All four main characters experience growth through their various experimentations. Andy’s run as a gigolo of sorts among the married women at the tennis club he works at results in him both losing his job and suffering a yeast infection, the latter of which prevents him from getting intimate with his online pen pal Danya when she comes to visit him. Nora breaks up with her new boyfriend Erland after she discovers he is cheating on her. Lionel’s experiments with drugs costs him his martial arts instructor job, so he renounces all intoxicants, believing that, as a trained fighter, he needs his mind to be sharp.
At the Summer Dump party that the quartet and Danya attend towards the end of the film, Lionel finds out from Danya that Andy hasn’t touched her at all and won’t tell her why, which he expresses disbelief over, and they wind up hooking up instead. Lionel also uses his martial arts skills to defend Andy from the husbands of the tennis wives when they show up to get revenge on him. Meanwhile, Mike gains his desired sexual release in the form of a handjob at the same party from fellow partygoer Margot.
In the end, Andy and Nora realize the feelings they had for each other all along and become a couple, despite them going to different colleges in different parts of the country.
As Andy (Nicholas Braun) is having one of his “private lessons” with Gabby (Ashley Judd) one of the many married women he has been sleeping with who are members of the tennis club he works at at her home, her husband (Eric Lutes) returns from an outing. Gabby forces Andy to leave naked since he won’t have time to dress, and she hides his clothes from her husband, but fails to notice that his headband has dropped on the floor. As Andy flees in his car, he gets pulled over by Officer Tirrell (Nick Chambers) for speeding, and is subsequently taken to the station after Tirrell sees his nude state. While there, Andy spots his old classmate Plymouth (Brett Wyman) being brought in, and after Plymouth makes a few math/sex combination jokes at him, Tirrell asks Andy if he knows about math and then has him help him sort out a problem he is having with a mortgage he is taking out. However, he makes it clear to Andy that his arrest is still on record. During his next shift at the tennis club, Andy is confronted by his boss Yaco (Demian Bichir) and co-worker Salvatore (Eduardo Baumann) about the driving incident, and they reveal that they, too, have been sleeping with Gabby. However, Yaco says he is displeased with Andy’s sloppiness, because if he ever gets caught, suspicion will fall on him, Salvatore, and all the other pros at the club, and he orders Andy to stop giving the women “lessons” or else he will fire him. Around the same time, a maid cleaning Gabby’s bedroom finds the headband and sets it on the nightstand.
As Andy is out at a party with Nora (Zoey Deutch), Lionel (Mateo Arias), and Mike (Israel Broussard) later on, he lashes out at Nora’s new boyfriend Erland (David Coussins) over how he can’t stand him (something he fought with Nora about before), so Nora talks to him privately and demands to know what is wrong with him. He blurts out that he wants to be the one dating her, revealing his true feelings for her, but she replies that they have been friends for so long that dating each other would be too weird. Andy later takes off to meet up with another tennis wife, but being somewhat drunk, he accidentally wanders into the house next to hers, where another former classmate, Conch (Dayo Okeniyi), is babysitting the son of the house’s owners. Conch invites him to sit, and after Andy tells him that he was just at a party, Conch expresses surprise that he went to one since neither he nor his friends ever went to any of the parties he and Plymouth hosted. Andy replies that they were never invited to them, prompting Conch to hit him with a revelation: Invitations weren’t necessary because ALL were welcome to attend the parties, including him and his friends, but they instead opted to stay away. However, he does give an official invite to him and the others to come to the Summer Dump party, scheduled for the last weekend of August in a local abandoned steel mill.
Nora later finds out that Erland is being unfaithful to her when she overhears him talking on the phone with another woman and arranging a time to have sex with her (Nora confronts Erland with this several days later before telling him to go screw himself). Lionel’s experiments with drugs end up costing him his job as a martial arts instructor after he tries to teach a class while high. Gabby’s husband notices Andy’s headband and calls the club asking if anyone had lost it, prompting Yaco and Salvatore to confront Andy again over the item before the former fires him and warns him that he will be giving his name to anyone trying to find information on who has been sleeping with their wives. Meanwhile, the husbands have a meeting over the headband and deduce that Andy is its owner, and decide to get revenge on him.
