Intrepid zoologist Nigel Marven thought he knew all about the world's most dangerous fish from his encounters with sharks, but that was before he went face-to-face with piranhas. Nigel travels through the waterways of South America to find out the truth behind this fish's ferocious reputation. From breeding to feeding, birth to death, all aspects of piranha natural history are covered. Topside, Nigel discovers dramatic ways to interact with them and the many animals whose lives are affected by piranhas. He finds frogs who have given up breeding in the rivers and lakes, and herons who use piranhas like guard dogs to keep their nests safe. As he swims near a piranha nest, Nigel is chased off by a territorial male piranha and he gets to feel the power of a feeding frenzy as he wades into the river to feed a shoal by hand. When it comes to maintaining the health of the waters of South America, piranhas just might be the most important animal in the river. But for all this they are still utterly impressive predators as Nigel finds out in the finale, when he goes underwater, right in the middle of a piranha feeding frenzy.