There appears to be dramatic evidence that some of the world's most iconic rivers are in declining health. Biologist, extreme angler and Animal Planet favorite Jeremy Wade (`River Monsters') investigates the disappearance of freshwater giants from the planet's largest waterways. He talks with people who live along the Amazon, the Ganges, the Yangtze, the Mississippi, the Danube and the Zambezi to understand how exploitation and pollution have degraded these rivers that were once home to his beloved monster fish. By gathering evidence, Jeremy learns about the rivers' health and sustainability and what is being done to save them.