Ben, a thoughtful American social psychologist based in Berlin after a painful separation from his wife in California, spends his days teaching at a local college and shaping books about how cults operate. His sixteen-year-old daughter Mazzy comes to live with him for a semester and study at a nearby English-speaking school, bringing a sense of distance and renewal to both their lives. The story follows their tentative reconnection as Berlin becomes the frame for a deeper investigation into how a tightly knit group can bend belief, loyalty, and fear toward a dangerous end.
On the day Mazzy is due to arrive, Ben is drawn into a chilling scene. His colleague Max informs him that a contact at the police department is at a suburban crime scene where a cult has carried out mass suicides. They are granted access to observe the aftermath: a house filled with meticulously laid-out bodies, each victim having swallowed cyanide in shifts so that the scene could be arranged with their personal belongings. A small seashell rests in each mouth and a dark smear marks their cheeks, a ritualized signature that hints at a shared, manipulative creed. With the gravity of the moment sinking in, Ben sends Mazzy a text asking her to take the train to his apartment, realizing he might not be there in time to welcome her in person.
Mazzy’s journey into this new city life continues as she navigates the unfamiliar transit system. A chance encounter with a young man named Martin leads to a spark of connection; he offers directions, shares his address, and they exchange numbers, sowing seeds for a complex web that will pull Mazzy deeper into Berlin’s underground currents. Back at work, Ben’s research intensifies. He conducts interviews with a cult member who insists on speaking through a barrier in her apartment door, and he collaborates with Max and Nina as they piece together the mechanics of group influence. Mazzy, meanwhile, begins to forge a quiet alliance with Martin, enjoying casual moments over coffee and later sharing a late-night experience in the woods that she’ll carry with her as a doorway into more risky explorations.
A family dinner at Max’s house introduces Mazzy to a social circle intertwined with secrecy. Max’s wife and their teenage daughter Elsa are present, and Mazzy senses a tension in the air as she probes who Nina is—only to learn from Elsa that Nina is a colleague, not a romantic interest. Flashbacks unfold to illuminate Martin’s past: a life shaped by his close bond with his grandmother, sorrow over her death, and a long-standing involvement with an environmental activist group led by a charismatic leader named Hilma. Martin brings a girl named Lotte to group meetings, and the narrative gradually reveals the fragility of his family history and the vulnerability he finds in his new circle.
As the days pass, the plot thickens with new incidents. Another girl is discovered dead by drowning in a lake, weighted by stones in her pockets. Nina and her team respond to the scene but determine that this death does not fit the earlier pattern: no shell, no black mark, no obvious linkage to the mass suicide. Nina and Ben grow closer, even as the investigation exposes cracks in their trust. Martin continues to involve Mazzy in protests with Hilma’s group, and Hilma herself welcomes Mazzy with an unsettling warmth, gifting her a necklace that becomes a symbol of belonging and danger.
One night, while Mazzy dozes to Hilma’s spoken lectures, Nina invites Ben to a drink, and he slips away briefly. When Mazzy wakes and discovers him at the bar kissing Nina, her heart shatters; she had assumed Ben was not seeing anyone. The next morning she confronts him, storming out to the center and seeking Martin, only to find his absence mirrored by Hilma’s comforting but cryptic presence. Mazzy’s day spirals into a series of confrontations and cultural explorations: she visits Elsa, meeting a friend named Larissa, and together they slip into an underground club where strangers mingle with alcohol and drugs. A disturbing moment occurs when Martin appears in the club and prevents a man from pressuring Mazzy into dancing; in the ensuing chaos, Martin wounds the man with a broken bottle and Mazzy, overwhelmed and vulnerable, ends up sleeping in a park with Larissa after an evening that felt perilously out of control.
The next dawn brings a new crisis. Martin carries Larissa to safety and dials emergency services, ensuring someone will come for her, while Mazzy is taken back to the center. Ben, meanwhile, learns that Max and the others have been trying to reach him to report Mazzy’s disappearance. Nina delivers a probing warning: do not contact the police yet, because the cult’s leadership has learned to exploit a system that requires a waiting period before action is taken. A tip arrives that a possibility exists of an unconscious girl being carried into the center, confirming that Mazzy’s disappearance is not merely a random event. The moment reveals a chilling truth: Nina is connected to the same cult as Martin, and her loyalty to Hilma’s cause is absolute. The group’s ringleader, Hilma, is orchestrating a broad recruitment strategy that leverages Ben’s public persona and his writings on groupthink to propagate their message.
The film builds toward a devastating revelation. The so-called Lotte—the girl whose death had haunted Martin’s past—has been present in various disguises within the group’s fabric, and the ritual necklace given to Mazzy is in fact the same seashell found with the suicide victims. The center’s walls become a stage for an initiation rite as Mazzy is enticed and then coerced to accept their help. She resists, begs to call her father, and is knocked unconscious with chloroform as Hilma and Martin escalate their coercive tactics, with Martin ordered to transport Mazzy to the lake to await further instructions. The confrontation between Ben, Nina, and Hilma brings a moment of sharp clarity: a line from the past echoes in Hilma’s words, allowing Ben to realize Hilma’s deeper identity and the role Mazzy has been primed to play in the planned mass end.
In the final act, the remaining members prepare for a collective suicide, while a shaken Martin retreats to his apartment, where the truth about his grandmother’s corpse remains on the living room rug. He lies beside her and takes a cyanide pill, a bleak gesture that underlines the personal history driving his loyalty to the group. Inside the center, Nina and the others reenact their devotion in ceremonial robes, placing seashells in their mouths and painting ash on their cheeks, before setting themselves aflame in a ritualized self-immolation that consumes the building. Hilma exits the scene as chaos and fire erupt behind the doors.
Mazzy wakes at the lakeshore, disoriented and still under the effects of the drug. The lake’s surface becomes a mirror for her fear as she imagines Lotte beckoning from the waters. She steps into the lake and begins to sink, slipping toward an ending she cannot foresee. Ben arrives with the police, diving into the water to reach her and pulling her into a tearful embrace. The aftermath is a tableau of loss and relief: Martin and the other cult members die, while Hilma’s influence remains shadowed by the center’s ruin. In the final image, Elsa has found a video of Hilma online, staring transfixed in the same way Mazzy once did, suggesting that the cycle may begin anew even as Mazzy survives this terrifying awakening.
