After she's kidnapped, a young woman begins to undergo a strange transformation.
A notorious cult kidnaps a young girl, and sacrifice themselves by the light of the locust moon. The next morning the girl awakes, caked in dried blood and surrounded by corpses...but safe - or so she thinks. Years later, the locust moon is about to rise again and the girl is captured once more by a surviving member of the cult. She is taken to a remote cabin where she learns that a demon has been growing inside of her all these years, and before the dawn it will rise.—unknown
Gloria Farrow wakes from a nightmare during which she flashes back to five years earlier, when she escaped a group of cultists who ritualistically slit their own throats while she was restrained to a sacrificial altar.
Gloria attends a group therapy session with her girlfriend Joan, who was supposedly whipped by her father growing up, before the two young women spend time serving food in a shelter with fellow volunteer Kent. In bed together later, Gloria and Joan reflect on the respective traumas that led them to each other.
Joan gifts Gloria with a necklace to commemorate their upcoming anniversary and they say their goodbyes. While walking home alone, Gloria is abducted by former cultist Thomas, who chains Gloria inside his RV and takes her to a remote family cabin in the woods. On the way, Gloria briefly recovers her cellphone and leaves a frantic message for her mother Ruth.
While chained in the cabin, Gloria dreams of waking from the ritual covered in the blood of cultists lying dead around the altar. Thomas promises Gloria that he intends to save her.
Joan calls Ruth looking for Gloria, which leads to Ruth's discovery of Gloria's voicemail. When Officer Carter claims there is little the police can do, Joan organizes her own search with Kent and other volunteers from the church.
Thomas explains to Gloria that he was part of the cult from five years ago, but seeing her face that night convinced him that the cult was delusional. Thomas faked his death during the ceremony because he no longer believed the cult's purpose.
Thomas and Gloria both begin having visions of the cult and a demon haunting them. Thomas tells Gloria that her ordeal will be over provided he protects her until sunrise.
While going door to door searching for Gloria, Joan puts her knife to the throat of a man who refuses to look at Joan's flyer. Ruth later calls Joan to her house after Officer Carter asks to question Joan about the necklace found at the abduction site and about Joan threatening the uncooperative man. Joan murders both Officer Carter and Ruth.
Thomas tells Gloria that five years ago, the cult intended to receive their dark lord Abaddon by the light of the locust moon so that he could be reborn under the same locust moon five years later. Thomas explains that not all of the cult members died, and by sacrificing Gloria, Abaddon can be brought to this world.
Joan is revealed as a cultist when she is seen flogging herself in front of a shrine to Abaddon as punishment for losing Gloria.
Thomas realizes there may be truth to the cult's beliefs when he sees Gloria transforming and growing holes in her shoulders. Haunting visions continue as Thomas is seemingly seduced by Gloria only for her to morph into a demon and vanish.
Kent introduces Joan to Mr. Vela, who provides a tip that he saw a suspicious RV around the time of Gloria's abduction. Gloria has Kent come with her as she begins tracking Thomas' trail.
Thomas reveals to Gloria that Joan is actually his sister Gwen who hid under their mother's name. Joan/Gwen was the cult's 'Chosen One,' the lone survivor who would complete the ritual after five years. Thomas also reveals that it was his sister who gave him the burn scars on his face.
A demon seemingly appears and Gloria escapes the cabin before regrouping with Thomas in the woods. Thomas returns Gloria to the cabin as her metamorphosis continues.
Joan and Kent finally locate Thomas' RV. Joan kills Kent.
Realizing that the demon has already taken her, Thomas prepares to shoot Gloria, but is thwarted by a vision of his burning by Joan and the cult. Thomas crawls outside and Joan captures him.
Joan confronts Gloria. Visions of cultists appear around a pentagram surrounding Gloria as Joan clutches a knife and recites an incantation.
Thomas escapes captivity and breaks into the cabin with an ax. As he crashes through the door, Thomas ends up in a vision of an open field where Gloria greets him. Thomas breaks free of the illusion and puts his ax into Joan's back in reality.
Visions continue slowing Thomas down. As Abaddon appears, Thomas kills Joan with a shovel and the illusions vanish. Thomas rescues Gloria as she is seemingly dying and he brings her outside to see the sunrise like he promised. However, Gloria suddenly attacks Thomas and sprouts wings from her back.
