Lisa and Lottie (original German title: Das doppelte Lottchen "The double Lottie") is a 1949 novel by Erich Kästner, which originally started out during WWII as an aborted movie scenario, about twin girls separated at birth who meet at summer camp. It has been adapted into film many times (see below).
Two nine-year-old girls—rude Lisa Palfy (orig. Luise Palfy) from Vienna, and respectful shy Lottie Horn (orig. Lotte Körner) from Munich—meet on a summer camp in Bohrlaken on Lake Bohren (orig. 'Seebühl am Bühlsee'). Lisa has curly hair; Lottie wears braids. Apart from that, they look alike.
They have never seen each other before, but soon find out that they are identical twins. It turns out that their parents divorced, each keeping one of the girls. The two girls decide it is unfair that neither of them has ever been told that she is a twin, or that her other parent is still living. They decide to trade 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. Lottie curls her hair, Lisa braids hers, and both go off to where they have never been before. The adventure begins.
While many adults are surprised at the changes in each of the girls after they return from camp ("Lottie" has suddenly 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 (pretending to be Lisa) finds out 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 that was taken while they were at summer camp. She quickly realizes what has happened and Lisa tells her the entire story. The discovery turns out to have come just in time. The girls' mother calls her ex-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 to be with her.
The four of them (father, mother, Lisa and Lottie) are still in Vienna for the girls' birthday. The girls tell their parents that they don't need any presents on this birthday or ever again as long as they don't have to be separated again. The parents talk it over, realize that they are still in love and decide to get remarried.
The story was adapted into film as: