The private investigator Maggie McNamara from Lyon Investigation is hired by the wealthy J.R. Randolph to find his niece that has disappeared with her boyfriend. Maggie seeks out the lonely environmentalist Paul Grogan to help her to look for the teenager. They head to an abandoned army facility and Maggie decides to drain the pools to see whether the body of the girl is there. They are assaulted by a woman with a crowbar but they subdue her. However she escapes and soon they learn that the woman is Dr. Leticia Baines, who is researching a hybrid species of piranha that is capable to survive in fresh and sea waters for military purpose. Further, Maggie has release the piranhas on the river and they are heading to the Lost River Lake Resort. Maggie and Paul inform the corrupt local Sheriff but Randolph tells him to lock them up since he does not want to jeopardize the party he has promoted to his resort. 所谓的吃人怪鱼，出现镜头少得可怜。 月色中一对恋人来到湖边夜游，但两人却双双失踪，前来调查的女主人公和一名当地男子意外发现此处原来是一秘密实验基地，实验变种食人鱼且这些食人鱼更危险、聪明。但两人不慎将全部食人鱼饵放入河道，附近的度假营地危在旦夕……
