Undine Wibeau, portrayed by Paula Beer, is a dedicated historian who specializes in the urban development of Berlin. Her life takes a dramatic turn when she is abruptly ended by Johannes, played by Jacob Matschenz, a married man she has been seeing. In a moment of desperation, she warns him that if he leaves her, she will be forced to "kill him," invoking the legend of the undine. She requests he wait for her at a café while she presents her lecture. However, upon returning, she discovers that Johannes has vanished.
In her search for him, she meets Christoph, an industrial diver portrayed by Franz Rogowski, who attended her lecture. Their encounter leads to an unexpected coffee invitation, but a mishap occurs when Christoph accidentally shatters the café's aquarium, resulting in their expulsion. Undine and Christoph quickly develop a deep and passionate bond.
While working in the Lingese, Christoph comes face to face with a massive Wels catfish known fondly as Big Gunther. Their relationship grows deeper as Christoph introduces Undine to the thrill of diving. In a tense moment during one of their dives, Undine drifts too far and nearly drowns, but Christoph bravely rescues her. Before she departs for Berlin, he gifts her a small diver statue, symbolizing their connection.
However, their path takes a dark turn when Christoph visits Undine's apartment in Berlin only to cross paths with Johannes and his new partner, Nora, played by Julia Franz Richter, resulting in an uneasy recognition. Later, Johannes reaches out to Undine, confessing he regrets ending their relationship, but she firmly rejects him.
After a tumultuous evening filled with uncertainty, Undine receives a phone call from Christoph, who inquires about Johannes. When he doubts her claim of not loving Johannes, the conversation leaves her unsettled. That night, she attempts to meet Christoph but learns that he has been involved in a tragic diving accident, leaving him trapped underwater without air. His colleague, Maryam Zaree, informs her of the grave situation.
Desperate for closure, Undine confronts Johannes at his home while he enjoys a swim with Nora. In a shocking twist, she enters the pool and drowns Johannes, immediately submerging herself in water. Meanwhile, Christoph awakens from his ordeal, desperately calling her name.
As time passes, Christoph finds himself in a new life with Monika, played by Anne Ratte-Polle, who is now pregnant. He gets an opportunity to work on the turbines in the Lingese once more, but while diving, he unexpectedly encounters Undine in the river. Though initially unable to capture her image on video, he remains drawn to her presence. That night, he plunges back into the water against Monika's wishes, determined to reclaim the connection that was lost. After embracing the vision of Undine, Christoph returns to Monika, holding the precious statue of the diver, symbolizing the complex intertwining of love, loss, and the unyielding pull of fate.
