During 1848 California's newly inaugurated statehood, a newborn justice system is put to the test in the trial of a hotheaded gunslinger who shoots a repected Hispanic citizen in a state that is a volatile balance beteween whites and Hispanics.

In the town of San Bartolo, California, in 1848, just after Mexico cedes California to the United States, Hatfield Carnes shoots Francisco Quiroga as Quiroga draws, then fires at him until Quiroga dies. Young sheriff Jim Ellison, who prefers using his fists rather than his guns because he realizes others are better shots than he, arrests Hatfield, who remarks that he "had to shoot me a Mexican." The distinguished New England judge Millard Isham, arriving with Deputy U.S. Marshal Ben Stroud to conduct the trial, is astounded to learn that Jim, formerly a corporal in the Marines, has not received his authority from the U.S. territorial government, but from Don Roberto de la Madrid, the former alcalde, who still runs most of the area. At a birthday celebration for Don Roberto's attractive daughter Elena, Ben and Jim select a jury that includes several new Mexican-American citizens. When Elena asks Jim, who is shy, to dance, he first asks her father's permission. The next day, during the trial, which is held in a converted animal hide warehouse, Jim testifies that although Hatfield frequently bullied Mexicans, none had ever tried to attack him. When Hatfield takes the stand and uses the term "Mex," the judge reprimands him, saying that the Mexicans present would find the word distasteful. Hatfield relates his version of the events: After he told Quiroga, who had been drinking in the same saloon as Hatfield and his friends, that he did not belong there, he assumed there would be a fight and told Quiroga to step outside. When Quiroga went for his gun, Hatfield beat him to the draw. After Hatfield admits that he had previously killed four Mexicans, Ben contends that he deliberately antagonized Quiroga. General store owner Clarence Tolliver, a witness to the killing, testifies that Hatfield told Quiroga, "You talk big, Mex. Let's see if you got guts enough to draw on a white man." Judge Isham instructs the jury that they must return a guilty verdict if they decide that Hatfield, knowing he was more adept with a gun than Quiroga, goaded him into drawing. If, however, Hatfield shot in self-defense, he must be set free. Mexican cowboys known as vaqueros, who have gathered in town because of the trial, believe Hatfield will not be convicted because killing Mexicans has never been considered murder in the area. Upon seeing the vaqueros, Hatfield worries about a lynching, but Jim maintains they are American citizens now and have a right to their opinion. When Don Roberto arrives in town with Elena, Jim takes him to meet the judge, who is honored. After Isham sends Jim to quiet a mariachi band hired by Lee Hearn, a man Ben has recognized as an outlaw from Fort Omaha, Jim bests Lee in a fight. When the judge berates Jim for leaving Hatfield alone and orders him to appoint a deputy to guard the prisoner, Jim convinces Lee to do the job. Over dinner, Don Roberto tells Judge Isham that the case puts American justice on trial; he wants to believe that Mexican-Americans are citizens entitled to all rights, as the Treaty of Guadaloupe Hidalgo states, but wonders if an American jury will be as stern with Hatfield as they would be if a Mexican killed a white American. Judge Isham frankly states that he does not know, but hopes the jury will be fair. When the judge apologizes for some rowdy whites in the cantina, Don Roberto acknowledges that ruffians are needed for a young land. Later, the rowdies from the cantina circle the jail, riding and shooting in the air, and when Jim sees the vaqueros reaching for their guns, he tries to send Elena home, but she refuses to go. He brings her to the jail for protection, but Hatfield embarrasses her with talk about his sexual exploits with "squaws." Lee, who sympathizes with Hatfield, offers to help him escape. Judge Isham invites Don Roberto to join him on the bench when the jury returns its verdict, as he wants the Mexican people to feel represented. Learning that Lee is a wanted man, the judge castigates Jim for choosing him as a deputy, but Ben attests that quite a number of lawmen started out with bad records. After the jury announces its guilty verdict, Jim disarms the whites and the vaqueros in the courtroom. Isham sentences Hatfield to twenty-five years in a federal prison, but because the crime was committed during a period of transition to constitutional law, he suspends the sentence on the condition that Hatfield never own, wear or touch a firearm again. As soon as Hatfield is set free, he grabs Lee's gun and hits him over the head. Jim gets a pistol and follows Hatfield outside, while Ben stops the whites and vaqueros from following. Unhappy with the quality of the pistol, Jim makes his way to the jail to get his rifle, as Hatfield boasts that after he kills Jim, he will come after Elena. At the jail, Elena urges Jim not to go out, saying they could barricade the door until help comes, but Jim says he would then be ashamed to look in the mirror. He kisses her for the first time, then goes out and orders Hatfield to drop his gun. After Hatfield is killed in the ensuing shootout, Judge Isham tells Don Roberto that American justice has been completely vindicated. He commends Jim, who, though slightly wounded, embraces Elena.