Ben, an American social psychologist, has built a life in Berlin after a separation from his wife in California. He teaches at a local college and spends his research hours studying how cults operate and influence people. His sixteen-year-old daughter, Mazzy, travels to Berlin to live with him for the semester and enroll in an English-language school, offering a window into a tense father-daughter dynamic as he balances work, study, and guardianship.
On the day Mazzy is set to arrive, Ben is pulled into a disturbing assignment by his colleague Max. A police contact at the scene of a cult mass suicide invites them to view the aftermath, having secured permission to witness it. They arrive at a quiet suburban house where members have methodically taken cyanide, lying among their belongings. Each corpse bears a small seashell in the mouth and a dark smear on the cheek, a ritualized detail that unsettles Ben and Max. Realizing the gravity of what they’ve seen, Ben texts Mazzy to head to his apartment, preferring she not be caught up in the tragedy.
Mazzy’s arrival sets off a separate current of tension and connection. She meets a young man named Martin on the train, who claims to live near Ben and offers his contact information. As Ben continues his fieldwork—interviewing a cult member who will only speak to him behind a closed apartment door, coordinating with Max and Nina—the Edith-like rhythms of Mazzy’s life unfold. She attends school, spends time with Martin in coffee shop chats and woodland trips, and later accompanies him to a protest led by a figure named Hilma. Meanwhile, the group’s dynamics deepen; Mazzy becomes drawn to Hilma’s charisma, and a necklace given by Hilma—later revealed to conceal the same shells found with the suicides—begins to symbolize a troubling lure.
Martin’s backstory emerges in flashbacks. He lived with his grandmother, whom he cherished after the loss of his parents. His activism with Hilma’s environmental group intertwines with private grief, and he starts introducing Mazzy to the circle that Hilma leads. Mazzy, enchanted by Hilma’s lectures, spirals into a quiet fascination that contrasts with the daylight reality of her father’s tense investigations and the increasing strain in their relationship, especially as Ben’s professional distance grows due to his dealings with Nina, a detective attached to the case.
A second tragedy surfaces when another girl is found dead by drowning in a lake, another ring of the same mysterious pattern but lacking a shell or a cheek mark. Nina and Ben discuss the possibility that this death might be unrelated, though the investigation soon uncovers deeper connections. Nina re-enters Ben’s orbit both professionally and personally, and Mazzy begins to drift into the hypnotic orbit of Hilma’s circle, listening to talks and lectures on her own. One evening, Mazzy wakes to find Ben missing and later discovers him at a bar with Nina, where a kiss confirms a betrayal that devastates her trust. The morning after, Mazzy confronts Ben about Nina and runs to Elsa’s house to escape the mounting tension.
In the days that follow, Mazzy encounters a web of social entanglements. She sneaks out with a friend to an underground club; Martin unexpectedly appears in the scene as a witness to a male’s attempt to pressure Mazzy into dancing. In a violent moment, Martin stabs the aggressor with a broken bottle, and the two girls collapse in exhaustion in a park. The next dawn sees Martin’s sense of responsibility pull him back to safety as he calls emergency services, escorting Mazzy to the center. The investigation now unravels a more startling truth: Nina is not merely a colleague but a member of the same cult, fully under Hilma’s control. Nina’s parents had entrusted her to Hilma as a teen so she could participate in what the group calls “the greater good,” and she had sought out Ben precisely because of his widely read books on groupthink and his belief in the importance of community.
Crucially, the case reveals that Lotte—the girl from Martin’s past who vanished from memory—was the true person behind the submerged threads: the girl who interacted with the woman who had committed suicide in the lake, and the one who masqueraded as multiple identities within the circle. The seashell necklace Mazzy wore, given by Hilma, is in fact the same shell that marked the group’s suicides. The earlier scenes—Ben’s presence at the crime scene at Mazzy’s arrival, Martin’s chance encounter with Mazzy at the station—were orchestrated by Hilma and her followers to pull Mazzy deeper into the ritual.
With Mazzy already drawn into initiation rites at the center, she resists as much as she can, pleading for her father to intervene. Hilma scolds Martin and declares that he has “failed again,” sending him to take Mazzy to the lake for the next instruction. The confrontation between Ben, Nina, and Hilma pushes Ben to realize Hilma’s true identity and the fate awaiting Mazzy. He phones Max and urges him to act, fearing a force stronger than law enforcement could handle.
As the group braces for a mass suicide, Martin walks away from the center, leaving a memorial all too real: the living room rug still bears the weight of his grandmother’s absence. The center is set aflame as Hilma leads the remaining members in a ceremonial robe swap—shells placed in mouths, ash smeared on faces, and gasoline poured for a final blaze. Nina, resisting the ultimate self-destruction, is ordered by Hilma to prove her loyalty, but she briefly resists before joining the others as a ritual endgame.
At the lakeshore, Mazzy wakes, still groggy from the drugging she endured earlier. She hallucinates Lotte calling from the water, and she wades in, allowing herself to sink as Ben arrives with the police and dives into the lake to rescue her. The crisis ends with Mazzy safe in her father’s embrace, tears spilling down their faces, while Martin and many of the cult members die in the center, leaving Hilma to depart as the flames burn behind her. The final image lingers on Elsa, who comes across a video of Hilma online and stares at it with a look of the same enthralled captivation that Mazzy once showed, hinting at a dangerous, enduring influence even after the immediate danger has passed.
