molly leaves after she can't find Amy. She walks home but is picked up by Triple A. Molly reluctantly gets in the car with her. She tells Molly that she got her nickname since she was said to give "roadside assistance" (blowjobs) to a guy. She admits to it, but doesn't like the reputation she's been given, nor does she like her nickname. Molly comes to an understanding with her and apologizes for using the name. She asks that they use her real name when they both go to Yale next year, which is Annabelle (although she isn't particularly fond of that name either). Back at the house, Amy is trapped with other teens that couldn't leave the house in time as the cops are searching the house. Amy tells them she is going to create a diversion, and she runs out to get the cops' attention.In the morning, Molly wakes up to see notifications on social media about Amy being a bad-ass. She sees a video of Amy getting arrested, so she heads to jail to bail Amy out before graduation. They apologize to each other for their behavior, and Molly says she can get Amy out since they have information on a criminal known as The Valley Strangler, as he is the pizza delivery guy they met. With mere minutes before graduation starts, the girls speed in Amy's car to the school, bursting through the gate before their fellow students, who cheer upon their arrival. Molly goes to the stage to deliver her speech, but not before kissing Jared in front of everyone. She then gives her speech, which everyone loves.Sometime later, Molly is helping Amy pack for her trip along with her parents. Hope shows up to Amy's house and forgives her for puking on her, and she even gives Amy her number so that they can hang out sometime. Amy gets excited while Molly is pleasantly surprised at this.Molly later drops Amy off at the airport after she gives Molly her car. They have a regular goodbye, with both of them appearing to tearfully go their separate ways. Molly starts to drive away, but Amy runs in front of the car to stop her. She hops back in and suggests they get pancakes before leaving, to which Molly readily agrees.