After the mysterious and untimely death of a scientist who was part of a highly classified bio-engineering project set in the murky swamps of the American South, the government dispatches a remarkable worker named Alice Cable to take his place. Upon her arrival, Alice quickly discerns that one of the essential swamp sensors has malfunctioned. Her guide, Charlie, introduces her to the project's supervisor, Harry Ritter, who is initially preoccupied with a troubling rumor. This rumor speaks of a nefarious paramilitary leader, Anton Arcane, who is planning to hijack their operation. As the situation unfurls, Charlie shares his insights with Ritter while Alice makes the acquaintance of Dr. Linda Holland and her brother, lead scientist Dr. Alec Holland. Alec happily takes Alice on an enlightening tour of the swamp, encouraging her to appreciate its natural beauty.
However, the collaborative atmosphere soon turns tense when Ritter and Charlie reprimand Alice for stealing Alec's focus. When Alice brings up the malfunctioning sensor, Ritter reveals the tragic truth: her predecessor had been killed while attempting to fix it. The group soon learns about the unsettling disappearance of one of their team members, which is suddenly interrupted by a thunderous bang. Rushing back to the lab, Linda reveals her cutting-edge discovery: a glowing, plant-based concoction that possesses explosive properties. Alec then takes the opportunity to showcase a remarkable hybrid of plant and animal cells, which he considers a monumental achievement.
As events unfold, Alec makes an intriguing observation that Linda's formula triggers rapid plant growth upon contact with various surfaces. The excitement leads him to kiss Alice, prompting her to seek out Ritter in the control room. When she arrives, she finds the room abandoned but stumbles upon chilling security footage of Charlie getting shot. Moments later, the tranquility is shattered as a band of paramilitary agents launches an assault on the lab, with a man resembling Ritter leading the charge. The tension escalates when Arcane shoots Linda in a desperate move to gain control over the situation. In the chaos, Alec grabs the vial but, tragically, his actions lead to an accident that ignites him. As Alec desperately douses himself in the swamp waters, a series of explosive blasts rock the surroundings.
Overnight, Arcane's merciless henchmen obliterate the entire project site, erasing any trace of the scientific team's endeavors. As dawn breaks, Alice finds herself captured by one of Arcane's men, who tries to drown her in the murky waters. Just when all seems lost, a green, humanoid creature emerges, rescuing her and scaring off Arcane's men in the process. Back at Arcane's mansion, he and his secretary realize to their dismay that Alec's crucial notebook has vanished amid the chaos. In a bid for assistance, Alice races to a nearby gas station to call her superiors. The operator connects her to Ritter, who claims he was away from the site during the attack. Meanwhile, keeping a low profile as she awaits Ritter, Alice befriends a local gas station attendant named Jude. Unfortunately, the tranquility is short-lived when Arcane’s men find her, leading to a tense chase through the forest.
In a thrilling turn of events, the Swamp Thing appears, deftly dispatching Arcane's mercenaries with remarkable resilience. After a brief moment of fear, Alice tries to evade the creature, but it retreats to the woods. Jude later helps Alice find sanctuary in a nearby cabin. Meanwhile, Arcane, intent on capturing Alice, orders his men to find her, believing she might lead them to the extraordinary creature.
Alice and Jude continue to navigate the treacherous waters of the swamp until they spot the Swamp Thing among the ruins, clutching Linda's locket. Determined, she vows to take Alec's notebook to Washington, D.C. However, Arcane’s forces soon close in again, forcing a confrontation that culminates in a dramatic maritime showdown led by the Swamp Thing, who outsmarts Arcane’s agents.
As tension builds, Alice is captured before she can react to Jude's distress cry. In a moment of desperation, the Swamp Thing finds Jude's lifeless body and uses its powers to revive him. Realizing that the creature is friend, Jude offers the notebook to the Swamp Thing for safekeeping. Onboard Arcane’s boat, Alice takes a stand against her captor, throwing him overboard and escaping into the water. Upon reaching land, she encounters the Swamp Thing, who recognizes her and reveals himself to be Alec. Their bond deepens as Alice experiences an intimate moment with Alec—now in his transformed state—as the creature presents her with a delicate orchid.
In a moment of vulnerability, as Alice enjoys a skin-baring swim, they are ambushed by Arcane’s men, who capture the Swamp Thing and seize the notebook. Later that evening, Arcane invites Alice to a lavish dinner where he boasts about replicating Alec and Linda’s formula. During the festivities, Arcane places the first dose of his version of the formula into the hands of Bruno, his lead henchman, setting off a horrifying transformation that shrinks Bruno into a grotesque shadow of his former self. Locking Alice in the dungeon with Bruno and the Swamp Thing, Arcane seeks answers from them regarding the failure of his experiments.
In a twist of fate, the Swamp Thing's missing arm regenerates in the sunlight, enabling it to break free along with Alice and Bruno. However, Arcane morphs into a monstrous creature and pursues them. Alice and the Swamp Thing escape through an underwater tunnel, eventually resurfacing. In a climactic confrontation, the Swamp Thing defeats Arcane, but not without turmoil. With a heavy heart, he prepares to leave Alice behind, despite her pleas to stay and rebuild the shattered remnants of their work together. Yet, just then, Jude reappears from the woods, and together they watch the Swamp Thing lumber away into the depths of the marsh, embodying both victory and sacrifice in this fantastical tale.
