Four days after a Weiberfastnacht's eve party (Wed. 20 February 1974), where Katharina Blum met a man named Ludwig Götten, she calls on Oberkommissar Moeding and confesses to killing a journalist for the newspaper Die Zeitung.
Katharina had met Götten at a friend's party and spent the night with him before helping him to escape from the police. The next morning, the police break into her house, arrest her and question her. The story is sensationally covered by Die Zeitung, and in particular its journalist Tötges. Tötges investigates everything about her life, calling on Katharina's friends and family, including her ex-husband and hospitalized mother, who dies the day after Tötges visits her. He paints a picture of Katharina as a fervent accomplice of Götten, and as a communist run amok in Germany.
Katharina arranges an interview with Tötges. According to Katharina, upon his arrival he suggests that they have sex, whereupon she shoots him dead. She then wanders the city for a few hours before driving to police headquarters and confessing to Moeding.
The book also details the effects of the case on Katharina's employers and friends the Blornas; Mr Blorna is her lawyer, and Mrs Blorna one of the designers of the apartment block where Katharina resides. Their association with Katharina leads to their exclusion from society.