Hired to accompany a transhumance towards Yosemite Valley, in the United States, in the summer of 1849, John Muir notes everything he sees, wanders, gets drunk on life in the great outdoors, on the wonderful freedom of camps. The more nature becomes wild, the more it is invaded, submerged, overwhelmed by the beauty of the world. A summer in the Sierra has a tone, a truly unique rhythm, the inimitable charm of the first times: this discovery, which he shares with us, of a splendor where everything, plants, animals, landscapes, seems to want to contribute to the same exultation.