Sara Price, portrayed by Natalie Dormer, is an American woman who receives distressing news from the Japanese police regarding her troubled twin sister, Jess. The authorities believe Jess has likely died after being spotted entering Aokigahara, a notorious forest at the base of Mount Fuji known for its haunting reputation as a suicide destination. Despite her fiancé Rob's concerns, played by Eoin Macken, Sara embarks on a journey to Japan to uncover the truth about her sister.
Upon her arrival at the hotel where Jess last stayed, Sara crosses paths with a fellow American journalist named Aiden, who shares a drink with her. During their conversation, she reveals the traumatic experience of losing their parents in a tragic drunk driving accident—a nightmare that Jess witnessed, but Sara did not. Aiden suggests they venture into Aokigahara with a park guide named Michi to search for Jess.
As they delve into the forest, Michi grimly informs Sara that her sister has likely taken her own life. As night falls, the trio stumbles upon a yellow tent that Sara instantly recognizes as belonging to Jess. Michi proposes they leave her a note and depart, but Sara, unwilling to give up, insists on waiting for Jess. Aiden decides to stay with her that night.
During the long hours of darkness, Sara hears mysterious rustling in the underbrush, leading her to believe it might be Jess. She chases after the sounds but instead encounters a young Japanese girl, Hochiko, who claims to have knowledge of Jess’s whereabouts. Warning Sara not to trust Aiden, she vanishes when Aiden approaches, prompting Sara to give chase—but she ends up losing sight of Hochiko.
On the following day, confusion sets in as Aiden and Sara become disoriented. Sara’s unease grows, prompting her to demand Aiden's phone, where she finds a disturbing picture of Jess, despite Aiden's denials of involvement. Feeling betrayed, she runs off into the forest, haunted by disembodied voices urging her to turn back. Her desperate flight leads her to discover a horrifying hanging body, triggering a panicked escape that results in her falling into an underground cave, where she briefly loses consciousness.
When Sara awakens, she finds herself back in the cave, confronting Hochiko, who has transformed into a menacing spirit. Fleeing deeper into the woods, she encounters Aiden who helps her out of the cave. However, her trust issues resurface when Aiden leads her to an abandoned ranger station he had found. There, Sara hears what she believes is Jess's voice coming from the basement and discovers a note suggesting Aiden is hiding Jess there. In a frenzied confrontation, Sara ends Aiden’s life with a kitchen knife, only to realize too late that her perceptions had been warped by hallucinations.
In a twisting revelation, Sara experiences a vision of the night her parents died through Jess's eyes. It becomes clear that their father was behind a murder-suicide, leading to a fresh wave of terror when her father's ghost appears, threatening her. In a desperate bid for survival, she manages to escape but realizes her hideous hallucinations have coalesced into a reality where she is fatally injuring herself instead.
In her final moments, an overwhelming sense of confusion engulfs Sara as she believes she sees Jess being pulled toward the safety of a search party. Yet, it dawns on her that her entire escape was a mere figment of her imagination, and she is indeed bleeding out in the basement. Just as the search party departs with a rescued Jess, Michi ominously senses the presence of Sara's spirit, caught between realms, as her dark fate is sealed.
