Richard Rory is a man who is down on his luck. Seated by a campfire in a deserted portion of the Florida Everglades, he reflects on the failure his life has become. He has lost his job, lost his girl, and now he has lost his way. While in deep thought, Richard Rory’s luck continues to get worse as he is attacked from behind by a huge alligator. As Richard wrestles for his life, the Man-Thing slowly lumbers out of the shadows and pauses, as if deciding whether to save the man or the beast. He chooses to save the man and grabs the alligator by the tail, swinging him around and finally through the air and into a tree. The scaly beast is dead and now the Man-Thing turns towards Richard. Mr. Rory is too weak to oppose the creature from the swamp, too tired to stop him. As the Man-Thing slowly approaches, Richard Rory’s world begins to fade into black.
Richard Rory slowly awakens from an unconscious state to see a young woman who has tended to his wounds and demonstrated compassion on him. Her name is Ruth Hart. She helps Richard to his feet and explains that she found him there the day before. His wounds had been covered with primitive dressings made from leaves. Richard is reminded of the Man-Thing he saw before he passed out and begins to tell Ruth Hart about the amazing creature. While she doesn’t put much stock in Richard’s story, Ruth still listens to him and then shares her own unbelievable tale.
Not too far away in Miami, a business meeting is taking place where a group of important-looking men discuss the Man-Thing and how he has prevented them from draining the Everglade swamps to build an airport. F. A. Schist, C.E.O. of a major construction company, offers a $50,000 reward to anyone who kills the Man-Thing. The men meeting with Mr. Schist are scientists who do not want to kill the Man-Thing; rather they want to capture him for scientific observation. They refuse Mr. Schist’s offer and leave the meeting. A lone scientist remains behind and promises a plan to destroy the Man-Thing.
Back in the the Man-Thing quietly lurks in the background, feeling and sensing the tender exchange before him. Richard Rory and Ruth Hart continue their conversation while eating a can of beans and franks. Richard persists with the negative picture of his life. He tells Ruth about his birth and how he was dropped by the doctor who delivered him in the hospital, setting the course for his miserable life. Ruth comforts Richard, reminding him that she is running for her life and that things will get better for him.
Man-Thing leaves the couple and comes across a gang of bikers known as “the Skull-Crushers.” These are the ones who are chasing Ruth Hart. They are an evil bunch and the Man-Thing openly approaches them. They respond by attacking the swamp creature. Guns are fired to no avail, for the bullets merely pass through his body without harm. A powerful chain is whipped at the Man-Thing, but again it offers no harm to the creature. The leader swinging the chain, called “Snake,” is grabbed by the Man-Thing and his hand begins to burn, “for whatever knows fear, burns at the Man-Thing’s touch.” The man is released and the Man-Thing lumbers off, the chain still hanging from his body. “Snake” and his cohorts vow to get revenge on the Man-Thing and Ruth Hart as well. They ride their motorcycles until twilight, searching for the monster and the girl, but neither can be found.
As dusk falls over the another group is looking for the Man-Thing. Mr. Schist and his scientist friend unpack a sophisticated weapon that is guaranteed to destroy the Man-Thing. A high voltage series of mirrors is set in a pattern that will electrocute the swamp dweller, turning him into a smoldering pile of ooze. A special frequency transmitter is set in a little shack as bait and the men go into hiding to await the creature’s arrival.
The scene shifts to Richard Rory and Ruth Hart. Ruth tells Richard why she is being hunted down—she has been accused of stealing money from the Skull-Crushers. She explains that the leader, Snake, stole the money himself in and blamed her for it. As the tale unfolds, motorcycles can be heard in the distance. Ruth begins to panic and runs into the swamp. Snake grabs Richard and begins beating him. Richard confronts Snake and tells the others the truth about the stolen money. Snake denies the accusations and runs into the swamp, determined to kill Ruth.
Meanwhile, the Man-Thing is suffering agony from the high frequency device set up by Mr. Schist’s scientist friend. Heading straight for the trap, the Man-Thing walks into the little shack and destroys the high frequency device. This sets off the high voltage trap and bolts of electricity are shot through the Man-Thing’s body. Outside the shack Ruth Hart is looking for a place to hide. She sees the shack, opens the door, and sees the Man-Thing being zapped by the trap. Snake runs up behind her and threatens to push her in to die with the creature. Close behind are the rest of the Skull-Crushers. They tell Snake that they have found $1000 worth of heroin under his bike seat, exposing the truth about the missing money. Snake is defiant and again wrestles with Richard Rory. With Snake’s chain in hand, the Man-Thing smashes the electronic devices that are shooting the powerful volts through his body. The bright lights dim and the Man-Thing slowly wanders out of the shack, throwing the heavy chain over his shoulder. Snake is struck in the back of the head by the flying chain and falls to the ground in a pool of blood. The prized possession of this leader was the very instrument that claimed his life.
Mr. Schist and his workers disappear, Richard Rory begins a new chapter of his life with Ruth Hart, and the Man-Thing quietly vanishes into the swamp.