Originally from the rivers of Eastern Europe, catfish have colonized almost all European lakes, canals and rivers since 1950. By the time they are adults, these giants have no competition and sit at the top of the aquatic food chain. Shads, pikes and ducks, all are prey to this rapacious fish. The catfish adapts quickly and has even learned to beach itself on purpose in order to catch pigeons! It has become Europe's largest freshwater fish, even longer and heavier than the fishermen who have aided its conquest. Having established their reign of terror over half a century, should this invader be declared harmful? To answer that question, this film documents the objective work of biologist Frederic Santoul who has been trying to solve a riddle: in all the rivers where it has been introduced, why has the catfish not eliminated any other fish species? With the help of free divers, he will follow the suspect's wrongdoings from the Danube Delta to the heart of the Tarn region in France and ...