Seth (Jonah Hill) and Evan (Michael Cera) are about to graduate from high school. Best friends for years, they face an uncertain future: Evan has been accepted into Dartmouth but Seth will have to attend the local state university. Both boys are notably horny but still virginal, something they want to correct before leaving for college at the end of the summer. Neither is particularly popular, as evidenced when a particularly obnoxious classmates stops them before school one day to not invite them to his graduation party. Seth and Evan do have female classmates to whom they are attracted. Seth is pretty sure that Jules (Emma Stone) returns his interest, but Evan is oblivious to the fact that Becca (Martha MacIsaac) wants to get with him. Seth resents Becca both for the trouble she caused for him in first grade when she revealed his obsession with drawing penises, and because she threatens his close relationship with Evan. In home economics class, Jules invites Seth and Evan to her graduation party that night. When painfully nerdy friend Fogell (Christopher Mintz-Plasse) reveals that he is about to get a fake ID, Seth is determined to provide the alcohol for Jules's party. Because Becca will attend Jules's party, too, Seth is convinced that he and Evan will have sex that night.
Things immediately go wrong. Seth's car is towed when he parks in the faculty parking lot. When Fogell arrives with his fake ID, Seth is appalled to learn that Fogell has adopted the name "McLovin." Believing that the ID won't be accepted, Seth plans to shoplift vast quantities of liquor from a supermarket but chickens out at the last minute. Instead, he and Evan send Fogell to a liquor store. Much to his surprise, Fogell successfully uses his fake ID to buy the liquor. But before he can leave the store, he is knocked unconscious by a robber. Because they went to ogle a buxom woman down the street, Evan and Seth don't see the robbery. They do see the police car arrive and assume that Fogell is being arrested for using a fake ID. Distraught at having neither the alcohol nor the $100 Jules gave him to buy alcohol, Seth panics. He is then hit by a car driven by Francis (Joe Lo Truglio). Francis is desperate for Seth not to report the incident to the police and agrees to procure alcohol for them. Even though Francis is creepy, Seth and Evan accompany him to a house party miles from the liquor store. Shortly after getting there, Francis is beaten up by the home owner, Mark (Kevin Corrigan). Evan is angry that Seth has dragged him to this dangerous party and announces he will no longer allow Seth to dictate the course of their friendship. Seth accuses Evan of deserting him as a friend. Seth returns to the party to steal alcohol. He dirty dances with Mark's menstruating girlfriend and winds up with a blood stain on his pants. When Mark sees this, he picks a fight with Seth. Evan tries to call Becca but can't get good reception on his cell phone so goes back into the house to use a landline. He finds himself trapped in a room with cocaine snorting thugs who mistakenly think he is the golden-throated brother of someone else. He sings "These Eyes" to placate them. Eventually, Evan and Seth manage to escape the party with large amounts of alcohol concealed in two large laundry detergent jugs.
Meanwhile, Fogell was not arrested by the police. In fact, Officers Slater and Michaels (Bill Hader and Seth Rogen) seem to be impressed with his taste in alcohol and ability to take a punch. Fogell winds up spending the night with them. He helps them subdue a violent drunk at a local bar and then shares a beer with them. They reveal a rather casual attitude towards police procedure and even let Fogell hold their guns. Slater and Michaels receive a call to break up a fight at the party Seth and Evan are trapped at. On the way to the party, Slater runs into Seth, knocking him to the ground. When he discovers alcohol in one of the detergent jugs, he plans to arrest Seth and Evan. But while Slater and Michaels try to devise a plan to explain the damage to the squad car, Fogell, Evan and Seth escape along with the detergent jugs and all of the alcohol Fogell bought, which he had been carrying with him while patrolling with Slater and Michaels. They take the bus to Jules's party. On the way, Seth learns that Evan and Fogell have already decided to room together at Dartmouth.
Becca is already there and already drunk. Evan learns she has spent the evening drinking heavily and announcing her intentions to have sex with him. She drags him to Jules's bedroom and drunkenly tries to seduce him. He resists, wanting their relationship to be more than physical. She throws up. Seth has no greater luck. Convinced that Jules will sleep with him only if they are drunk, he gets inebriated but is surprised to learn both that Jules doesn't drink and that she would prefer they hook up when both are sober. Confused, Seth passes out; on the way down, his head slams into Jules's, giving her a black eye. When he comes to, he sees Slater and Michaels have arrived to bust the party. He rushes inside and, finding Evan drunk and passed out, carries him to safety. Michaels and Slater find Fogell in a bedroom about to have sex with Nicola (Aviva). Slater is angry that Fogell abandoned them but Michaels calms him down. They tell Fogell that they knew his ID was fake but remembered how much they hated police officers when they were Fogell's age. Wanting to demonstrate to him that cops could be cool, too, they took him out on patrol. They all agree to be friends. To impress the ladies, Fogell has Slater and Michaels drag him away as though he were a dangerous criminal. Then they go to a deserted parking lot, do donuts in the squad car, then trash it.
Evan and Seth go back to Evan's house. They make miniature pizzas and climb into their sleeping bags. They confess that they will always love one another as friends. The next day, they go to the shopping mall; Evan wants a new comforter to take to Dartmouth and Seth needs new pants. There they see Becca and Jules. Becca thanks Evan for not taking advantage of her drunkenness and Seth apologizes to Jules for his own embarrassing behavior. Becca and Evan decide to go shopping for comforters and Seth will help Jules buy makeup to cover up her black eye. Secure in their friendship, Seth and Evan are finally comfortable being apart.
Summary: Two high school best friends forever (Jonah Hill, Michael Cera) go on a mission to provide alcohol for a graduation party in hopes of losing their virginity.
After facing the fact that both Seth [Jonah Hill] and Evan [Michael Cera] will both be going to different colleges next year, it seems more imporant than ever that they go to a high school party, and achieve some of their long time goals; including, but not limited to, losing thier virginity with the girl of their dreams by getting them wasted and being their "regret hookup." Their awkard friend Fogell [Christopher Mintz-Plasse] gets a fake ID, with just the name McLovin [it was either that or Mohammid], and agrees to get liquor to bring to Jules' [Emma Stone] aka Seths dream girl's graduation party. All seems to be going well until McLovin is suckerpunched in the liquor store by a robber. When the police come, they try to get a story out of the clerk, but she is too worried about her exam tomorrow, so their question McLovin, which they give offer to give a ride home [more hillarity ensues].
All in all the boys almost / briefly get laid, and learn that its just better to man up and confront the girl than try to / count on her geting wasted if you like her.