In the Florida Everglades, a pair of larger-than-human, intelligent leeches are living in an underwater cave. They begin dragging local people down to their cave where they hold them prisoner and slowly drain them of blood. One of the first victims is the local vixen, Liz Walker, played by Yvette Vickers. After a couple of gratuitous displays of flesh (Vickers appeared as the centerfold in the July 1959 issue of Playboy), and some running around on her husband (Bruno VeSota), Liz finds herself a prisoner of the leeches along with her current paramour. Game warden Steve Benton (Ken Clark) sets out to investigate their disappearance. Aided by his girlfriend, Nan Grayson (Jan Sheppard), and her father, Doc Grayson, he discovers the cavern. The monsters are finally destroyed when Steve, Doc, and some state troopers blow up the cavern with dynamite.