Over the last few decades, systematic concreting and the use of chemical products have seriously damaged the biodiversity of cities. In Strasbourg's old historic city center, on the Cathedral of Notre Dame, Mathieu Baud, in charge of the conservation of the building, watches over the remaining representatives of a rich and unsuspected wildlife. Collaborating with filmmakers Pauline Bugeon and Cédric Chambin, Mathieu embarks on an investigation to document, inventory and attempt to preserve the wild species that populate the most visited religious monument in France, the Cathedral of Strasbourg.