A lost box is a strange one that nobody knows. It appears in the middle of an apartment exchange involving the Ilse and Rudi Karschek family and another couple. Since no one can be assigned to this box, it is opened; the contents: the portrait of an attractive girl who dedicated it to "her" Rudi. Rudi, however, is quite sure that this must be a misunderstanding. But then his wife Ilse receives a phone call in their new apartment that is not at all misleading, which brings the situation in the Karschek family to a head. In this respect, the whole affair becomes a total "mess".