It is the summer of 1987 and James Brennan (Jesse Eisenberg) has just graduated with a comparative literature degree from Oberlin College. He is looking forward to touring Europe for the summer and going to Columbia University for graduate school in journalism when his parents (Wendie Malick and Jack Gilpin) announce they won't be able to help him financially.
James looks for summer jobs, but can only find one, at Adventureland, a local amusement park in his hometown of Pittsburgh, Pennsylvania where his childhood friend Frigo (Matt Bush), who has a habit of punching James in the testicles, works. James is hired immediately by the assistant manager, Bobby (Bill Hader), and is assigned to the games area. He meets some of his co-workers: the dry and sarcastic Joel (Martin Starr), Bobby's wife and manager of Adventureland, Paulette (Kristen Wig), Sue O'Malley (Paige Howard), Mark (Mark Miller), Lisa P. (Margarita Levieva), and the park's repairs technician Mike Connell (Ryan Reynolds), who is a part-time musician and claims to have once jammed with Lou Reed.
James meets another games worker, Emily "Em" Lewin (Kristen Stewart), after she saves him from being stabbed by a disgruntled customer. She later invites him to a party at her house. During the party, Em invites him to join her in the pool. After the party ends, it's revealed that Em has been having an affair with Connell, although he is married. James goes out to a bar for a drink with Em, and while they are talking at the bar, Em is surprised to learn James is still a virgin. Connell arrives at the bar and Em wants to leave, so they go to one of Pittsburgh's three rivers close to one of the city's bridges where James kisses her. The following day, James tells Connell about his strong feelings for Em, which Connell reports to Em. Em later tells James she wants to take it slow.
The next morning, Lisa P. asks James out on a date, but only lets a few people know. After James returns from the date, he learns Em had called to tell him that she regrets having rejected his feelings on their past date. James gives Em some of his marijuana to bake pot cookies to cheer her up. After a pot-induced game-playing streak, Joel is attacked by an angry customer who learns the games are rigged. When Em steps in, the customer pushes her and James defends her by punching him in the balls. The customer's friend starts chasing James who hides in Bobby's office, who then scares the customer off by threatening him with a baseball bat. Em tells James that she doesn't want to lose him.
Joel sees them walking together and decides to quit. James tries to talk him out of it but Joel is angry at James for dating Lisa P. when he already loves Em. James wants to be honest and tells Em about the night he went out with Lisa P. Shocked by his honesty, Em goes to Connell's house that night to end their affair. Prompted by Frigo, the park's mentally challenged parking official tells James he saw Em and Connell doing "push-ups without any pants on" in the back of Connell's car a few weeks earlier. James goes to Connell's house and sees Em leaving, having just broken up with Connell. When James confronts Em, she is so shocked to see him there that she becomes tongue-tied and does not explain; James becomes angered by the assumed betrayal, and leaves after Em calls herself an "idiot". Later, James tells Lisa P. about the affair, but asks her not to tell anyone.
Lisa P. tells her friend Kelly, and soon everybody in the park has found out and Em quits. James is heartbroken and drunkenly crashes his dad's car into a tree and passes out. The next morning, his mother wakes him up and tells him that he has to pay to fix all the damage. After getting his last paycheck, Connell tells James that he has not spoken to Em since she quit. James doesn't have enough money to go to graduate school, but heads for New York anyway. In New York City, James waits outside Em's apartment. She's surprised to see James waiting for her and is reluctant to talk to him, feeling she has ruined everything between them. James tells her that he sees her in a different way than she sees herself, that he sees the girl who saved him from getting knifed, the girl who stood up for Joel, the girl who introduced him to psychotropic cookies, and the girl who doesn't make apologies for herself. Touched, Em brings James up to her apartment. Once there, James gets out of his wet clothes, and finds she still has an Adventureland shirt. They kiss and begin to take their clothes off as the film ends, and James presumably finally loses his virginity to Em.