Plot
In 1985, Megan arrives at the home of the Gambels to babysit their daughter, Ashley. As she enters the basement, she finds Ashley being eaten by an inhuman creature in the form of Mrs. Gambel. Terrified, she attempts to escape, but Mr. Gambel shuts the door on her, trapping her down there with the creature. A strange sign is then shown to be etched on the door.
In the present, 17-year-old Ben Shaw comes to live with his father, Liam, as his parents are in the middle of a divorce. Liam gives Ben a job working at the marina, where he befriends a fellow employee named Mallory. During his shift, Ben notices Liam kissing his co-worker, Sara, and gets angry. During a hike in the woods, Dillon, the son of Liam's neighbor, Abbie, finds a tree with the same symbol seen as that at the Gambel house. He hears what sounds like Abbie's voice coming from the tree, beckoning him to approach it before the real Abbie appears behind him. As they return home, they hit a buck, and Abbie decides to bring it with them. When Abbie attempts to gut it, putrid, black entrails spill out. Later that night, an unknown creature crawls out of the buck's corpse.
That night, Ben investigates strange noises he hears on the roof. He follows the sounds to Abbie's house, but discovers it was only a raccoon. He then catches a glimpse of a witch on the porch, but cannot be sure what he saw due to Abbie's husband, Ty, turning on the porch light. Ben befriends Dillon and makes him promise to tell him if he sees anything strange around their house. Later, Abbie goes to check on her infant son, Sam (Dillon's younger brother), in his crib. However, she finds that he has disappeared and has been replaced by a bundle of sticks. She is then attacked by the witch. Ben returns home from a local party later that night and notices Abbie walking into the forest with a child, but is interrupted by an angry Liam who had been searching for him after Ben didn’t return any of his calls.
The next morning, Abbie, disheveled and filthy, returns to her house, where she ominously stares at Dillon as he watches TV. While at work, Ben dumps trash in the boat of the local bully's father as revenge for him and his friends cruelly pranking him at the party. That night, he is driven home by Sara who reveals she's lactose intolerant and tries to bond with him. Ben finds Dillon hiding in his room, claiming that there is something wrong with his mother. Abbie comes looking for Dillon, threatening Ben and attempting to get into the house. Fortunately, Ty soon arrives and takes Dillon home. Dillon tries to warn Ty that Abbie is acting weird, but he dismisses his son's concerns. Afterward, Ty visits Sam's nursery, which is now empty. However, he doesn't seem phased by this.
Ben becomes concerned that something strange is happening with Abbie and attempts to spy on her until interrupted by Liam. Later, as Abbie is shown having sex with Ty, it is revealed that her flesh has begun to decay. At work the next day, Ben learns from Mallory that Dillon never showed up for his sailing lesson. When he goes to Dillon's house to check on him, he is greeted by Ty, who strangely denies even having children. Before leaving, Ben finds the ominous symbol carved into the porch.
Ben researches the symbol and finds information about an evil forest witch said to wear the skins of her victims and "feed on the forgotten". He confides in Mallory as they observe Abbie suspiciously emerging from the cellar with a trash bag. But she doesn't take Ben seriously and mocks him, even going so far as to slip a taunting note under Abbie's door, proclaiming they know what's in her cellar. Meanwhile, Ben begins spreading salt around his house, as it is apparently capable of repelling witches. When Ty finds the note and attempts to investigate, Abbie appears and whispers something demonic to him, causing his ears to bleed. That night, as Mallory locks up the marina office, it is revealed that one of her family pictures has mysteriously disappeared from the employee bulletin board.
Later, Ben becomes even more suspicious after discovering the trash bag Abbie was carrying contained various items from Sam's nursery. The next day, as Abbie steps out of the shower, it is revealed that her entire body has now almost entirely decayed. After seeing Abbie emerge from the cellar again, Ben breaks in and finds a deer skull and a picture of the family with Ty's face scratched out, along with the missing picture of Mallory's family (with her and her mom's faces scratched out as well). He realizes that Mallory's younger sister, Lily, is the witch's next victim. He calls Mallory to warn her that Lily is in danger, but Mallory doesn't seem to remember her sister. Ben rushes to save Lily, but is interrupted by the local bully, who attacks Ben for vandalizing his father's boat. After escaping, Ben tries to catch up with Lily, but he is too late, arriving just as the witch pulls her into the tree. He tries to pull Lily out, but he loses his grip and gets knocked out after hitting his head on a rock.
Upon returning home, Ben finds that Liam has called the police to report him missing. Ben attempts to explain what is happening with the neighbors, but given his recent strange behavior, Liam merely accuses him of being on drugs and refuses to listen. Sara appears and Ben tries to talk to her, but is shocked to see her pouring creamer in her coffee. Noticing that the flowers around her are decaying, Ben realizes the witch is now inhabiting her skin. When Sara tries to attack him, Ben slashes her arm with a kitchen knife. But the witch reverts to normal and makes it look like Ben attacked her for no reason.
Ben is then taken into custody. From the police car, he sees Sara whispering something in the officer's ear. He tries to warn Liam that Sara is the witch and pleads with him to check the neighbors' cellar. Instead of taking Ben to the station, the officer drives Ben to the beach, where he attempts to drown him until he gets attacked by a dog. The officer shoots the dog and then himself after realizing that he is being controlled by some dark force. Meanwhile, Liam explores the neighbors' house, where he discovers the dead bodies of Ty and Abbie. He also finds a disturbing shrine containing the neighbors' family picture Ben saw before, along with a picture of his own family, where his, Ben's, and his wife's faces are scratched out.
The now-possessed Sara arrives and attacks Liam, causing him to drop his lamp and start a fire. Just as she is about to kill him, Ben arrives and shoots her with the officer's gun. The witch (now in her true form) crawls out of Sara's corpse and goes after Ben, but he takes Liam to the safety of the latter's car as the neighbors' house burns down, burning all the pictures the witch stole. When the picture of Ben's family is destroyed, Ben suddenly remembers that he has a little brother, Nathan. All Ben's true memories come back, revealing Nathan had been present throughout all the events so far, and he has been taken by the witch.
Mallory arrives looking for Lily, and the two head to the tree to rescue their siblings. Ben descends into the witch's lair while Mallory stays behind to spread salt around the tree. After Ben manages to save Nathan and Lily, he is about to escape when the witch emerges and grabs him. Just as she begins pulling Ben back in, Liam drives into the forest, distracting the witch long enough for Mallory to throw salt in her face and free Ben. Liam then rams into the witch with his car.
Several weeks later, Lily has begun seeing a therapist to deal with the traumatic event. It is revealed that after escaping the witch, Ben and Mallory burned the tree, then aided the injured Liam. Back in the present, Ben and Nathan plan to move back to their mother's house. Mallory gives Ben a flower she took from a bouquet Lily sent to the marina office, and the two share a kiss goodbye. Mallory then takes three children out on the water for a sailing lesson. Liam explains that he is going to stay with his brother, Rory, while he recuperates and thinks things through. As they depart, Ben attempts to smell the flower Mallory gave him, and is shocked to discover it's fake. As only real flowers decay in the witch's presence, it is implied that she survived and has possessed Mallory, who is now alone, in the middle of the lake, with three young children.
