Accompanied by a child, the mathematician Galileo observes the firmament with a telescope. Ten years ago, the philosopher Giordano Bruno was burned in Rome for having supported the idea of an infinite and centerless universe, based on the work of Copernicus. By dint of observations and calculations, Galileo seeks proofs for his hypothesis of a cosmic system where the Earth is "an ordinary celestial body, one among thousands". From Padua to Venice, the mathematician shakes certainties by confronting the power of a Church which wishes to maintain its absolute power in the "crystal spheres" where Ptolemy has hitherto locked up the world.