A mute Scotswoman named Ada McGrath is sold by her father into marriage to a New Zealand frontiersman named Alisdair Stewart, bringing her young daughter Flora with her. The voice that the audience hears in the opening narration is "not her speaking voice, but her mind's voice". Ada has not spoken a word since she was six years old and no one, including herself, knows why. She expresses herself through her piano playing and through sign language, for which her daughter has served as the interpreter. Flora later dramatically tells two women in New Zealand that her mother has not spoken since the death of her husband who died as a result of being struck by lightning. Ada cares little for the mundane world, occupying herself for hours every day with the piano. Flora, it is later learned, is the product of a relationship with a teacher with whom Ada believed she could communicate through her mind, but who "became frightened and stopped listening," and thus left her.
Ada, Flora, and their belongings, including a hand crafted piano, are deposited on a New Zealand beach by a ship's crew. As there is no one there to meet them, they spend the night alone on the beach amongst their crated belongings. The following day, the husband who has bought her, Alisdair, arrives with a Māori crew and his white friend, Baines, a fellow forester and retired sailor who has adopted many of the Maori customs, including tattooing his face. Alisdair proves to be a shy and diffident man, who is jokingly called "old dry balls" by his Māori neighbors. He tells Ada that there is no room in his small house for the piano and abandons the piano on the beach. Ada, in turn, is cold to him and is determined to be reunited with her piano.
Unable to communicate with Alisdair, Ada and Flora visit Baines with a note asking to be taken to the piano. He explains that he cannot read. When Flora translates her mother's wishes, he initially refuses, but the three ultimately spend the day on the beach with Ada playing music. Baines, whose wife is far away in England living a separate life, is taken by the transformation in Ada when she plays her piano. Baines soon suggests that Alisdair trade the instrument to him for some land. Alisdair consents, and agrees to his further request to receive lessons from Ada, oblivious to his attraction to her.
Ada is enraged when she learns that Alisdair has traded away her precious piano without consulting her and complains that she does not want a man with filthy hands and no ability to read touching her piano. Alisdair shouts the finality of his decision and demands that she fulfill the contract of providing lessons. On the day she arrives at his hut, she attempts to make an excuse that she can't play the piano because it is out of tune. She is stunned to find that Baines has had the piano put into perfect tune. She begins by asking him to play anything he knows, but he asks to simply listen rather than learn to play himself. It becomes clear that he procured the piano not for his own interest in music, but because he likes who Ada becomes when she plays. During one session, Baines proposes that Ada can earn her piano back at a rate of one piano key per "lesson," provided that he can observe her and do "things he likes" while she plays. She is not anxious to accept the deal, but cannot turn down the opportunity to regain her piano. She agrees, but negotiates for a number of lessons equal to the number of black keys only.
While Ada and her husband Alisdair have had no sexual, nor even mildly affectionate, interaction, the lessons with Baines become a slow seduction for her affection. Baines requests gradually increased intimacy in exchange for greater numbers of keys. Ada reluctantly accepts but does not give herself to him the way he desires. Realizing that she only does what she has to in order to regain the piano, and that she has no romantic feelings for him, Baines gives up and simply returns the piano to Ada, saying that their arrangement "is making you a whore, and me wretched," and that what he really wants is for her to actually care for him.
Despite Ada having her piano back, she ultimately finds herself missing Baines watching her as she plays. She returns to him one afternoon, where they submit to their desire for one another. Alisdair, having become suspicious of their relationship, hears them making love as he walks by Baines' house, and then watches them through a crack in the wall. Outraged, he follows her the next day and confronts her in the forest, where he attempts to force himself on her, despite her intense resistance. He then boards up his home with Ada inside so she won't be able to visit Baines while Alisdair is working on his timberland. After this, Ada realizes she must show affection with Alisdair if she is ever to be released from her home prison, though her caresses only serve to frustrate him more because when he tries to touch her, she pulls away. Eventually resolving to trust her, he removes the barriers from the house, and exacts a promise from Ada that she won't see Baines.
Soon afterwards, Ada sends her daughter with a package for Baines, containing a single piano key with an inscribed love declaration reading "Dear George you have my heart Ada McGrath". Flora does not want to deliver the package and brings the piano key instead to Alisdair. After reading the love note burnt onto the piano key, Alisdair furiously returns home with an axe and cuts off Ada's index finger to deprive her of the ability to play the piano. He then sends Flora to Baines with the severed finger wrapped in cloth, with the message that if Baines ever attempts to see Ada again, he will chop off more fingers.
Later that night, while touching Ada in her sleep, Alisdair hears what he believes to be Ada's voice inside of his head, asking him to let Baines take her away. Deeply shaken, he goes to Baines' house and asks if she has ever spoken words to him. Baines assures him she has not. Ultimately, he decides to send Ada and Flora away with Baines and dissolve their marriage once she has recovered from her injuries. They depart from the same beach on which she first landed in New Zealand. While being rowed to the ship with her baggage and Ada's piano tied onto a Māori longboat, Ada asks Baines to throw the piano overboard. As it sinks, she deliberately tangles her foot in the rope trailing after it. She is pulled overboard but, deep underwater, changes her mind and kicks free and is pulled to safety.
In an epilogue, Ada describes her new life with Baines and Flora in Nelson, where she has started to give piano lessons in their new home, and her severed finger has been replaced with a silver finger made by Baines. Ada says that she imagines her piano in its grave in the sea, and herself suspended above it, which "lulls me to sleep." Ada has also started to take speech lessons in order to learn how to speak again. The story closes with the Thomas Hood quotation, from his poem "Silence," which also opened the film: "There is a silence where hath been no sound. There is a silence where no sound may be in the cold grave under the deep deep sea."