In Brüsel, the world is thrown into chaos. Out of nowhere, stones appear in an apartment, all with the exact same weight, which is also a prime number. At the same time, gourmet chef Maurice, despite the good food, is becoming lighter and threatens to float away. The city fathers are at a loss and turn to Mary von Rathen, once a quirky girl and now an expert on inexplicable phenomena, for advice. Indeed, Mary is the only one who can truly see what is happening... Another fantastic philosophical episode from the universe of the mysterious cities.