Andy, Nora, and Lionel meet up with Mike at his job later that day and give him the scoop on the unfortunate events that have befallen them, and Lionel decides to renounce intoxicants forever on the grounds that, as a trained fighter, he needs his mind to be sharp. Andy tells them that they got invited to the Summer Dump, and all are enthused to go except for Nora, who votes that they cut their losses now. Mike disagrees, saying that their group has been disintegrating this summer, and this party will be the last time they will be hanging together, so they are going and that’s that. Suddenly, Andy gets a call from his online pen pal Danya (Tasie Lawrence), who tells him that she is at the shuttle bus stop in Hyannis and has come to visit him, something he wasn’t anticipating since he hadn’t checked the spam folder in his e-mail account recently. He goes and picks her up, and she greets him with a kiss.
Andy later takes Danya to a bed and breakfast to get some alone time with her, but just before they can do anything physical, he excuses himself to go urinate. However, he discovers that he has a burning sensation when he does so, prompting him to abruptly leave Danya to go see a doctor. He finds out that he has a yeast infection, brought about by his surfeit of sex and his alcohol consumption suppressing his immune system, and he is told to abstain from sex for the next week. He doesn’t tell Danya about any of this and makes excuses to halt any intimate moment she tries to initiate, and she comes to believe that he doesn’t like her.
The four friends go to the Summer Dump party with Danya, where they are greeted with open arms. As the evening goes on, Mike gets approached by Margot (Angela Jaymes), a partygoer, who develops an interest in him after she sees him use his culinary skills to alter the punch to make it taste better, and she later gives him a handjob, his desired form of sexual release that he had been seeking. Andy and Nora also find themselves growing closer in various ways, taking too much of a toll on Danya’s emotions and causing her to go away from everyone to mope. Seeing that she is hurt, Lionel goes to her, and she tells him that she thinks Andy doesn’t like her at all because he hasn’t touched her since she arrived. Lionel expresses disbelief over this, telling her that she is so beautiful that that doesn’t make sense. The two of them instantly click after this and end up making out in another part of the mill.
The tennis wives’ husbands then make a surprise appearance and walk straight up to Andy. Sensing an imminent demise, Andy tells Nora he loves her, and after trying and failing to smooth things out with the husbands, Gabby’s husband starts punching him. Suddenly, Lionel having sensed that Andy was in trouble comes to Andy’s aid and uses his martial arts skills to beat all of the husbands unconscious. Mike appears asking what is going on, and Andy tells him that maybe they all ought to call it a night now, and Mike is cool with that, then tells them of how he attained release. Officer Tirrell then appears with his partner and sees the beaten men, and Lionel confesses to being the assaulter when asked, with Nora adding that he did it to save Andy’s life. Tirrell then recognizes Andy and tells him that he got his mortgage situation fixed thanks to him, so he has decided to wipe his arrest from his record. He then offers to give the gang (and Danya) a ride home, and on their way out, the partygoers give the four of them a huge ovation.
A few days later, the quartet take Danya back to the bus depot so she can resume her journey out to Los Angeles to attend UCLA, and she and Lionel who had sex a few times in the days following the party fondly bid each other adieu and agree to shoot for a meetup during the winter break. The gang then drive back to Nora’s house, and after Lionel and Mike say their farewells first, Andy and Nora get to talking. She finally reciprocates his love, explaining that the Summer Dump wasn’t the right place to say so, and they promise each other that they won’t try to find love with anyone else while they are apart. They then give each other a big kiss goodbye.
Andy is dropped off at Dartmouth College by his parents, and while he waits in line to register for his classes, he catches a gummy bear in his mouth that gets thrown to him from a distance by his friend Tinsley (Julia Garner), who is also attending the college. He gets praised by one of the other students in line, so Andy makes his introductions with him and his friends.