Abby Arcane finds a mysterious creature when she investigates a deadly virus in the Louisiana swamps.

Alec tries to understand his metamorphosis into Swamp Thing.

Driven to cure the worsening epidemic in Marais, Abby searches Alec's lab for a cure when her friend, Harlan, becomes stricken with the "green flu"; Swamp Thing uses his strange new powers to intervene, but he is desperate to find his humanity.

Swamp Thing senses a growing darkness within the swamp and starts to understand his role in the struggle for balance between growth and decay; Abby must accept that the things she has witnessed in Marais are evidence of supernatural forces at play.

Haunted by the ghost of her childhood friend who re-emerges from the swamp, Abby is forced to confront both her past with the Sunderlands and the dark forces at play in Marais.

Increasingly alienated from humanity after being hunted by sheriff's deputies and animal trackers, Swamp Thing struggles to understand his strange condition; Matt learns the truth about Alec Holland; Avery pressures Woodrue to deliver results.