In the fictional town of Ebbing, Missouri, Mildred Hayes is grieving over the rape and murder of her teen daughter, Angela, seven months earlier. Angry over the lack of progress in the investigation, she rents three disused billboards near her home and posts on them: "Raped While Dying", "And Still No Arrests?", "How Come, Chief Willoughby?". They attract attention, so Bill Willoughby, the local chief of police, visits Mildred, but is unable to persuade her to take them down, even by revealing he has terminal pancreatic cancer. He renews his efforts to solve the case, but does not get anywhere.
Many townspeople are upset by the billboards, including Jason Dixon, an alcoholic police officer, who unsuccessfully tries to intimidate Red Welby, who rented Mildred the billboards, into taking them down. Mildred's dentist is sympathetic to Willoughby and menaces her during an appointment, so she drills a hole in his thumbnail. Willoughby brings her in for questioning and accidentally coughs up blood into her face. He has her released and is hospitalized, though he soon checks himself out against medical advice.
The billboards have further strained Mildred's relationship with her son, Robbie, and she recalls that her last interaction with Angela was an argument. Her abusive ex-cop ex-husband Charlie confronts her about the billboards and ends up revealing that, shortly before Angela's murder, he had turned down her request to come live with him.
At his mother's suggestion, Dixon arrests Mildred's friend Denise on trivial drug possession charges to put pressure on Mildred. Willoughby spends an idyllic day with his wife, Anne, and their two daughters, and then commits suicide later that night to spare his family from watching him die slowly. Dixon reacts to the news by assaulting Welby and throwing him out a second-story window. This is witnessed by Abercrombie, Willoughby's replacement, who fires Dixon.
Before his death, Willoughby wrote several letters, including one to Mildred. Anne delivers it, interrupting an unknown man who was menacing Mildred at work. In the letter, Willoughby tells Mildred that she was not a factor in his suicide and that he secretly paid to keep the billboards up another month.
After the billboards are destroyed by arson, Mildred retaliates by tossing Molotov cocktails at the police station, which she believes to be unoccupied for the night. However, Dixon is inside reading Willoughby's letter to him, which advises him to let go of hate and embrace love if he wants to be a detective someday; he manages to escape the blaze with Angela's case file. James, an acquaintance of Mildred, happens by and extinguishes Dixon's burning clothes before providing Mildred with an alibi. Dixon is put in the same hospital room as Welby, to whom he apologizes.
Jerome, who was part of the team that put up the billboards, brings Mildred a set of copies and helps her restore the signs. Discharged from the hospital, Dixon overhears the man who menaced Mildred bragging in a bar about raping a girl in the same manner as Angela. He notes the number on the man's Idaho license plate and then scratches the man's face to get a DNA sample, passively accepting the resulting beating.
Mildred is on a date with James to thank him for his help, when Charlie enters with his 19-year-old girlfriend Penelope and apologizes for burning the billboards when he was drunk. Unnerved that she retaliated against the wrong target, Mildred abruptly calls off the date, but James misinterprets her decision as embarrassment to be seen with him and leaves the restaurant in disgust.
Abercrombie informs Dixon that the DNA sample is not a match and the man was overseas on military duty at the time of Angela's death. Dixon gives Mildred the disappointing news and, believing the man to be guilty of some other rape, the pair plan a trip to Idaho to kill him. As they set out, Mildred confesses that she set the police station on fire, which Dixon had already assumed. They both express uncertainty about their mission, but Mildred says they can decide what to do along the way.
