After local-moonshine swilling trapper Lem Sawyer (George Cisar) sees a giant creature, people start disappearing. While searching for illegal traps Steve Benton (Ken Clark) and Nan Greyson, his girl-friend (Jan Shepard) find Lem dying with giant sucker wounds on his body. One couple Liz Walker (Yvette Vickers) and Cal Moulton (Michael Emmett), forced into the water by her enraged husband Dave Walker (Bruno Ve Sota), gets taken by the leeches. When police refuse to believe Dave's story, he hangs himself. Soon after this, 2 more trappers disappear, the local Game Warden Steve Benton gets involved. He and Nan's father Dr Greyson (Tyler McVey) realize that the people were taken by the leeches and the leeches live in caves under the swamp. Using dynamite, the 4 missing bodies are discovered and the leeches are destroyed.