Katharina Blum is an innocent woman who works as a housekeeper for a famous corporate lawyer, Hubert Blorna, and his wife Trude. She is nicknamed "the Nun" due to her prudish lifestyle which makes her acquaintances very surprised by her suspected involvement with a criminal. Her life is ruined by an invasive tabloid reporter, Werner Tötges, who works for a tabloid simply known as The Paper. Katharina lands in the papers when the police begin to investigate her in connection with Ludwig Götten, a man she has just met and quickly fallen in love with, and who is accused of being an anarchist, a bank robber, and an alleged terrorist. Police suspect Katharina of aiding and abetting Götten.
Katharina meets Werner at a costume party attended by her friend. Her friend is with her boyfriend (a police informant, unknown to the others) who is dressed like a sheikh. He radios back to the police with information regarding Werner's whereabouts and his meeting with Katharina. The police search Katharina's apartment the next day but do not find him there. They take her in for interrogation which makes her very unhappy. They know he could not have gone far since he was in her apartment last night. They lie to her and use cruel investigation tactics to get her to confess his whereabouts but she will not budge.
Throughout the film, Katharina's limits are tested, and her dignity, as well as her sanity, is on the line as she tries her best to make her voice heard and the truth known. Lie after lie is printed by The Paper and everyone, including Katharina's former friends begins to believe it. After Tötges visits Katharina's mother, who is recovering from surgery in the hospital, her mother dies. He fabricates her last words in his newspaper to give the impression to the readers that she despised her daughter with her dying breath. This aggravates Katharina greatly. Ludwig is captured; Katharina had allowed him to hide out at the country house of Alois Sträubleder, a political leader who was pursuing her romantically and had given her the key to his country villa. The police had earlier taken an expensive ring from her as evidence that she was in contact with the bank robber but it is revealed it was in fact Alois's private gift to her. It turns out that Ludwig was not a bank robber but instead a deserter from the Bundeswehr who stole two regiments' pay.
Unable to find justice for herself or make the negative press coverage stop, Katharina murders Tötges and his photographer. Katharina and Ludwig see each other once more, passionately clinging to each other as they pass in the basement of the prison where they are initially held.
In an epilogue, at Tötges's funeral, his editor delivers a hypocritical speech about how his murder was an attack on democracy and the freedom of the press. The film's final image is a block of text that appears over Tötges's funeral wreath and casket, linking the film's depiction of The Paper's yellow journalism to the practices of actual German tabloid Bild-Zeitung. This text also appears at the beginning of Heinrich Böll's book. It reads:
The characters and action in this story are purely fictitious. Should the description of certain journalistic practices result in a resemblance to the practices of Bild-Zeitung, such resemblance is neither intentional, nor fortuitous, but unavoidable.
