It is the summer of 1987, and James Brennan, played by Jesse Eisenberg, has just graduated with a literature degree from Oberlin College. All set for a European tour and prospective studies at Columbia University, his plans take a turn when his parents, portrayed by Wendie Malick and Jack Gilpin, reveal they can no longer provide financial support.
As James searches for summer jobs, he finds an unexpected position at Adventureland, a local amusement park in his hometown of Pittsburgh, Pennsylvania. There, he reconnects with his childhood friend Frigo (Matt Bush), who is notorious for playfully punching James in the testicles. The assistant manager, Bobby, hires him and assigns him to the games area. On his journey through this new chapter, he meets colorful co-workers such as the dry-witted Joel, Bobby's wife and park manager Paulette, as well as the park's technician Mike Connell (Ryan Reynolds), who leads a double life as a part-time musician boasting tales of jamming with Lou Reed.
One day, James encounters Emily "Em" Lewin, portrayed by Kristen Stewart, when she heroically saves him from a disgruntled customer. Their bond begins to blossom when Em invites him to a party at her house. Amidst the festivities, they share some intimate moments, including a flirtatious pool session. However, complications arise when it’s revealed that Em is entangled in an affair with Connell, despite his marital status. During a heartfelt outing at a local bar, James reveals to Em that he is still a virgin, and their chemistry ignites further when a kiss happens as they linger near one of Pittsburgh's three rivers.
The next day, when Lisa P. asks James out, he learns that Em regrets the pace of their relationship. As they navigate miscommunication and jealousy, James finds himself defending Em against an angry customer incident at the amusement park, strengthening their connection. Despite this, personal conflicts mount as Joel expresses his frustrations over James dating Lisa P., all while trying to unravel the complexities of their feelings.
Fueled by the revelations and emotions, James heads to Connell's house, only to stumble upon Em leaving after ending her affair. The confrontation leaves both hurt, leading to tensions that culminate in Lisa P. unintentionally spreading the news of Em's past relationship, causing her to quit the park altogether. In a haze of heartbreak, James crashes his dad's car and wakes up to the consequences of his actions.
Though James's dreams of graduate school are dimmed by financial woes, he follows his heart to New York City, waiting outside Em's apartment. Their reunion is bittersweet; Em feels responsible for the fallout. Yet, James reassures her, sharing how he sees her as the courageous girl who intervened when it mattered, and the one who introduced him to a different way of experiencing life. This heartfelt exchange rekindles their connection, and as they retreat to her apartment, a new chapter unfolds—one where James finally embraces intimacy with Em as the film comes to a close.
