Sara Price (Natalie Dormer), an American woman, receives a phone call from the Japanese police telling her that they think her troubled twin sister Jess is dead, as she was seen going into Aokigahara, a forest at the northwest base of Mount Fuji in Japan known as a popular destination for the suicidal. Despite the concerns of her fiancé, Rob (Eoin Macken), she journeys to Japan and arrives at the hotel where her sister was staying.
At her hotel, Sara meets a fellow American reporter named Aiden (Taylor Kinney). They drink together, and she tells him of her parents' death in a drunk driving accident years earlier, which her sister witnessed, but Sara did not. Aiden invites her to go into the forest with him and a park guide, Michi, so she can look for her sister.
The next day, as the three enter Aokigahara, Michi tells Sara that Jess has most likely killed herself. Deep in the woods, the group discovers a yellow tent that Sara recognizes as Jess's. With nightfall approaching, Michi suggests they leave a note for Jess and leave. Sara refuses, and Aiden volunteers to stay with her through the night.
That night, Sara hears rustling in the bushes and, believing it may be Jess, rushes into the woods after them. Sara finds a young Japanese girl, Hochiko, who claims to know Jess. The girl warns Sara not to trust Aiden and flees at the sound of his voice. Sara attempts to chase after her but falls and loses her.
The next day, Aiden and Sara become lost and begin to walk around the forest. As they walk, Sara's suspicions are raised and she demands Aiden to give her his phone and finds a picture of Jess on it. Aiden denies any involvement with Jess, but Sara runs into the forest alone. While running she begins to hear voices telling her to turn around. She appears to be unfazed by this until she hears the voice directly behind her. She turns around to see a hanging body and continues to run away. She then falls into an underground cave and goes unconscious, later she wakes up and discovers that she is in the cave with Hochiko, who turns out to be a yrei. Hochiko then turns into what appears to be a "demonic figure". Sara then runs the opposite way towards what appears to be Aiden. Aiden finds her and helps her out of the cave and, with some convincing, they continue to walk together.
Aiden brings Sara to an old ranger station he'd discovered while looking for her. Inside, Sara hears her sister's voice coming from the basement and finds a note which implies that Aiden is holding Jess captive in the basement. Convinced that he is a threat, Sara attacks and kills a horrified Aiden with a small kitchen knife. As he dies, Sara realizes that Aiden was telling the truth and that the picture on his phone, the voice at the basement door and the note itself, had all been hallucinations.
In the basement of the ranger station, Sara sees a vision of the night her parents died from Jess's point of view. It was actually a murder-suicide initiated by her father. The ghost of her father suddenly appears and lunges toward her, grabbing her wrist. She cuts his fingers away from her wrist and escapes the station. Running into the forest, she sees Jess running toward the lights of the search party. Sara calls to her sister, who is unable to hear her. She realizes that her escape from the ranger station was another hallucination. When she cut at her father's fingers she was actually cutting deep into her own wrists, and is now dying from blood loss in the basement. As she succumbs to her wounds, a yrei pulls Sara into the forest floor. Her very much alive sister is rescued by the search party and explains that the feeling of her sister is gone and it is assumed that Jess knows Sara is dead. As the search party leaves, Michi is seen staring at a figure and realizes, too late, that it's Sara's evil spirit who lunges at him.