Two nine-year-old girls, bold Lisa Palfy (orig. Luise Palfy) from Vienna and shy Lottie Horn (orig. Lotte Körner) from Munich meet in a summer camp in Bohrlaken on Lake Bohren (orig. 'Seebühl am Bühlsee'), where they discover that they are identical twins whose parents divorced, each keeping one of the girls.
The girls decide to swap places at the end of the summer so that Lottie will have a chance to get to know her father and Lisa will get to meet her mother. While many adults are surprised at the changes in each of the girls after they return from camp ("Lottie" has apparently forgotten how to cook, gets in a fight at school, and becomes a terrible student, while "Lisa" has begun to keep a close eye on the housekeeper's bookkeeping, will no longer eat her favorite food, and becomes a model student), no one suspects that the girls are not who they claim to be.
When Lottie (under the name of Lisa) finds that her father is planning to remarry, she becomes very ill and stops writing to her sister in Munich. Meanwhile, Lottie's mother comes across a picture of the two girls at summer camp, and Lisa tells her the entire story. The girls' mother calls her former husband in Vienna to tell him what has happened and to find out why Lottie has stopped writing. When she hears that her daughter is ill, she and Lisa immediately travel to Vienna. At the daughters' request, the parents are reunited.