A young nurse, Alma, is put in charge of Elisabeth Vogler: an actress who is seemingly healthy in all respects, but will not talk. As they spend time together, Alma speaks to Elisabeth constantly, never receiving any answer. Alma eventually confesses her secrets to a seemingly sympathetic Elisabeth and finds that her own personality is being submerged into Elisabeth's persona.
Swedish actress Elisabet Vogler, while on stage during a performance of Elektra, becomes mute. The day following, she is in a near catatonic state. The doctors in charge of her case diagnose no medical reason that would cause these disabilities. A nurse named Alma is assigned to be Elisabet's caretaker, she who believes that there is some sort of inner strength that Elisabet possesses to choose not to speak. Since Elisabet is not allowed visits by either her husband or her son, the doctor provides her own secluded beach side summer house as a respite for Elisabet and Alma. To break the silence during the respite, Alma speaks continually to Elisabet, ultimately about personal details of her life. Elisabet responds to Alma's talking, and Alma uses these situations as her own therapy sessions. Alma admires Elisabet and wants to be like her. As the women spend more time alone together, Alma learns more about her patient. But Alma also begins a transformation, which she now knowingly tries to stop because of those things she has learned about Elisabet.
A nurse is put in charge of a mute actress and finds that their personas are melding together.
- Persona begins with images of camera equipment and projectors lighting up and projecting dozens of brief cinematic glimpses, including a crucifixion, an erect penis, a tarantula spider, clips from a comedic silent-film reel first seen in Bergman's Prison (depicting a man trapped in a room, being chased by Death and Satan), and the slaughter of a lamb. The last, and longest, glimpse features a boy who wakes up in a hospital next to several corpses, reading Michail Lermontov's A Hero of Our Time ("Vår Tids Hjälte" in the film), and caressing a blurry image of Elisabet and/or Alma's faces.
A young nurse, Alma (portrayed by Bibi Andersson), is summoned by the head doctor and charged with the care of stage actress Elisabet Vogler (portrayed by Liv Ullmann), who has, despite the lack of any diagnosed impairment, become mute. The hospital administrator (portrayed by Margaretha Krook) offers her own seaside cottage as a place for Alma to nurse Elisabet back to health. Though Elisabet is nearly catatonic when the film begins, she does react with extreme panic upon seeing a Vietnamese Buddhist monk's self-immolation on television, and laughs mockingly at Alma's radio soap opera. As the two women leave the hospital together, Alma reads aloud a letter Elisabet's husband has sent her, which includes a photograph of her young son.
Together in the administrator's cottage, Elisabet begins to relax, though she remains completely silent and non-responsive. Alma speaks constantly to break the silence, at first about books she is reading and trivial matters, then increasingly about her own anxieties and relationship with her fiancé, Karl-Henrik, who scolds her for lacking ambition "though not with my career, I suppose in some greater way." Alma constantly compares herself to Elisabet and begins to grow attached to her. As the act closes, Alma confesses to cheating on her fiancé in a ménage à quatre with underage boys. She became pregnant, and had Karl-Henrik's friend abort the baby; "and that was that". She is not sure how to process the abortion mentally. Elisabet is heard to say "You ought to go to bed, or you'll fall asleep at the table", but Alma dismisses it as a dream. Elisabet later denies speaking.
Alma drives into town, taking Elisabet's letters for the postbox, but parks by the roadside to read what she wrote. She discovers in Elisabet's letters that Elisabet has been analyzing her and "studying" her. Alma returns distraught, accidentally breaks a drinking glass on the footpath, and leaves the shards there to cut Elisabet. When Elisabet's feet start to bleed, her gaze meets Alma's knowingly, and the film itself breaks apart: the screen flashes white, scratch marks appear up and down the image, the sound rises and screeches, and the film appears to unwind as brief flashes of the prelude reappear for fractions of a second each.
When the film resumes, it is following Elisabet through the house with a thick blur on the lens. The image clears up with a sharp snap when she looks out the window before walking outside to meet Alma, who is weepy and bitter. At lunch, she tells Elisabet she has been hurt by Elisabet talking about her behind her back, and begs her to speak. When Elisabet does not react, the nurse flies into a rage. Alma tries to attack her and chases her through the cottage, but Elisabet hits her during the ensuing scuffle causing Alma's nose to start bleeding. In retaliation, Alma grabs a pot of boiling water off the stove and is about to fling it at Elisabet, but stops after hearing Elisabet wail "No!" Alma explains that Elisabet wouldn't have spoken had she not feared death. Alma goes to the bathroom, washes her face, and tries to pull herself together. She then goes to Elisabet and frustrated by her unresponsiveness tells her, "You are inaccessible. They said you were healthy, but your sickness is of the worst kind: it makes you seem healthy. You act it so well everyone believes it, everyone except me, because I know how rotten you are inside." Elisabet tries to walk away, but Alma pursues and continues to accost her. Elisabet flees, and Alma chases her begging for forgiveness. That evening, Elisabet opens a book she is reading and finds a famous Stroop Report photograph of Jews being arrested in the Warsaw Ghetto. Elisabet stares at details in the photograph, but mostly at the boy with his hands raised.
That night, Alma watches Elisabet sleep, analyzing her face and the scars she covers with makeup. She hears a man yelling outside, and finds Elisabet's husband, Mr. Vogler, in the garden. Mr. Vogler (portrayed by Gunnar Björnstrand) mistakes Alma for his wife, and despite her repeatedly interjecting with "I'm not your wife", delivers a monologue about his love for her and the son they have together (repeating words he wrote to Elisabet in the opening act "We must see each other as two anxious children"). Elisabet stands quietly beside the two, holding Alma's hand, and Alma admits her love for Mr. Vogler and accepts her role as the mother of Elisabet's child. The two make love with Elisabet sitting quietly next to the bed with a look of panic on her face, and afterward, Alma cries. The image of Elisabet becomes blurry.
The climax of the film comes the next morning; Alma catches Elisabet in the kitchen with a pained expression on her face, holding a picture of a small boy. Alma then narrates Elisabet's life story back to her, while the camera focuses tightly on Elisabet's anguished face: at a party one night, a man tells her "Elisabet, you have it virtually all in your armory as woman and artist. But you lack motherliness." She laughs, because it sounds silly, but the idea sticks in her mind, and she lets her husband impregnate her. As the pregnancy progresses, she grows increasingly worried about her stretching and swelling body, her responsibility to her child, the pain of birth, and the idea of abandoning her career. Everyone Elisabet knows constantly says "Isn't she beautiful? She has never been so beautiful", but Elisabet makes repeated attempts to abort the fetus. After the child is born, she is repulsed by it, and prays for the death of her son. The child grows up tormented and desperate for affection. The camera turns to show Alma's face, and she repeats the same monologue again. At its conclusion, one half of the face of Alma and the other of Elisabet's visage are shown in split screen, such that they appear to have become one face. Alma panics and cries "I'm not like you. I don't feel like you. I'm not Elisabet Vogler: you are Elisabet Vogler. I'm just here to help you!" Alma leaves, and later returns, to find that Elisabet has become completely catatonic. Alma falls into a strange mood and gashes her arm, forcing Elisabet's lips to the wound and subsequently beating her. Alma packs her things and leaves the cottage alone, as the camera turns away from the women to show the crew and director filming the scene.