On a hot and dry evening in September, a group of men is gathered in a barbershop in Jefferson, Mississippi, discussing the rumor that a black man, Will Mayes, has attacked Minnie Cooper, an unmarried white woman. The barber, Henry Hawkshaw, attempts to convince the other men that Mayes is innocent, but the others angrily argue that a white woman must be telling the truth. The war hero John McLendon enters the shop, a gun protruding from his pocket, to round up a group of men to find Mayes. As the men leave with McLendon, Hawkshaw decides to follow them and try to stop them from hurting Mayes.
Minnie Cooper, who lives with her aging mother and aunt, is nearly forty. She spends her days sitting on her porch in the mornings, window shopping with her friends in the afternoons, and at the moving pictures in the evenings. Though she was an attractive young woman, she never married. She dated a widowed bank clerk for a time, but he left her when he got a job in Memphis. According to local gossip, Minnie regularly drank whiskey she bought from the clerk at the soda fountain, and had in the past accused another man of watching her undress.
Hawkshaw finds the men on their way to find Will Mayes and joins them, still intending to keep the others from causing harm. The men arrive at the ice factory where Mayes works, drag him out, handcuff and beat him, and then put him in the back seat of the car next to Hawkshaw. Mayes attempts to talk to the men, maintaining his innocence, but the men ignore him as they drive out to an abandoned area of town. Feeling sick, Hawkshaw begs McLendon to stop the car and let him out, to no avail. He finally decides to open the door while the car is moving and jump out. Hawkshaw then walks along the road, hiding in a ditch when the car passes him on its way back to town.
In the meantime, Minnie is preparing to go to the movies for the evening. She is suddenly the center of attention: her friends come to help her dress and press her for more information about the crime, and the men downtown watch her and whisper about her and Mayes. The news has spread that McLendon and his gang have retaliated against Mayes for his presumed crime, and it is then that everyone notices that there are no black men in the square that evening. When Minnie goes in to watch the film, she has a fit of uncontrollable laughter, and is escorted out of the theater. Her friends take her home, undress her, put her into bed with an ice pack, and call for the doctor.
McLendon returns home at midnight. His wife has stayed awake, which angers McLendon and leads to a confrontation between the couple. McLendon abuses his wife verbally, then grabs her shoulder and throws her onto the chair. He walks to the bed, undresses, puts his gun on the bedside table, and attempts to wipe the sweat from his body.