Sixteen-year-old Steve Harmon recounts his and James King’s trial for the killing of Mr. Nesbitt, a drugstore owner, in a botched robbery in Harlem six months prior. Through personal notes and a screenplay he writes in his notebook, Steve recounts the 11 days between the start of the case and the jury’s verdict. He names the screenplay “Monster” after what the state prosecutor Sandra Petrocelli called him in court.
On the first day of the trial, Monday, Steve sits with his attorney Kathy O’Brien and listens to Petrocelli make her opening remarks: according to the state, late last December, James King and Richard “Bobo” Evans entered a drugstore, tried to rob Mr. Nesbitt, and accidentally shot the man with his own handgun. According to the prosecution, Steve Harmon and 14-year-old Osvaldo Cruz both acted as lookouts during the robbery, and are thus legally culpable for the man’s murder, as well. Nobody actually witnessed the murder, but Petrocelli presents her first key witness, a man who claims to have information that connects King and Bobo Evans with the murder. The man himself is a convict who testifies so that his own prison sentence will be reduced. King’s attorney Asa Briggs challenges the witness’s ability to be objective since he is benefiting personally from testifying at the trial. Steve’s mind wanders back to violent scenes from his childhood growing up in Harlem, even though he himself never sought out violence.
On Tuesday, Steve writes about how much he hates jail and how afraid he is—everyone there is violent and only talks about hurting each other. They attack people for no reason, and one of them carries a knife. In court, Petrocelli produces another witness, also a criminal who tells the same story as the first, in exchange for a reduced sentence. Once again, Briggs challenges the witness’s objectivity and moral character, and the judge adjourns the hearing for the day when Briggs starts to get heated. That evening, Steve lies in bed listening to two men beat and rape another inmate. He thinks about his younger brother Jerry and how much he misses him.
On Wednesday, Steve wakes up thinking about how in jail, they take people’s shoelaces and belts so inmates can’t kill themselves. Steve can’t help but think of himself as a monster, just as Petrocelli branded him. O’Brien told him that her job was to make the jury see Steve as a human being instead, and Steve understands why. In court, Petrocelli brings Detective Karyl in to testify, who (supposedly) investigated the murder and made the arrests, even though he never found any actual evidence at the crime scene. Steve recalls the night Karyl and his partner first questioned him. Karyl automatically assumed he was guilty and said he hoped Steve would get the death penalty, even though he’s just a kid. Back in the courtroom, Briggs accuses Karyl of not actually investigating at all, but just finding a few convicts who’d testify for him instead. O’Brien worries that none of this makes Steve look any more innocent, since half the jury will automatically think he’s guilty just because he’s a young black male. Osvaldo Cruz, a 14-year-old kid whom Steve had to be careful not to offend in Harlem, since he is part of a dangerous gang, testifies that he was pressured into participating in the robbery against his will by Bobo, who threatened him.
On Thursday, Steve writes about his relationship with O’Brien. He can tell O’Brien wants to know who he truly is, and Steve wants her to know that he’s a good person, but he doesn’t know how to make her see that. In the courtroom, Osvaldo continues his testimony against King, Bobo, and Steve, which he is giving in exchange for an acquittal, since he is young and claims he was coerced into participating. However, Briggs and O’Brien cross-examine Osvaldo and force him to reveal that not only is he a gang member with a violent history, but he has also at least once committed savage violence against strangers without reason, which ruins the credibility of his claim that he was afraid of Bobo. Later, Steve meets with his father Mr. Harmon, but realizes that their father-son relationship has broken. He thinks that his dad now sees a monster where his son should be. Steve also recalls watching the murder reported on the news and being arrested by the detectives two weeks later.
On Friday, four minor witnesses testify while Steve thinks about Mr. Nesbitt, lying on the floor, knowing he is about to die. Through the medical examiner’s testimony, Steve learns that Mr. Nesbitt was shot through the lung and died after drowning in his own blood. He is horrified.
On Saturday, Steve thinks about how horrible it would be to spend the next two decades of his life in prison, which seems the most likely outcome. He knows O’Brien privately thinks that he’s guilty, even though she’ll still defend him. Mrs. Harmon visits Steve in jail, but he knows it’s too painful for her to see her son as a prisoner. At night, as Steve lies in bed, he questions his own innocence and recalls King telling him that he was going to rob a place and asking Steve if he wanted to be in on it.
On Sunday, Steve attends a church service in the jail until a fight breaks out and everyone is put on lockdown for the morning. He thinks about how nothing feels real anymore outside of jail, not his memories of his old life or the baseball game on TV. Steve’s parents walk Jerry past the window so Steve can see him, though Jerry is not allowed in the jail because he is a child. If he wasn’t an inmate, Steve wouldn’t be allowed in either. His parents visit briefly, and Steve worries about Monday, which will be a critical day for the prosecution.
On Monday, a woman testifies that she was in the drugstore shortly before the murder and saw King and one other man enter, though she admits she had difficulty identifying King. Once she saw the two men fighting with Mr. Nesbitt, she fled the store. Bobo Evans testifies next, also in exchange for a reduced sentence. From what King told him, Bobo understood that Steve was supposed to be their lookout, and he saw Steve enter and exit the drugstore and walk away. After that, he and King entered and fought with Mr. Nesbitt. When Mr. Nesbitt took out a handgun, King wrestled it from him and shot the man, stole cash and cigarettes, and then both of them went to a fast-food restaurant to buy some food and lay low. Briggs and O’Brien cross-examine Bobo, forcing him to admit that Bobo never actually spoke to Steve himself, nor did he ever threaten Osvaldo to help them with the robbery; Osvaldo wanted to be in on the heist. Petrocelli announces that the prosecution has concluded.
On Tuesday, O’Brien admits that it doesn’t look good for Steve. Bobo’s testimony was damning and Briggs is going to try to associate King with Steve, since it will make King look better, as Steve is obviously a decent kid. King’s cousin testifies and gives a weak alibi for King on the day of the murder. O’Brien wants Steve to testify and present himself to the jury as a good kid. She coaches him on what sort of answers to give and Steve realizes that the truth is less important than making the right case.
On the stand, Steve testifies that he was nowhere near the drugstore on the day of the murder (though he’s privately admitted that he was) because he was working on a film project all week. He also testifies that his relationship with King is minimal; he’s just some guy he saw at the playground occasionally when people were playing ball. When Steve is finished, his film teacher Mr. Sawicki provides a character witness and testifies that Steve is an honest, sensitive kid who makes uplifting films about his neighborhood. Briggs makes his closing remarks, claiming that his client King has no connection with Bobo Evans and did not participate in the robbery in any way. O’Brien claims the same for Steve in her own closing remarks, and adds that there is not enough evidence against Steve to lock up a young kid for the rest of his life. The jury leaves to make their decision and Steve and King are taken back to jail.
That Friday, Steve and King are brought back to the courthouse to hear the jury’s verdict. King is found guilty and duly sentenced for a felony murder charge. Steve is found not guilty. He spreads his arms to hug O’Brien, but she turns stiffly away. He remains with his arms outstretched as the image blurs and fades until Steve’s silhouette looks like “some strange beast, a monster.”