It's supposed to be a big event in their lives, but the first day of high school for best friends Wade (NATE HARTLEY) and Ryan (TROY GENTILE), who now wants to be known as T-Dog, turns out to be a nightmare. And that's because Wade decides to step in and tell senior bullies Filkins (ALEX FROST) and Ronnie (JOSH PECK) to stop picking on diminutive freshman Emmit (DAVID DORFMAN).
Accordingly, Filkins and his accomplice make the three boys their target for continued harassment. Too embarrassed to tell their parents and unable to get the school principal (STEPHEN ROOT) to believe their story -- especially since Filkins has everyone fooled that he's a good kid -- Wade and Ryan decide they have to hire a bodyguard for protection.
After a round of interviews that go nowhere, they end up hiring former Army man Drillbit Taylor (OWEN WILSON), not realizing he's really a homeless man who will say anything to get what he needs or desires. That includes enough money to get him to Canada, but his fellow homeless friend, Don (DANNY McBRIDE), convinces him to stay since gaining the boys' confidence will also gain them access to their homes and the possessions therein.
Even so, and realizing the boys truly do need his help, Drillbit decides to pose as a substitute teacher so that he can watch over them. He's distracted, however, by English teacher Lisa (LESLIE MANN) with whom he carries on a torrid affair. From that point on, and as Wade finds himself smitten with fellow student Brooke (VALERIE TIAN), the boys hope that the pointers and training Drillbit has delivered will allow them to handle the bullies who are terrorizing them.