Down an out loser, Richard Rory minds his business while camping in the Florida swamps when he accidentally spills coffee on an alligator. The gator naturally attacks him, which prompts the Man-Thing (who had been silently watching Rory) to rescue the man from his reptilian attacker. When the Man-Thing kills the gator, Richard tries to get the Man-Thing to administer first aid to his injuries, but he blacks out due to blood loss. When he comes out of it, he finds that the Man-Thing is gone but has been replaced by a woman named Ruth Hart. Ruth has patched Richard up, and when Richard asks her about the muck monster, she tells him she saw him alone. She tells Richard she needs to find somebody to stop people from killing her and has elected Richard.

Meanwhile, at the office of F. A. Schist, Schist has a board meeting where he and his executives try to figure out how they can destroy the Man-Thing. Hargood Wickham, an M.I.T. graduate, offers to deal with the Man-Thing and is hired by Schist to do the job. While back at the swamp, Ruth, and Richard get to know each other Richard explains his hard luck in life to her. Silently watched by the Man-Thing, the muck-monster eventually leaves the two for privacy and wanders the swamp. In its travels, it runs into the Skull-Crushers, the biker gang of which Ruth used to be a part. When they see the creature, they attack it, but nothing works. When their leader Snake, tries to attack the creature with a chain, the Man-Thing grabs him, and his momentary fear causes him to be burned by the muck monster. When the Man-Thing finally departs, Snake vows to get the creature and orders his gang back on their bikes to find Ruth and the money she took.

While at the F. A. Schist construction site, Schist is shown Hargood's invention: The Slaughter Room, which will lure the Man-Thing in with sounds and then kill the muck-monster. While back with Ruth and Richard, Ruth tells Richard she's on the run from the gang because she's blamed for stealing the gang's treasury, but she confesses it was her former lover, Snake. When the gang finally finds her, Ruth flees into the swamp, and Snake attacks Richard. However, when Richard questions Snake about the truth regarding the gang's money, Snake denies it and runs into the swamp after Ruth.

Meanwhile, the Man-Thing is drawn to the Slaughter Room, pained by the sonic vibrations inside it. Inside, the muck-monster is attacked by flame devices designed to incinerate the Man-Thing. However, the creature destroys the device by throwing chains at its controls. At that moment, Ruth fled onto the construction site, followed shortly by Snake and Richard. Both end up at blows, but when the Man-Thing indifferently throws away the chains it used to free itself, they strike Snake, the blow to the head killing him. Sickened by the sight of the dead man, Schist and Hargood flee the scene. A grateful Richard believes he'd beaten the odds and that his luck will finally start changing.

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