Laguna Negra follows a mule driver, a mountain guide, and a park ranger as they prepare for the historic opening of this Andean lake in Cajón del Maipo, Chile, after 150 years of inaccessibility. A vital water source for Santiago, its history is intertwined with a recreation of the 1873 expedition led by Governor Benjamín Vicuña Mackenna. Amid climate change, this documentary amplifies the voices of mountain communities demanding fair and sustainable access to their waters.