After being informed that his lack of trial experience threatens his chances of achieving tenure, State University law professor David Blake decides to spend his summer vacation interning for a local attorney. In the nearby resort town of San Juno, California, David is unsuccessful until he happens into the small law office of Barney Castle. Barney enthusiastically offers to pay David's expenses in exchange for assistance on his biggest case to date, the pro bono defense of Angel Chavez, a Mexican-American teenager accused of murdering Marie Wiltse, a local white girl. Marie's body was discovered the evening before on San Juno's private beach after beachgoers heard her screams. Nearby, police found a trembling and frightened Angel and immediately hauled him off to jail. Angel admits that he trespassed onto the beach and ran into Marie, an acquaintance from school. According to Angel, he and Marie kissed, but Marie suddenly became frightened of getting caught and bolted. Noting that Marie suffered from a serious heart condition, Barney declares the state's case weak and refuses to accept District Attorney John Armstrong's offer of a plea bargain. Meanwhile, the simmering racial tension between San Juno's white and Mexican-American communities threatens to explode, leading Barney and David to visit the dead girl's grieving parents to request that they hold a small, private funeral. Mrs. Wiltse agrees to their request, but the following day, two of the town's most outspoken racists, Ralph Castillo and Cap Grant, show up at Marie's funeral and incite the crowd with calls for vengeance and racial segregation. Transformed into a lynch mob, the crowd heads over to the town jail to demand that Angel be handed over. Barney and David, aware that the jailer, A. A. "Fats" Sanders, is sympathetic to the mob, rush to the jail to demand Angel's protection. Eventually, Sanders convinces the assembled townspeople to disperse by promising them a "legal hanging." David begins preparing for the upcoming trial, working after hours at Barney's beach house with Abbe Nyle, Barney's attractive secretary. Meanwhile, in order to raise funds for Angel's defense, Barney travels to New York City with Consuela Chavez, Angel's mother. Judge Theodore Motley, a black man, is assigned to preside over the case, arousing David's suspicion that the choice of judge has been influenced by the powerful town bigots in an attempt to give the trial the appearance of fairness. When David cautiously approaches Judge Motley with his concerns, the judge, greatly insulted, accuses David of racism. Jury selection begins, after which Barney summons David to New York in order to make a speech at a fundraising rally for Angel. David suspects that Barney's New York colleagues are Communists and confronts Barney with his suspicions. In response, Barney cynically proclaims that he does not care whether the money he raises for Angel is "clean, American money." At the Madison Square Garden rally, David reluctantly delivers the short speech Barney has prepared for him, but when he attempts to speak out against Communism, he is drowned out by a large, brass band. Back in California, a disillusioned David is furious with Abbe for not warning him about Barney's political leanings and, to make matters worse, his presence at the New York rally catches the attention of the zealous Senator Battle, chairman of the State Un-American Activities Committee. David soon confronts Abbe, with whom he was beginning to fall in love, with his suspicion that both she and Barney are Communist Party members. Abbe admits that Barney is a Communist, and confesses that she was a "fellow traveler" in her idealistic college days, but insists that she no longer supports the Party. David accepts Abbe's tearful apology and their romantic involvement deepens. With jury selection completed, Angel's trial begins. In his opening statement, Armstrong asks for the death penalty, declaring that Marie died in the act of defending herself from a sexual assault by Angel. David decides against calling defense witnesses, preferring instead to raise doubts about the prosecution's case, a strategy which proves successful as he rigorously cross-examines first Marie's cardiologist and then an eyewitness. Barney returns from New York and, over David's objections, inexplicably insists that Angel take the stand. Soon realizing that Barney plans to sabotage any chance of Angel's acquittal in order to make the boy a martyr for the Communist cause, Abbe advises David to resign from the case, but David refuses to abandon Angel. On the stand, Angel is convincing under David's gentle questioning, but begins to falter when Armstrong catches him in a number of lies, most notably concerning the extent of his sexual education. The jury returns a guilty verdict and David begins preparing an appeal, but Barney promptly fires him. Later, David and Abbe visit Mrs. Chavez hoping to convince her to dismiss Barney from the case, but Barney has successfully manipulated her into believing that the sacrifice of her son will benefit the fight for racial equality. The next morning, as Angel's sentencing commences, David bursts into the courtroom and demands to be heard as a "friend of the court." After Barney's attempts to silence him fail, David makes an impassioned speech revealing Barney's plan to engineer Angel's execution in order to drum up support for the Communist Party. The judge believes David, as do the prosecutor and assembled townspeople, who greatly fear becoming pawns in a Communist plot. Judge Motley sentences Angel to a short term in reform school, causing Barney to denounce him as an "Uncle Tom." Barney's outburst earns him a thirty-day sentence for contempt, while David learns that Sen. Battle's committee is no longer investigating him. A relieved David and Abbe leave the now empty courtroom arm in arm, as a pensive Judge Motley looks on.
