Brick

High school student Brendan Frye lives a lonely existence following his break up with ex-girlfriend Emily Kostich and the betrayal of his friend Jerr to the authorities. Brendan discovers a note leading him to a pay phone, where he receives a phone call from a terrified Emily begging him for help. She mentions a "brick", "poor Frisco", "Tug", and the "Pin", before abruptly hanging up. Her fear appears to have been caused by a passing black Ford Mustang, from which a distinctive-looking cigarette was thrown. Upon asking for information from his acquaintance named Brain, Brendan's search for Emily leads him to Kara, another ex-girlfriend - which further leads him to a Halloween party attended by a flirtatious Laura and her boyfriend, Brad Bramish.
Laura points Brendan in the direction of a local diner, where he arranges a meeting through Dode, leader of a stoner clique that Emily belongs to. Upon meeting, Emily recants what she had said on the phone and tells Brendan to let her go. Brendan steals her notepad during the encounter and finds a note that leads him directly to her dead body. Emotionally distraught by her death, Brendan takes it upon himself to solve her murder, enlisting the aid of Brain. Brendan hides the body to avoid police intrusion. Brendan discovers that "The Pin" is a local drug baron. After finding out that Brad is a regular customer of Pin's, Brendan sets about getting his attention by beating up Brad Bramish. Afterwards, Brendan is beaten up by an unknown young man while speaking on the phone with Brain.
Brendan visits Vice Principal Trueman to ask the vice principal if he could investigate the events, without mentioning Emily's death, as a favour for turning in Jerr. While Trueman allows Brendan to continue his investigation, he warns that if Brendan gets caught, Trueman will "throw him under the bus". Brendan visits Kara, to request more information about the Pin, although she fobs him off. Later while walking, Brendan sees the same black Ford Mustang in a parking lot. Before attempting to break into the car, he is noticed and beaten up by the car's owner, who is the man who beat him previously. Brendan asks the man several times to meet The Pin. Reluctant at first, going so far as to drive away and then come back again, the young man takes Brendan to The Pin.
Brendan meets with The Pin and persuades him to consider Brendan for a spot in his operation. It is also revealed that the unknown man is Tug, The Pin's main grunt and muscle. The Pin tells Brendan he will either hire him or rub him out by the next day. On the walk back home Laura tells Brendan that The Pin had previously rejected Emily's attempt to join, so she stole The Pin's brick. Laura then offers to help Brendan, but he distrusts her. While Brendan awaits a response from The Pin at school, he is slashed by a knife wielding man. After a chase, Brendan incapacitates the assailant and The Pin accepts him. Brendan gets a call from Dode, who says he saw Brendan hide Emily's body and, believing Brendan is the murderer, vows to ruin him. Brendan meets with The Pin, who suspects an uprising from Tug.
Brain reports that "poor Frisco" is Frisco Farr, a student who fell into a coma after injecting poorly-cut heroin. At The Pin's house, Tug tells Brendan that The Pin received a shipment of ten bricks and sold eight; one was stolen and replaced with another that had been doctored with detergent, causing Frisco's coma, and the 10th brick remains to be sold. The Pin arrives and tells Tug about hearing from someone who knows what happened to Emily. Brendan, weakened from several recent fistfights, intercepts Dode before the meeting and discovers Emily was pregnant when she died. Dode hints to Tug and The Pin that he has information about who killed Emily, saying it is someone very close, but Tug goes berserk and beats Dode before shooting him in the head. Tug then threatens The Pin, who walks away as Brendan faints from a coughing fit. Brendan awakens in Tug's bedroom, where Tug says he's at war with The Pin.
Brendan confronts Kara, accusing her of manipulating Dode by telling him Emily was carrying his baby and pushing him to sell his information to The Pin. Brendan arranges a meeting between Tug and The Pin, and waits in Tug's bed; Laura enters to comfort him as he sobs over Emily, and they have sex. Brendan recognizes her post-sex cigarette as the same distinctive brand that was dropped from Tug's black Mustang after Emily was frightened during the first phone call. At the meeting, chaos erupts when it is discovered that the tenth brick is now missing. Tug beats The Pin to death while Brendan flees, escaping just as police arrive. As he goes he passes the trunk of Tug's car, where he has hidden Emily's body to ensure that police pin her murder on Tug.
The next day, Brendan meets with Laura in the school's football field. Brendan explains to Laura that he knows she set Emily up to take the fall for Laura's theft of the ninth brick. She further manipulated Emily into meeting Tug who ultimately killed her, after letting him believe he was responsible for Emily's pregnancy. It is revealed that Laura later stole the tenth brick as well. Brendan tells Laura he has put this truth in a note to Vice Principal Trueman of the school, who will find the brick in Laura's locker if, in fact, what he says is true. Laura vindictively tells Brendan that Emily expressed regret that she couldn't keep her pregnancy because she did not love the prospective father, and that Emily was three months pregnant when she died, implying that the baby was his. Brendan watches Laura walk away.

Question: Whose murder does Brendan want to solve?
Answer: Emily.