Ben is an American social psychologist living in Berlin in the aftermath of his separation from his wife in California. He teaches at a local college and studies how cults work, writing books that explore the mechanics of group influence and conformity. His teenage daughter, Mazzy, arrives in Berlin for the semester to live with him and attend a nearby English-language school.
On the day Mazzy is due, Ben’s colleague Max calls with urgent news from a police contact who is at the scene of a cult mass suicide. Ben and Max hurry to a suburban house where a group has meticulously taken their own lives. The forensic detective Nina explains that the suicides happened in shifts so everyone could be laid out with their belongings. Each body bears a small seashell in the mouth and a black smear on the cheek. Because Ben is at the scene, he texts Mazzy to take the train to his apartment, since he knows he won’t get there in time to meet her.
Mazzy begins her journey and, while trying to navigate the train, meets a young man her age who offers to help and introduces himself as Martin. He explains that he lives near Ben’s apartment, and they exchange numbers. As days pass, Mazzy starts school, and Ben continues his research, conducting interviews with a cult member who speaks to him only behind the apartment door, while working with Max and Nina. Mazzy also spends time with Martin, sharing coffee and later wandering in the woods while they do shrooms together. One evening, Mazzy, Ben, Nina, Max, and Max’s wife Elsa share a meal at Max’s house; Mazzy asks Elsa who Nina is, suspecting a romance, but Elsa says Nina is simply a colleague.
Flashbacks reveal Martin’s past: he lived with his grandmother, who was the closest family he had. He becomes involved with an environmental activist group led by a woman named Hilma, participating in group therapy, meditation, and protests. Martin brings a girl around his age, Lotte, to the meetings. He eventually returns home to find his grandmother has died, deepening his sense of loss. He had told Mazzy that his parents died in a car accident when he was young and that he is currently living alone, raised in part by his grandmother.
A few days after the mass suicide, another girl is found drowned in a lake, with stones in her pockets. Nina and her team respond, but she concludes there is no shell in her mouth and no facial mark, and therefore it appears to be an isolated tragedy, not linked to the earlier event. Nina and Ben begin spending more time together, and Martin brings Mazzy to a protest alongside Hilma and the activist circle. Hilma welcomes Mazzy, even giving her a necklace.
Mazzy starts listening to Hilma’s lectures on her own, drawn in by the charisma of the group. One night, while Mazzy is asleep, Nina asks Ben to meet for a drink, and he leaves a note for Mazzy saying he’ll be at the bar down the street. She wakes up and finds him kissing Nina, which deeply upsets her, since he had told her he wasn’t seeing anyone. The next morning, Mazzy confronts Ben about Nina and leaves to go to Elsa’s house. Before leaving, she stops by the center where the group meets and is comforted by Hilma as she talks about her own parents—Hilma does not dispute Mazzy’s fear, instead encouraging her to distance herself from her family.
Mazzy then goes to Elsa’s house but leaves when Elsa doesn’t want to go clubbing. She meets Larissa, a school friend, and the two head to an underground club where they drink and do drugs with strangers. Unbeknownst to them, Martin is also in the club and witnesses a man pressuring Mazzy to dance with him. Larissa pulls Mazzy away and the two leave, while Martin stabs the man with a broken bottle. Intoxicated and exhausted, Mazzy and Larissa sleep in a nearby park.
Dawn comes, and Martin covers Larissa with a blanket, then uses her phone to summon help before bringing Mazzy to the center. Meanwhile, Ben, after spending time with Nina, learns that Max has been trying to reach him about Mazzy’s disappearance. They rush to Elsa’s and press Elsa for any information, but she only knows that Mazzy wanted to go to a club. Ben considers calling the police, but Nina convinces him to wait 24 hours, claiming to have detective contacts who can help. She leaves to take a call and returns with a tip: an unconscious girl was seen being carried into the center. They hurry there and discover Hilma and several group members inside, but Martin and Mazzy are missing.
The truth emerges: Nina is part of the same cult as Martin and is under Hilma’s control. Nina’s parents entrusted her to Hilma so she could be groomed for the group’s cause, including the mission to recruit Ben, whose writings on groupthink and community have given Hilma hope to spread their message. The woman Ben interviewed through the door and the drowned girl from the lake are revealed to be Lotte, whom Martin had previously recruited. Everything—from Ben’s presence at the crime scene to Martin’s chance encounter with Mazzy at the train station—has been orchestrated by Hilma’s circle. The necklace Hilma gave Mazzy is revealed to be a seashell, the emblem of the cult’s planned suicides.
When Martin brings Mazzy to the center after she’s found unconscious, Hilma, Martin, and the others persuade her to “let them help,” then take her to an underground chamber for an initiation ritual. Mazzy resists, begs to call her father, and finally lashes out at Hilma, who orders Martin to take her to the lake and await instruction. Ben’s realization deepens as a line Hilma uttered during an interview echoes in his memory, and he rushes to Max to alert the authorities.
As the group prepares for a mass suicide, Martin abandons Mazzy on the lakeshore and returns to his apartment, where he reveals the living room rug still bears his grandmother’s body, a reminder that life’s ties are unbroken even amid catastrophe. Inside the center, Hilma chastises Nina, who insists she has “always been faithful.” Hilma gives Nina one last test of loyalty, taking away the cyanide pill and pushing her to prove her devotion. The others don ceremonial robes, place the seashells in their mouths, smear ash on their faces, pour gasoline over themselves, and strike matches, setting the center ablaze as Hilma quietly departs.
Mazzy awakens on the lakeshore, dazed and still under the influence of the night’s events. She nearly drowns, but Ben arrives and dives into the water to pull her free. They embrace with tears in their eyes as most of the cult members die, Martin among them, leaving Hilma’s fate somewhat implied as the only survivor who has escaped into the night. The film closes on Elsa, who has found a video of Hilma online and stares at it with the same entranced gaze Mazzy once showed, suggesting the pull of Hilma’s ideas continues to haunt those left behind.
