Having just graduated from college, James Brennan (JESSE EISENBERG) is looking forward to a European trip with his friend with whom he'll live in New York City come fall. But the summer trip is scuttled when his parents (JACK GILPIN & WENDIE MALICK) inform him that they can no longer afford it due to his dad being demoted at work.
With no useable skills that might land him a decent summer job, James ends up working at Adventureland, the local amusement park run by Bobby (BILL HADER) and Paulette (KRISTEN WIIG). Besides lamenting the notion of working all summer with a kid he grew up with, Tommy Frigo (MATT BUSH), who has a thing for punching him in the crotch, James isn't happy when Bobby turns down his request to run a ride.
Instead, he ends up assigned to work one of the carny games, much like Joel Schiffman (MARTIN STARR) who has a realistic but droll view of himself and general outlook on life. He's attracted to sexpot Lisa P (MARGERITA LEVIEVA) who's returned for another summer of work at the park -- but he eventually ends up drunkenly making out with another young employee, Sue (PAIGE HOWARD) -- while James finds himself drawn to Em Lewin (KRISTEN STEWART), a pretty girl who's disillusioned with her life. Little does James know, however, that she's secretly having an affair with Mike Connell (RYAN REYNOLDS), the park's married maintenance man who becomes something of James' older mentor.
As the various young people schlep through their menial tasks at the park day and night through the summer, they must contend with various issues, including budding romances and thoughts about their futures.