After a violent incident with a special chemical, a research scientist is turned into a swamp plant monster.
In the swamps of Louisiana, Dr. Alec Holland works with his sister Linda on a top-secret bio-engineering project to create a plant/animal hybrid capable of thriving in extreme environments. Government agent Alice Cable arrives just as Holland makes a major breakthrough, and begins to develop feelings for him. However, a paramilitary group led by the evil Dr. Anton Arcane, who is obsessed with immortality, kills Linda while trying to steal the formula for their own purposes. During the attack, Alice escapes and Alec is covered in chemicals, caught on fire, and runs screaming in the swamp, presumably to die. However, he returns as a monstrously mutated plant creature.
As the Swamp Thing, Holland battles Arcane's forces to protect Cable, and eventually takes on the mad doctor himself, who has also mutated from his incomplete understanding of the Holland formula.