In 1985, Megan Sydne Mikelle arrives at the Gambels' home to babysit their daughter Ashley Kenzie Jones. Deep in the basement, she discovers a creature feasting on little Ashley. Terrified, Megan tries to flee, but Mr. Gambel Tug Coker deliberately shuts the door on her, and she is presumed dead as a strange sign appears on the door.
In the present, Ben Shaw John-Paul Howard comes to stay with his father Liam Jamison Jones as his parents navigate a painful divorce. Ben starts a job at the marina and quickly befriends Mallory Piper Curda. He also notices Liam growing closer to Sara Azie Tesfai, a co-worker. In the woods, Dillon Blane Crockarell, the neighbor Abbie’s son, uncovers a tree bearing the same ominous symbol from the Gambels’ house. He hears what sounds like Abbie Zarah Mahler speaking from within the tree before the real Abbie appears. They bring home a buck they hit, and that night something crawls out of the buck’s corpse.
Ben befriends Dillon, and that very night Abbie checks on Sam Owen Thomas Pierce in his crib. In his room, a crunching sound and a window left open precede Abbie discovering sticks wrapped in a blanket and then being attacked. Ben later sees Abbie slip into the forest with a child. At work, he learns that Dillon never showed up for sailing lessons. When he goes to Dillon’s house, Ty Kevin Bigley denies having a son. Later, Abbie whispers something demonic into Ty’s ear, causing him to bleed, and she then takes a shower, after which her body begins to decay.
Ben grows suspicious and reads about a witch known for possessing people and abducting children. He searches Abbie’s cellar and finds a family photo along with a photo of Mallory and Lily Ja'layah Washington, realizing the witch is targeting Lily next. He rushes to save Lily, but the witch has already pulled her into the tree. While Ben is with Sara, he notices flowers around her wilting and realizes she is possessed by the witch. When Sara lunges, he slashes her arm, but the witch makes it look like Ben attacked Sara. He is taken into custody, and the officer driving him to the station Officer Guthrie attempts to drown him at the beach until a dog attacks. The officer shoots the dog and then himself, realizing something is controlling him to kill Ben.
Meanwhile, Liam discovers the bodies of Ty and Abbie at their home. Sara attempts to stab Liam, but Ben arrives and shoots Sara. The witch crawls out of Sara’s corpse and pursues Ben, who manages to get Liam to safety as their house burns. Ben then remembers he has a younger brother, Nathan Judah Abner Paul, a detail the witch had caused him to forget.
Ben and Mallory head to the tree to rescue their siblings. As Ben pulls Nathan and Lily from danger, Liam arrives and rams his car into the witch. Later, Ben and Mallory share a goodbye kiss, with Mallory tucking a flower into Ben’s hair before she leaves to teach sailing lessons. Ben notices the flower is fake, revealing that the witch is still alive and has begun possessing Mallory, who is now alone with three children.
