Ben, an American social psychologist living in Berlin, finds himself grappling with the emotional aftermath of a recent separation from his wife in California. He dedicates his time to teaching at a local college and researching the fascinating, often disturbing world of cults, aiming to understand how they operate and influence their followers. His daughter, Mazzy, a 16-year-old girl, comes to Berlin for the semester, eager to attend a local English school and experience life abroad.
The story takes a haunting turn when Ben receives a call from his colleague Max, informing him about a gruesome scene at a suburban house—a mass suicide by a cult group. Arriving with Max, Ben encounters Nina, a forensic detective, who explains that the members took cyanide pills in shifts, ensuring everyone could die together while laying out their personal belongings. Strangely, each body had a small seashell in their mouth and a black smudge on their cheeks, details that hint at a ritualistic aspect to the tragedy. As Ben processes the horror before him, he texts Mazzy, urging her to come to his apartment, unaware of the dangerous path the cult's influence is about to lead her down.
Mazzy, meanwhile, misses her train but is helped by a young man named Martin, who introduces himself as someone living nearby Ben’s apartment. They exchange contact information, and she begins to settle into her temporary life. As she attends her new school, she starts to explore her surroundings and her own feelings, spending time with Martin, whose background unfolds through flashbacks. Martin's childhood was marked by loss—his grandmother, who was his only family, recently passed away, leaving him emotionally shattered. He had previously claimed he had lost both parents in a car accident and now lives alone.
Ben continues his research, interviewing a cult member who refuses to speak openly unless behind a closed door. His interactions with Nina also deepen, especially as they spend more time together, leading to an awkward but genuine connection. Meanwhile, Mazzy gets involved with Hilma, a charismatic leader of an environmental activist group she begins to listen to on her own. This group practices meditation, attends protests, and shares a communal sense of purpose. Hilma, wearing a distinctive necklace, welcomes Mazzy warmly, further pulling her into their ideology.
Amidst these unfolding events, more ominous incidents occur. A girl is found drowned, with no signs linking her to the mass suicide, but Detective Nina dismisses it as unrelated. As Mazzy immerses herself in the group's teachings, she attends protests and even shares a meal with Max, his wife Elsa, and their teenage daughter Elsa—who comments on Nina’s role, unaware of the secret relationship brewing.
Jealousy and suspicion stir when Mazzy witnesses Ben kissing Nina outside a bar—an encounter Ben hadn’t disclosed, leading to her storming out and seeking solace elsewhere. She confides in Hilma, who subtly encourages her to distance herself from her parents, revealing her own perspective on abandonment and trust. That night, Mazzy cuts loose further, visiting an underground club with her friend Larissa, where she drinks and experiments with drugs. In this vulnerable state, she encounters a man attempting to pressure her into dancing; Martin, witnessing the situation, intervenes violently, stabbing the man with a broken bottle. Exhausted and shaken, Mazzy and Larissa spend the night in a park.
The next morning, Martin shows kindness, covering Larissa with a blanket and helping her contact emergency services, then bringing Mazzy back to the cult center. Meanwhile, Ben and Nina realize that Mazzy is missing. Their frantic search leads them to Elsa, but she knows nothing beyond Mazzy’s desire to go clubbing. As hours pass, Nina cautions Ben not to alert the police prematurely, hinting at a darker conspiracy. Their suspicions intensify when they hear that an unconscious girl was reportedly taken into the cult’s center, but they find that the victim of the lake drowning isn’t connected to the group, suggesting a random act or separate tragedy.
It’s soon revealed that Nina is deeply embedded in the cult, a loyal follower of Hilma, and part of the sinister plan. Her history is dark—her parents handed her over to Hilma’s control, believing they were doing so for a "greater good." Her involvement was strategic; she approached Ben because of his bestselling book on groupthink and social influence, with the cult aiming to use his reputation to spread their message further. The woman Mazzy encountered through the door and the drowning victim from the lake are both Lotte, a girl Martin had previously convinced to join the cult.
The narrative reaches a terrifying climax as Mazzy is lured into an underground chamber at the cult’s center, where she refuses to participate in the ritualistic mass suicide. Her pleas for her father’s help are ignored, and she is knocked out with chloroform. Hilma, the cult leader, orders Martin to take her to the lake, where he leaves her as part of the ultimate sacrifice. Meanwhile, Ben, realizing the danger, rushes to save Mazzy, calling Max to coordinate rescue efforts.
At the lakeshore, Mazzy, still drugged, begins to hallucinate that Lotte is in the water, beckoning her to join her. Struggling with her fear and confusion, she walks into the water and sinks beneath the surface. It takes heroic effort from Ben, who dives in to rescue her, emerging with her in tears. The cult members, led by Hilma, conduct their mass self-immolation, dousing themselves in gasoline and igniting flames in a chilling ritual of devotion.
In the aftermath, Martin, overwhelmed by grief and guilt, stays alone in his apartment, where he is shown lying beside his grandmother’s corpse—never having moved her from the living room rug—and taking a cyanide pill. The tragic ending highlights the impact of manipulation and loss, with Hilma and her followers perishing in flames. The final scene reveals Elsa discovering a video of Hilma, her gaze fixed on it with an unsettling fascination, mirroring Mazzy's initial intrigue. The film leaves viewers with a haunting sense of the destructive power of cult influence and the fragile boundaries between belief and control.
