Through encounters with a hydrologist, a farmer, and citizens mobilized around the Marais Poitevin, the director helps us understand the effects—on biodiversity and the state of rivers—of substitution reservoirs, also known as megabassins. For several years, Anne-Morwenn, David, Patrick, Yann, and others have been fighting for fair water sharing and devising ways to convince as many people as possible that it is time to change the agricultural model.