Kein Auskommen mit dem Einkommen (1957)

To supplement their pension, the eccentric couple August and Ida Bodendiek rent a room to young Klaus. His father wants to remarry, so Klaus moved out of his parents’ house and now works the night shift as a truck driver. This gives the Bodendieks the opportunity to double their income: at night, they rent the room to Lisa Franzen, who works as a secretary during the day. When Klaus loses his job, he inevitably runs into Lisa in their shared apartment. He also tells a white lie, pretending to be Lisa’s husband, which leads the two to fall in love without knowing that they are step-siblings—because Lisa’s mother is Klaus’s father’s new wife.