In recent decades, systematic concreting and the use of chemicals have seriously damaged the biodiversity of cities. However, Mathieu Baud, in charge of the conservation of Notre-Dame cathedral in the historic centre of Strasbourg, is watching over the last examples of a fauna as rich as it is unsuspected.

Accompanied by filmmakers Pauline Bugeon and Cédric Chambin, Mathieu embarks on an investigation to document, list, and attempt to preserve the wild species that inhabit the most visited religious monument in France - Strasbourg Cathedral.