Diary of a Lost Girl (German: Tagebuch einer Verlorenen) is a 1929 silent film directed by G. W. Pabst and starring American silent star Louise Brooks. It is shot in black and white, and various versions of the film range from 79 minutes to 116 minutes in length. This was Brooks' second and last film with Pabst, and like their prior collaboration, Pandora's Box, it is considered to be a classic by many film historians. It is based on the controversial and bestselling 1905 novel of the same name by Margarete Böhme. A previous screen adaptation of the novel, directed in 1918 by Richard Oswald, is currently classified as a lost film.
Thymian Henning, the innocent, naive daughter of pharmacist Robert Henning, is puzzled when their housekeeper, Elisabeth, leaves suddenly on the day of Thymian's confirmation. It turns out that her father has got Elisabeth pregnant. Elisabeth's body is brought to the pharmacy later that day, an apparent suicide by drowning, upsetting Thymian. Thymian's father's assistant Meinert, promises to explain it all to her late that night, but instead takes advantage of her and she also becomes pregnant. Though Thymian refuses to name the illegitimate baby's father, the relatives find out from her diary, and decide that the best solution is for her to marry Meinert. When she refuses because she does not love him, they give the baby to a midwife and send her to a strict reformatory for wayward girls run by a tyrannical woman (Valeska Gert) and her tall, bald assistant (Andrews Engelmann).
Meanwhile, Thymian's friend, Count Osdorff (André Roanne), is cast off and left penniless by his rich uncle, also Count Osdorff (Arnold Korff), after he proves unsuccessful at every school and trade. Thymian begs her friend to persuade her father to take her back, but Thymian's father has married his new housekeeper, Meta (Franziska Kinz), and Meta wants no rivals for Robert's affection. Rebelling against the reformatory's rigid discipline, Thymian and her friend Erika (Edith Meinhard) escape with Osdorff's help. When Thymian goes to see her baby, she is told the child has just died. After despondently wandering the streets, she re-unites with Erika.
Osdorff, who had been counting on the money to rebuild a life for himself too, throws himself out the window to his death when she tells him what she has done. The uncle, grief-stricken, decides to make amends by taking care of Thymian. He introduces her to his cousin as his niece, Countess Osdorff. In a strange twist of fate, Thymian is invited to become a director of the same reformatory where she herself was once held. When Erika, her old friend, is brought before the directors as an "especially difficult case", Thymian denounces the school and its "blessings" and takes Erika out of the room. Count Osdorff follows the two women; but before leaving he pauses, turns back toward his startled cousin, and declares, "A little more love and no-one would be lost in this world!"