Two weeks ahead of their high school graduation, long-time best friends Evan and Seth are faced with the reality of attending different colleges. Their mutual friend, Fogell, intends to procure a fake ID, prompting Seth to volunteer to buy alcohol for a party hosted by his crush, Jules (Emma Stone). Meanwhile, Evan has aspirations to buy Goldslick vodka for his own crush, Becca (Martha MacIsaac). Tension arises between Evan and Seth, as both will head to Dartmouth while Seth is bound for a state school.
During the alcohol purchase, Fogell encounters a hooded robber, leading to a chaotic situation where police officers, Michaels (Seth Rogen) and Slater (Bill Hader), mistakenly believe Fogell's fake ID, which amusingly identifies him as “McLovin,” is legitimate. Instead of taking him home, they offer Fogell a ride to the party, making unexpected detours and showcasing their poor judgment as officers by stealing beers and misusing their police authority.
Through a series of misunderstandings, Evan and Seth worry that Fogell has been arrested after Seth is nearly hit by a car. The driver, Francis (Joe Lo Truglio), is reluctantly drawn into their plan to attend the party, but things go awry as he is unwelcome and ends up getting attacked by the party host, Mark (Kevin Corrigan). Seth and Evan sneak out with stolen beer hidden in detergent bottles, only to stop later to catch their breath.
During a heated exchange, Seth confronts Evan about his perceived lack of loyalty. In a moment of frustration, Evan asserts that Seth has restricted his opportunities for the past three years, accusing him of spending too much time on trivial matters. Their argument escalates into a physical altercation, which culminates with Seth being hit by the police cruiser containing Slater, Michaels, and Fogell. As the officers attempt to apprehend them, Evan makes a break for it, allowing Seth and Fogell to escape with the alcohol.
Upon reaching the party, a tipsy Becca makes a bold move toward Evan, but he declines, concerned for her state and ending up cleaning up after her unfortunate bout of vomiting. Seth, inebriated, tries to kiss Jules but passes out, unintentionally giving her a black eye in the process. Meanwhile, Fogell successfully attracts his crush, Nicola (Aviva Farber), but is quickly interrupted by the arrive of Slater and Michaels, who unveil that they were aware of his forged ID the whole time yet chose to overlook it, seeing a bit of their youthful selves in him.
To further complicate matters, Fogell convinces the officers to pretend to arrest him in front of the partygoers to elevate his stature among his peers while contriving a story to explain the state of their damaged police vehicle. In the midst of the chaos, an unconscious Evan is whisked away by Seth, who avoids the cops searching for them.
The next day, Evan and Seth run into Becca and Jules at the mall, leading to a hilariously awkward exchange that reflects on their wild night. As they navigate the fallout from their escapades, Seth takes Jules to obtain makeup to cover her bruise, while Evan and Becca search together for a new comforter.
