Swamp ThingSave

Government agent Alice Cable (Adrienne Barbeau) is sent to a Louisiana bayou-based research facility to evaluate Dr. Alec Holland's (Ray Wise) experimental plant-mammalian biology research. Holland believes he can engineer plants capable of surviving in deserts, ending world hunger; his sister Linda (also played by Wise in a brief flashback) is helping with the research. The facility is heavily guarded — Holland is on the verge of a breakthrough. Cable's arrival coincides with the secret infiltration of the facility by Anton Arcane (Louis Jourdan), a wealthy and obsessive scientist who wants Holland's research for himself.

Arcane infiltrates the facility with mercenary support. In the chaos of his attack, Holland's lab is sabotaged — chemicals from his plant-biology experiments are spilled over Holland himself; the chemicals react violently with the open flames Arcane's men have set. Holland is engulfed in green flame; he flees into the surrounding swamp, his body transforming as the plant-biology serum begins to integrate with his skeletal and muscular structures. Cable, witnessing the transformation, escapes into the swamp with a portion of Holland's research notes. Arcane survives the lab explosion and begins hunting both Cable and the transformed Holland.

Cable hides in the bayou as Arcane's mercenaries hunt her. She encounters the transformed Holland — now a green, muck-skinned, plant-haired creature of the swamp — and recognisizs him as Holland through his ability to communicate in his own voice. Holland, struggling with his new physical form, eventually rescues Cable from the mercenaries. Their relationship deepens through the swamp sequences; Cable adapts to communicating with the transformed Holland, while Holland struggles with the loss of his human form and identity.

Arcane discovers Holland's transformation and decides he wants the plant-mammalian serum for himself — to achieve immortality. He captures Holland's research notes and uses them to engineer his own version of the serum, but his attempts produce mutant rabbit-and-rodent abominations rather than the perfect plant-mammal hybrid Holland achieved. Frustrated, Arcane decides to capture and dissect Swamp Thing himself, harvesting Holland's biological tissues for further research. The middle act consists of Arcane's mercenaries hunting Swamp Thing through the bayou using sonar equipment and trained dogs.

Swamp Thing's healing factor is the film's defining superpower. When wounded, the creature can regenerate within hours; when severed (Arcane's mercenaries reportedly cut off one of Swamp Thing's arms during a hunt), the severed body part dissolves into plant matter and the lost arm regrows. The healing-factor establishes Swamp Thing as effectively immortal within the bayou's plant-rich ecosystem. Cable, recognisizing this, devises a plan to use the swamp itself as Swamp Thing's weapon against Arcane.

The climactic confrontation takes place at Arcane's swamp-mansion stronghold. Cable is captured; Arcane has now consumed his own modified serum and is in the process of transforming into a deformed reptilian creature similar to Swamp Thing but driven by his original ambition rather than Holland's noble intent. Swamp Thing arrives to rescue Cable; the two transformed beings engage in a brutal physical battle that destroys substantial portions of the swamp-mansion. Arcane is killed by Swamp Thing's superior physical strength; his deformed body sinks into the bayou.

The film's epilogue is bittersweet. Cable and Holland share a quiet moment in the bayou; Holland, accepting that he can no longer return to human form, encourages Cable to leave the swamp and live her life. Cable promises to return; she walks back to civilisization. Swamp Thing remains in the bayou as a permanent inhabitant — an immortal protector of the ecosystem. The film ends on Swamp Thing watching the sunrise from the bayou's water, content in his new form. A sequel, The Return of Swamp Thing (1989), would continue the franchise with Dick Durock again playing the title creature.