Ian Lafferty (Josh Zuckerman) is an 18-year-old recent high school graduate. He searches for a girl online making it look as if he's buff and suave, although he's sweet and unassuming. He soon meets "Ms. Tasty" (Katrina Bowden) and agrees to meet her in person. She lives in Knoxville, while he lives in Brookfield. With his best friends Lance (Clark Duke) and Felicia (Amanda Crew), they go in a Pontiac GTO Judge borrowed without permission from Ian's arrogant homophobic older brother Rex (James Marsden).
On the way to Knoxville, they come across a hitchhiker (David Koechner), as the radiator in the Judge overheats. They attempt urinating in the radiator, which only works briefly as they try to leave the hitchhiker in the dust. The hitchhiker, frustrated at Ian's lack of concern for his well being, leaves. As Ian and Felicia wander to find help, Lance is waiting with the car as Ezekiel (Seth Green) happens by in his horse-drawn buggy. Ezekiel and his Amish buddies repair the car while they join a Rumspringa party where Fall Out Boy is playing a concert and at which Lance meets an attractive Amish girl, Mary (Alice Greczyn). The three promise to come again on the way back to do some work in return.
They go to jail due to Ian throwing a tire iron into a state trooper car (due to his increasing frustration at trying to put a possum he hit out its misery), and are released after Mary pays the bail. They find a hotel that sports a wide variety of role playing rooms. Rex arrives angrily and insists that they go back and that Ian cannot visit Ms. Tasty. After Ian pretends to be gay, Rex allows him to see Ms. Tasty, hoping this encounter will change Ian's mind. Ian finally meets Ms. Tasty, However, when he tells her about Felicia, her seduction for Ian to take off his clothes becomes a threat as her psychotic boyfriend Bobby Jo (Dave Sheridan) puts a gun to Ian's head. It becomes apparent that the couple work at a chop shop and attempts to rob Rex's Pontiac GTO. During the comic confrontation, Ian manages to save Felicia, who then is able to run off and report to the police. Ms. Tasty tries to escape, but is stopped by Andy and Randy, two students from Ian's high school she failed to manipulate. Bobby Jo, on the other hand, is treated after being shot by Ian in self defense. Felicia tells the police about the chop shop location and the couple is arrested.
In the end, Lance refuses to come back home, decides he prefers Amish life, and stays behind to marry Mary, while Ian and Felicia realize their love for each other. On New Year's Eve Ian gives his virginity to Felicia. It is also revealed Rex's homophobia stemmed from the fact that he was hiding his own homosexuality. In the final frame of the film, a picture is shown of Lance and Mary being married, accompanied by Ian and Felicia. He's shown sporting a beard much like Ezekiel. A clip of Ezekiel and Fall Out Boy arguing over the fact that the Amish fixed Fall Out Boy's tour bus for just "a five song set" in form of compensation serves as a running gag during the credits of the film.