In December 2015, the climatic future of the planet will be decided in Paris. During COP21, one of the largest conferences ever organized on the topic, the UN invites the nations of the world to sign an agreement to limit global warming. For the first time, scientists from around the world are uniting to influence these negotiations. Led by Jeffrey Sachs, a committed and influential economist, their mission is to convince the decision-makers of the world that the catastrophe can be avoided without returning to the Stone Age.