Julia de Weert, an affluent heiress, lives in her late parents‘ residence alongside her uncle and legal custodian Josef Kuhbrot (known as Josse) and aunt Wilhelmine (known as Wimpel). Julia eagerly awaits the day she reaches adulthood – and financial independence. Having gotten used to a life of luxury, Josse on the other hand fears for the day on which the family wealth will be signed over. To circumvent the loss of status and comfort, he has sent for a nephew of the Kuhbrot clan as a potential suitor for Julia, ignoring that her heart is already taken: Julia is in love with her cousin Roderich, who was her childhood sweetheart before leaving for Batavia in the Dutch colonies of Java seven years ago. Before his departure, they swore to look up to the moon and think of each other every night – an oath Julia has taken seriously ever since.
There’s another candidate for Julia’s hand: Egon von Wildenhagen, son of the district administrator, who has been unsuccessful in wooing her so far, but arrives with good news. He managed to declare Julia a major with immediate effect. Giddy with joy, Julia and her friend Hannchen invite a stranger into house to join them for an evening of boundless amusements!
The next morning: the stranger queries Hannchen about Julia and learns about her longing for absent lover Roderich and her disdain for alternatives such as nephew August Kuhbrot. Seizing the opportunity, he presents himself as said Roderich. Julia is beside herself with excitement and discards all initial doubts. This is when Egon arrives with more news – no less exciting than before, but rather worrying: according to his research, the stranger cannot possibly be Roderich, as the boat from Batavia is only due to arrive today. The stranger admits not to be Roderich and Julia, disheartened, sends him away.
While Julia bemoans the loss of the wrong Roderich, Josse sketches up his own theory: the stranger is bound to have murdered his nephew August, who is overdue. Enter a second stranger, who is immediately smitten with Hannchen – and vice versa. The union seems perfect until Hannchen finds out that the man before her is, in fact, the true Roderich. Just off the boat from Batavia, where he acquired great wealth under dubious circumstances, he has all but forgotten about his oath to Julia. Hannchen, keen to hold on to the millionaire, cooks up a plan: the real Roderich must introduce himself to Julia as August Kuhbrot while the fake Roderich, who has revealed himself to Hannchen as the real August Kuhbrot, is waiting for his happy end in the wings …