Bacteria, viruses, but also fungi, algae, pollen, and even insects: micro-organisms thrive and circulate constantly in our sky. How can so many living beings find their way into the air and circulate? How do they survive? And what influence do they have on our lives and the living world? Biodiversity, health, climate: it is only recently that scientists have begun to understand how this discreet aerial "plankton" affects our lives and our ecosystem. But despite their many virtues, some of these micro-organisms are now threatened by human activities. With the help of experts and 3D models, this scientific investigation plunges us into the heart of a still mysterious world, and reveals the diversity and fragility of the air we breathe.