Something unnatural is happening in the swamps outside Marais, Louisiana. When a mysterious illness strikes the town, CDC investigator Abby Arcane is sent to investigate.
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.
Driven to cure the worsening epidemic in Marais, when her friend and co-worker Harlan is stricken with the "green flu," Abby searches Alec's lab for a cure, but instead finds the latest supernatural reanimation from the Rot. Although Swamp Thing uses his strange new powers to intervene, he's more desperate than ever to find his humanity. Meanwhile, Avery Sunderland is facing increasing pressure from financial sources and Sheriff Cable who questions him about the death of Alec Holland.
Realising that his new abilities allow him to communicate with the life around him, Swamp Thing senses a growing darkness within the swamp.
Haunted by the ghost of her childhood friend who reemerges from the swamp, Abby's forced to confront both her past with the Sunderlands as well as the dark forces at play in Marais. While she continues to search for a cure for Alec, he struggles to come to terms with his transformation with the help of a mysterious Phantom Stranger.
Increasingly alienated from humanity after being hunted by sheriff's deputies and animal trackers, Swamp Thing struggles to understand his strange condition.
Still taking on the human appearance of Alec Holland, Swamp Thing guides Abby to an area of the swamp blighted with the encroaching Rot.
After a harrowing journey of survival through the swamp, Avery comes face to face with Swamp Thing, who rescues the injured man.
After being captured and taken to a Conclave facility, Swamp Thing is experimented upon by Jason Woodrue, who makes an incredible discovery.
The swamp fights back in the season finale. Still reeling with the revelation of the truth behind his existence, Swamp Thing goes on the offensive against Ellery's mercenary team hunting him. Meanwhile, Abby comes up against Jason Woodrue who's crossed the line to save his wife's life.