When the Delta Psi Beta fraternity moves into the neighborhood, Mac and Kelly Radner (Seth Rogen, Rose Byrne), a young couple with a newborn baby, want to appear cool. They meet Teddy (Zac Efron), the charismatic president of the fraternity, who appears to be a nice guy. He promises the noise will be kept to a minimum.

However, after one more extremely loud party, complete with fireworks, Mac and Kelly end up calling the police for a noise violation. When asked who called them, the police point the finger at the Radners, and the frat boys declare war.