Lotte Körner, one of two identical twins separated in infancy after their parents' divorce, lives in Munich with her mother Luiselotte Körner, an editor for a weekly newspaper. Reserved and domestically adept, Lotte contrasts sharply with her sister Luise Palffy. She demonstrates responsibility through household skills like cooking and cleaning, excels academically in mathematics, and plays piano proficiently. Her shy maturity manifests in braided hair, differing from Luise’s curls.

During an Alpine summer camp, Lotte and Luise overcome initial hostility to discover their twin bond through matching birth details. They engineer an identity swap: Lotte moves to Vienna as Luise to live with their father Ludwig Palffy, while Luise assumes Lotte’s life in Munich.

In Vienna, Lotte diligently performs unfamiliar chores, astonishing the housekeeper Resi. She excels academically where Luise struggled but endures profound emotional turmoil. Her anxiety escalates upon learning of Ludwig’s engagement to Irene Gerlach, whom she views as an obstacle to family reconciliation. Failed attempts to halt the engagement trigger a debilitating stress-induced illness, leaving her bedridden and mute.

The twins’ deception unravels when Luiselotte uncovers camp photographs and confronts Luise in Munich. Luiselotte and Luise journey to Vienna, exposing the switch and reuniting the family. Lotte reclaims her identity but remains temporarily in Vienna with her mother and sister. At their tenth birthday celebration, Lotte and Luise jointly advocate for their parents’ reunion. This prompts Ludwig and Luiselotte to rekindle their affection and remarry, concluding with Lotte fully integrated into the restored family unit alongside Luise and both parents.