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In a small community near the Florida Everglades, Lem Sawyer relates to skeptical cronies his sighting of a strange, water-bound creature with tentacles and suction-cup eyes in the swamp. A few nights later, game warden Steve Benton and his girl friend, Nan Greyson, are searching the marsh for poacher's traps when they hear a scream and discover attractive Liz Walker, who has come upon Lem's horribly mutilated and bloody body. After Lem's autopsy, Steve questions Sheriff Kovis about the unusual markings on Lem's body, but Kovis insists the man died from an alligator mauling. Angered by Kovis' refusal to investigate, Steve goes to the Greysons' to inform Nan and her physician father of his intention to search the swamps in hopes of confirming Lem's mysterious sighting. Steve and Nan spend the next two days carefully searching the swamp, but find nothing. When Lem's death continues to disturb the locals, Liz and her husband, store owner Dave, quarrel bitterly over his refusal to move into town away from the swamp. Unknown to Dave, Liz is having an affair with handsome young Cal Moulton, with whom she rendezvous one afternoon after Dave has gone to work. The couple goes out to a spot near the marsh and Cal asks Liz why she ever married the older, heavyset Dave. Liz explains that her first husband was a drunken abuser whom she divorced and that Dave was the first man to show her real kindness. Dave, who has followed Liz, interrupts the couple's tryst and, at gunpoint, forces them to walk into the swamp. While Cal pleads for his life, Dave is startled when a large creature comes up behind Cal and drags him underwater, and a moment later another drags Liz below. Horrified, Dave runs back into town. Kovis disregards Dave's wild description of the couple's disappearance and, when Liz and Cal cannot be located, arrests Dave for their murder. Later that night, Dave hangs himself in his jail cell. That same evening, Steve discusses the incident with Greyson and Nan and rejects the doctor's suggestion that they dynamite the swamp to kill any creatures within it. When Steve reluctantly admits he believes that Dave killed Liz and Cal, Greyson insists that he spoke with Dave an hour before his suicide and found him in a state of terror, not despair. After a day goes by with no sign of the dead bodies, Dave's friends Sam Peters and Porky Reed take a boat to explore the swamp. Unsettled by the eerie lifelessness in the marsh, the older men decide to return to shore when the creatures attack their boat and drag them into the swamp to an underwater cave, where Liz and Cal lie unconscious. The creatures then attack the men, sucking all their blood from them. Upon learning of Sam and Porky's disappearance, Steve joins an official, eventually vain search arranged by Kovis. When a member of the search party notes the unusual absence of alligators or fish in the swamp, Steve wonders if the water is contaminated. Greyson again recommends using explosives to kill any unusual life or dislodge the bodies that might be trapped underneath the water, but Steve insists he must maintain the preserve's ecological system. The next day, Steve finds Greyson and Nan at the swamp setting a small charge of dynamite and reluctantly does not intervene. The explosion jars the bodies of Cal, Sam and Porky off the cave ledges into the water and in moments they rise to the surface. Later, Steve goes to the Greysons' to charge the doctor with vandalizing the preserve, but is startled when Greyson informs him of the men's autopsy results. Despite Korvis' attempt to suppress the results, Greyson has learned that Cal did not die from Dave's gunshots, but, like the others, as a result of having no blood in his body. Each man had strange, unidentifiable marks on him and had only recently died. Amazed by these revelations, Steve wonders if the men somehow survived in underground caves and if Liz might possibly remain there. Determined to explore the waters, Steve contacts Mike, a former war buddy, to assist him in making a dive into the swamp. The next day with the Greysons, a cynical Kovis and other locals observing, Steve and Mike take a small boat to the middle of the swamp where Steve dives in armed with a spear gun. Finding the entrance to the caves, Steve witnesses one of the giant creatures attacking Liz, who is still alive. Steve fires the spear gun, but when the creatures come after him, he tugs on the rope attached to the boat and Mike quickly pulls him to the surface. Steve calls to shore to inform the others what he has seen, then, taking a second spear gun, returns below. When Nan wonders how the creatures came to be in the swamp, Greyson speculates that the nearby space program has contaminated the area with radiation. At the underground cave, a weakened Liz revives slightly when she sees Steve swimming by and tumbles off the ledge into the water, rising to the surface. Mike pulls her into the boat, noting the strange suction marks along her neck and face. Meanwhile, below, Steve struggles with one of the creatures. When Mike is unable to pull Steve up, he dives below and stabs one of the creatures with a knife. The group on the shore gasps when the creature's body floats to the swamp surface. Moments later, Steve and Mike return to shore and set a large charge of dynamite, which they toss into the swamp and detonate. The second creature's body rises to the surface and Steve and the others move off relieved, unaware that a surviving creature watches them from the edge of the marsh.