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.

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.

Certain that Alec’s disappearance is more than an accident, Abby and Matt venture into the swamp, where she encounters Swamp Thing. Scientist Jason Woodrue comes to Marais.

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.

An outsider’s arrival forces Alexandria to rehash devastating old wounds; eye-opening secrets from the past are revealed.

Dr. Brenner holds Hopper and Joyce for questioning while the boys wait with Eleven in the gym. Back at Will’s, Nancy and Jonathan prepare for battle.