Monster follows the trial of sixteen-year-old Steve Harmon from Monday, July 6th, to Tuesday, July 14th, with the final chapter covering the verdict on Friday, July 17th. There is a short prologue in which Steve reflects on life in prison, which feels unreal to him. It seems like a film, but without the plot or any other such structure. He decides to write a screenplay based on his prison experience and gives it the title Monster, which is what the prosecutor called him during his trial. The rest of the novel takes the form of this screenplay.

On Monday, Steve describes his journey to the courthouse from the Manhattan Detention Center. He employs the language of cinema, noting the camera angles, cuts, and other technical details of the scene. At the courthouse, Steve’s lawyer, Kathy O’Brien, explains that the prosecutor, Sandra Petrocelli, is seeking the death penalty against him for murder, claiming that he acted as a lookout during a drugstore robbery in which Mr. Nesbitt, the owner of the store, was shot. Although the robbery and the shooting were actually carried out by two older men, James King and Bobo Evans, Steve will also be legally culpable of murder if found guilty. Evidence is adduced that connects King and Evans to the murder, though the witness is a criminal who has been promised a reduced sentence for testifying. Steve listens to some of it while his mind occasionally flashes back to his childhood, which was full of violence despite his own attempts to avoid trouble.

On Tuesday, there is another witness who gives similar testimony to the first, and for the same reason: he is a career criminal who has been promised a reduced sentence if he testifies. Asa Briggs, the lawyer representing James King, points to this incentive and questions the objectivity and truthfulness of his account. In the detention center, both before and after the trial, Steve hears the sounds of fighting, and at night he hears a gang-rape taking place.

On Wednesday morning, Steve begins the day thinking about how even shoelaces and belts are taken away from prison inmates, since suicide is a common—and understandable—occurrence in these circumstances. In court, the testimony comes from the investigating officer, Detective Karyl. Steve’s mind drifts back to the night Karyl questioned him. The detective immediately assumed Steve was guilty because he was a young Black man, and Kathy O’Brien is concerned that the jury will share this prejudice and believe the detective, despite the fact that his investigation has been slipshod. Osvaldo Cruz, who has also been accused of taking part in the robbery, testifies that he did so only when threatened by Bobo Evans.

On Thursday, the questioning of Cruz continues. Although he has been charged with keeping a lookout for the robbers, the same crime as Steve, he has subsequently been acquitted in exchange for his testimony, partly because of his youth (he is fourteen). However, Cruz is also a dangerous gang member with a history of violence and intimidation, which undercuts his claim to have been coerced. During this day, Steve becomes increasingly concerned that others see him as a monster, including his lawyer, the jury, and even his father, who comes to visit him.

On Friday, four witnesses take the stand, including the medical examiner, who testifies that the bullet which killed Mr. Nesbitt caused extensive internal bleeding, meaning that he died by drowning in his own blood. Steve is horrified by this, but it does not seem to affect the rest of the court much. James King, in particular, appears bored and indifferent.

On Saturday, the court is in recess and Steve is in the detention center all day. He thinks of what O’Brien has told him about the likelihood of his spending decades in prison and wonders whether he would be able to endure this, especially given what he has heard of the rape and physical injury that are common in prison life. He ruminates on the deep unfairness of his having to go through this ordeal because he happened to be in the neighborhood of the drugstore where Mr. Nesbitt was killed. His mother visits him in prison and gives him a Bible, but she is unable to comfort him. She says she knows he is innocent, but Steve is unsure whether to believe her.

On Sunday morning, Steve goes to the prison church service, but a fight breaks out and the inmates are all put on lockdown. After it ends, Steve’s parents visit. Their attempts to cheer him up are futile, and he thinks that they are already preparing for the worst and mourning him as if he were dead.

On Monday, Petrocelli questions a witness who says that she saw James King arguing with Mr. Nesbitt in the drugstore, but King’s lawyer casts doubt on her identification, and she has to admit to some uncertainty. Bobo Evans, who, like several other witnesses, has made a plea bargain, testifies next. He says that King pointed out Steve as their lookout and that, after Steve left the drugstore, he and King began fighting with Mr. Nesbitt. When Nesbitt pulled a gun, King managed to take it from him and shoot him with it, after which they both fled with money and cigarettes. In O’Brien’s cross-examination, Evans admits that he did not speak to Steve himself and only has King’s word for his involvement.

On Tuesday, Steve himself takes the stand. Having realized that the impression he makes is more important than telling the truth (which so many other witnesses have not done), he says that he did not go into the drugstore on the day in question. Mr. Sawicki, the film teacher from Steve’s school, appears as a character witness, testifying that Steve is a decent person whose main interest is in filmmaking. The case concludes, and Steve returns to the detention center to await the verdict.

The previous chapters have all taken place on consecutive days, but the final chapter occurs on Friday, July 17th, when Steve returns to court for the verdict. The jury finds James King guilty, and he is placed in handcuffs and led from the courtroom. They find Steve not guilty. He attempts to hug Kathy O’Brien, but she turns away, leaving Steve with arms outstretched. The final cinematic image is of Steve silhouetted in this posture, looking like “one of the pictures they use for psychological testing, or some strange beast, a monster.”

The screenplay ends here. A postscript written five months later reveals that Steve is still making films, though he is distant from his parents and wonders obsessively what caused O'Brien to turn away from him.