High school student Brendan Frye (Joseph Gordon-Levitt) discovers a note directing him to a pay phone, where he receives a call from his terrified ex-girlfriend Emily Kostich (Emilie de Ravin), begging for help. She mentions a “brick”, “Tug”, and “the Pin” before abruptly hanging up, apparently afraid of a passing black Ford Mustang, from which a distinctively-branded cigarette is thrown.
Unable to locate Emily, Brendan enlists his friend Brain (Matt O’Leary) for help. An encounter with another ex-girlfriend, Kara (Meagan Good), leads him to a party held by flirtatious upper-class girl Laura Dannon (Nora Zehetner) and her boyfriend, Brad Bramish (Brian J. White). Laura points Brendan to Dode (Noah Segan), who arranges a meeting with Emily.
Emily dismisses the phone call and tells Brendan to let her go. Brendan steals her notepad and finds a note that leads him to her dead body in a tunnel the following morning. Distraught, Brendan decides to investigate her murder, hiding the body to avoid police intrusion. Brendan discovers that “the Pin” (Lukas Haas) is a secretive local drug baron. As Brad is a frequent drug user, Brendan picks a fight with him, hoping to attract the Pin’s attention. Later, a man wearing a beanie attacks Brendan.
Brendan sees the black Mustang in a parking lot and tries to break into it. He is beaten up by the car’s beanie-wearing owner. Rather than fight back, Brendan repeatedly asks to meet the Pin. The man is Tug (Noah Fleiss), the Pin’s main enforcer, who reluctantly takes Brendan to the Pin’s house.
Brendan asks the Pin for a job, and the Pin says he will investigate him and either hire or kill him by the next day. Laura reveals herself and drives Brendan back to school. She explains that Emily stole a “brick” of heroin after being rejected by the Pin’s operation. Laura offers to help Brendan, but he distrusts her.
The next day, Brendan learns that the Pin has hired him. Dode calls Brendan and says he saw Brendan hide Emily’s body. Believing Brendan is the murderer, he threatens to ruin him. Brendan meets the Pin, who suspects that Tug is planning to betray him.
At the Pin’s house, Tug tells Brendan that the Pin recently bought ten bricks of heroin. He quickly sold eight wholesale. The ninth was stolen and later returned contaminated, and the final brick remains to be sold. The Pin arrives and says that someone wants to meet to discuss Emily, revealing that Tug was romantically involved with her.
Brendan intercepts Dode on the way to the meeting and discovers Emily was pregnant when she died; Dode believes it was his baby. Brendan passes out from his accumulated injuries and arrives at the meeting late, where Dode is demanding money to reveal who killed Emily. Tug goes berserk and shoots Dode in the head, then threatens the Pin, who walks away as Brendan faints again. Brendan awakens in Tug’s bedroom, and Tug tells him they are at war with the Pin.
Brendan arranges a meeting between the two and waits in Tug’s bedroom. Laura comforts him as he grieves Emily, and they kiss. Brendan recognizes her cigarette as the same brand that was dropped from the Mustang during the call with Emily. At the meeting, chaos erupts when it is discovered that the tenth brick is missing. Tug beats the Pin to death while Brendan flees, escaping just as police arrive. As he goes, he passes the partly-open trunk of Tug’s car, where he has placed Emily’s body to ensure that police blame her murder on Tug.
The next day, Brendan meets Laura at the school. Brendan explains that he knows Laura set Emily up to take the fall for Laura’s theft of the ninth brick, then manipulated Emily into meeting Tug, who panicked and killed her after she told him he was the father of her unborn child. Brendan has written a note to the school administration stating that the tenth brick is in Laura’s locker. Laura vindictively tells Brendan that Emily did not want to keep the baby because she did not love the father, and that Emily was three months pregnant when she died, meaning the unborn child was his.