Mildred Hayes lives in Ebbing, Missouri, and for the past few months has been grieving the rape and murder of Angela, her teenage daughter. She is frustrated and angry because it seems that the investigation has stalled already. Mildred rents three billboards near her home and posts "STILL NO ARRESTS", "HOW COME, CHIEF WILLOUGHBY?" and "RAPED WHILE DYING" on them. Chief Willoughby is angry about the billboards and it seems that most of the townspeople are disquieted by them too. Although it hasn't been announced, it is a worst-kept secret in town that Chief Willoughby is dying of incurable pancreatic cancer, and the general feeling seems to be that Mildred is kicking a dying man when he is down. Mildred, though, is resolute, keeping the billboards in place despite threats and abuse from the neighbors.
Willoughby is not unsympathetic to Mildred's situation and her feelings of grief and anger. He does resent the slight against his personal character and morality. Officer Jason Dixon, an alcoholic racist cop given to outbursts of violence, decides that the way to get the billboards taken down is to threaten their owner and he seeks out businessman Red Wellby. He also arrests Mildred's friend Denise on trifling drug charges.
Mildred's dentist threatens her and she defends herself by grabbing a drill and drilling a hole through his thumb. Willoughby calls her in for questioning, but begins to cough up blood during the interview. He spends the day with his wife and children and then commits suicide because he does not want his family to have to watch him die. He leaves a note for Mildred telling her that she and her billboards were not a factor in his suicide. He had secretly paid to keep the billboards up for another month because he wanted to keep attention on the murder. Dixon assaults Welby and blames the billboards for Willoughby's suicide; he is fired by Willoughby's successor, Abercrombie, who witnesses the assault.
Things begin to spiral out of control. Mildred's billboards are destroyed by an arsonist and she retaliates by throwing a Molotov cocktail through the police station window at night when she believes it to be empty; however, Dixon has stayed after his shift to read the letter that Willoughby had left for him. The letter tells him that he needs to put aside his hate and learn to work from a place of love, which is the only way in which he will be able to become a detective, his goal in life. Dixon escapes the building, grabbing Angela's case file, and Mildred's acquaintance, James extinguishes his burning clothes. Although he saw Mildred start the fire, he tells police that they were on a date together, giving her an alibi. Dixon is patched up and put in a room at the hospital with Welby. He takes Willoughby's letter to heart and apologizes for assaulting him.
Dixon goes to a bar after he is discharged from hospital. He overhears a man bragging about having raped and killed a girl before; the man had threatened Mildred in her store earlier that week. He sees that the man's car has Idaho plates, which he makes a note of before starting a fight with the man. He scratches him in the course of the fight, deliberately, so that he retains a sample of the man's DNA under his fingernails.
The DNA sample from the man in the bar does not match the DNA from the man who attacked Angela. Dixon concludes that the man must still be responsible for the rape and murder of someone and so sets out to Idaho with Mildred to kill him. Mildred admits that she set fire to the police station which Dixon admits he has already realized. They are not sure what to do about the man in Idaho, but keep driving, hoping that they will figure it out along the drive.
