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 (series star CRYSTAL REED) is sent to investigate. At the hospital, she encounters biologist Alec Holland (series star ANDY BEAN) who believes the bizarre illness might be connected to his scientific work in the swamp for powerful businessman Avery Sunderland (series star WILL PATTON).
Abby has a history with Avery and Maria Sunderland (series star VIRGINIA MADSEN), who still blames the young Arcane for the tragic death 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 (series star HENDERSON WADE), Abby once again crosses paths with Alec, but this time they encounter a terrifying, dark force that’s not only killed intruders, but is also taking control of its victims.
At Delroy's Roadhouse, Abby consults her old friend and local reporter Liz Tremayne (series star MARIA STEN) 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 (series star DEREK MEARS), 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.
