Ben is an [American social psychologist](actor/eric-bana) living and working in Berlin, trying to rebuild his life following his separation from his wife in California. He spends his days teaching at a local college, researching, and writing books on the workings of cults and how groupthink influences human behavior. His daughter, [Mazzy Lizzie Sink](actor/sadie-sink), a 16-year-old girl, comes to Berlin to spend a semester with him while attending an English school.
The story begins with a disturbing discovery: a group has carried out a carefully planned mass suicide in a suburban house. [Nina](actor/sylvia-hoeks), a forensic detective and a member of the police, informs Ben and his colleague [Max](actor/stephan-kampwirth) that the victims took cyanide in shifts to ensure everyone died peacefully, and each body contains a small seashell in the mouth along with a black smear on their cheeks—details that hint at a ritualistic element. Ben, concerned for Mazzy's safety, asks her to take the train to his apartment, but she becomes confused and meets a young man named [Martin](actor/jonas-dassler), who shows her where to go and introduces himself as someone living nearby. They exchange contact information, and over the following days, Mazzy begins to settle into her new environment, attending school and spending time with Martin.
Meanwhile, Ben continues his investigations, conducting interviews that reveal deeper insights into the cult’s beliefs and practices. One such interview is with a cult member who only speaks behind a closed door, hinting at the group’s secretiveness. Mazzy also befriends a girl named Lotte and explores the city, even engaging in activities like note-sharing over coffee and experimenting with psychedelic mushrooms in the woods. Her interactions with Martin and the cult group deepen her curiosity and sense of connection.
Throughout the story, flashbacks reveal Martin’s background. He lived with his grandmother, who was his most significant family figure, and was very close with her until her sudden death. His parents died in a car accident when he was young, which he mentions casually in conversation, and he now lives alone. Martin was involved in environmental activism, joining protests with an activist group led by a woman named Hilma. As he becomes more involved, he introduces Lotte to the group, which uses meditation, group therapy, and activism as part of its community-building efforts.
One day, a girl is found drowned in a lake with stones in her pockets—another suicide, but with enough differences to suggest it is unrelated to the earlier mass death. Nina and her team are called to investigate, but Nina quickly concludes that this death isn’t connected and dismisses it as an isolated incident. Over time, she and Ben grow closer, sharing moments together, yet their relationship becomes complicated when Ben sees Nina kissing another woman outside a bar, revealing tensions and secrets.
As Mazzy continues to explore, she visits Elsa, [Max’s] daughter, but leaves after Elsa refuses to go out. Instead, Mazzy meets Larissa, a friend from school, and they wind up at an underground club. While there, Mazzy is approached by a man trying to pressure her into dancing, prompting Larissa to intervene. The altercation results in Martin stabbing the man with a broken bottle, and the girls, overwhelmed and intoxicated, decide to sleep in a park. The following morning, Martin helps Mazzy and Larissa before calling emergency services, showcasing his protective side.
The plot thickens as Mazzy’s parents and the police search for her. Her friends and Ben are frantic, but Nina advises caution, informing Ben that the police’s official response will be to wait 24 hours before taking action. As they search, Nina reveals her true allegiance: she is part of the same cult as Martin, having been handed over by her parents to Hilma in her youth. She confesses her commitment to Hilma’s vision of a collective sacrifice for the greater good. It’s uncovered that the woman Ben interviewed behind a door, as well as the girl from the lake, was actually Lotte, who had been recruited into the cult.
The film reveals a meticulously orchestrated plan: everything from Mazzy’s encounter with Martin at the train station to her being brought to the cult center was manipulated. Hilma, the leader, and her followers have been preparing for a final act—self-immolation. Mazzy, captured and drugged, resists an underground initiation ritual, begging to contact her father but is knocked out with chloroform. Hilma orders Martin to take her to the lake where the cult plans their mass suicide. Ben rushes there upon discovering the truth.
Martin, feeling conflicted, leaves Mazzy on the lakeshore, where she begins to hallucinate and ultimately sinks into the water. Ben dives in and rescues her just in time. As the cult members—dressed in ceremonial robes, with seashells in their mouths and ash on their faces—set themselves ablaze in a ritualistic burning, only Hilma remains among the flames. Meanwhile, Martin, overwhelmed by grief and guilt, succumbs to the same fate inside his apartment, lying next to his grandmother's remains and taking a cyanide pill.
In the final scenes, Mazzy and Ben are reunited at the lakeside, embracing in tears after Ben rescues her from drowning. The cult members, except for Hilma who has left the scene, are shown dying in the fire. The movie closes with a haunting image of [Elsa](actor/sophie-rois), who finds a video of Hilma online, staring entranced—mirroring Mazzy’s initial fascination—highlighting how the influence of cult mindsets can persist beyond the immediate tragedy.
