Something unnatural is happening in the swamps outside Marais, Louisiana. When a mysterious illness strikes the town, CDC investigator (and former Marais native) Abby Arcane is sent to investigate. At the hospital, she encounters biologist Alec Holland who believes the bizarre illness might be connected to his scientific work in the swamp for powerful businessman Avery Sunderland. Abby has a history with Avery and Maria Sunderland, who still blames the young Arcane for the tragic passing of her daughter years before. But with a deadly swamp-born virus out there, something is wrong in Marais right now. Along with Sheriff's Deputy Matt Cable, Abby once again crosses paths with Alec, but this time they encounter a terrifying, dark force that's not onlyhuged intruders, but is also taking control of its victims. At Delroy's Roadhouse, Abby consults her old friend and local reporter Liz Tremayne who has a lead deep in the swamp. When Alec goes missing after investigating the unnatural experiments deep in the swamp, something else rises in his place: Swamp Thing, a mysterious creature born of the depths of the swamp's mystical and terrifying secrets. With nature wildly out of balance and coming for the people of Marais, in the end, it may take some Thing from the swamp to save it.

While Sheriff Lucilia Cable searches for Alec unsuccessfully, Abby theorizes that it was more than an accident and retrieves his video journals from Daniel Cassidy. When one of her patients, Susie Coyle, escapes from the hospital, Abby and Matt follow her into the swamp. There, Abby encounters Swamp Thing - a creature that Susie later identifies as Alec Holland. While Alec struggles to understand his metamorphosis into the monstrous, plant-based creature, known as Swamp Thing, Avery meets with Jason Woodrue, the scientist who initially developed the biological accelerant responsible for the "green flu" and orders him to halt the epidemic. And Maria Sunderland reaches out to Madame Xanadu in order to communicate with her dead daughter, Shawna.

When her friend and co-worker is stricken with the green flu, Abby searches Alec's lab for a cure, but instead finds the latest supernatural reanimation from the Rot.

While Swamp Thing senses a growing darkness within the swamp, Abby's return dredges up her dark history with the Sunderlands. We learn more about the mysterious Dan Cassidy.

While Swamp Thing struggles to come to terms with his transformation with the help of a Phantom Stranger, Jason Woodrue shares a revelation with Avery Sunderland.

While Swamp Thing finds a new way to communicate with Abby, Matt learns the truth about Alec Holland and Avery pressures Woodrue to deliver results.

While Maria meets with the shadowy finance group known as the Conclave, Lucilia leads Avery out into the swamp under false pretenses.

After a harrowing journey of survival through the swamp, Avery comes face to face with Swamp Thing, who rescues the injured man and helps his recovery.

While Abby and Liz track down the secret Conclave facility, Jason Woodrue conducts experiments on Swamp Thing and makes an incredible discovery.

Still reeling from the revelation of the truth behind his existence, Swamp Thing goes on the offensive against Ellery's mercenary team hunting him; Abby comes up against Jason Woodrue, who has crossed the line to save his wife's life.