At a little private dance held by her godmother, Else Woltersheim, Katharina Blum becomes acquainted with Ludwig Götten. Reticent and almost prudish since her failed marriage, Katharina at once falls in love with Ludwig and takes him home with her. Next morning – it is the last Thursday before Ash Wednesday, the first highlight of the crazy carnival in Cologne – a special police squad storm Katharina's apartment in order to arrest the long wanted Götten. Since the latter has succeeded in escaping through the sewers, Katharina is suspected of helping him and is arrested. In intensive questioning Commissioner Beizmenne tries to pin Katharina down, but the picture crystallizes of a lonely young woman whose main interest is paying off her own flat. She earns her money as a home help for a solicitor and with occasional weekend jobs.