The previous wave of piranha attacks finally came to an end. At this time, scientists were horrified to find that the piranhas that had turned people upside down were still in their primary form. God knows what kind of trouble they will cause when they grow up! The last tragedy left a deep shadow in Shelby's (Danielle Panabaker) heart. When she learned that her uncle was going to build a large luxury water park somewhere, Shelby felt strongly uneasy. Sure enough, in order to save money, the unreliable uncle directly introduced the nearby lake water. In this way, a large wave of piranhas also followed. For them, a feast is about to begin. Facing the screaming and crying people, Shelby's strong sense of justice made her unable to sit idly by. She bravely launched a vigorous rescue operation together with her surviving partners.