Asta is looking for his brother who disappeared fifteen years ago. Already despairing of finding him in her native Rittersheim, she accidentally comes across the city of Arnelm, which is not on the map. Here, houses float in the air, and the river talks to people, you just have to listen. An unexpected meeting with the Arnelmians - the historian Theo, his grandson Lin and the blacksmith Sven - opens up Asta's path beyond the boundaries of the familiar world, where she will have to find answers to her questions and learn a lot new. For example, what are floating houses made of? Why do foxes need wings? Why do her new friends sometimes behave extremely strangely? And finally, who will return the Heart to the river, stolen many years ago?..