The Mega Piranhas are genetically modified piranhas created in Venezuela which double in size every 72 hours. After escaping into a river, they attacks many persons, including an american ambassador. An american special agent named Jason Fitch comes to investigate and he is rapidly confronted to the Piranhas, but he manage to kill one of them, which convince the Venezuelan authorities to organize a bombardment in the river. However, many Piranhas survives and attacks the city. Later, they manage to go in the ocean but Fitch and many others militaries fight against them. Finally, Fitch succeeds to kill one of them, causing them to devour each others.