In the mid-1800s, Ada McGrath, a Scottish woman who suffers from elective mutism, embarks on a journey to colonial New Zealand with her daughter, Flora. Their voyage is for an arranged marriage to settler Alisdair Stewart. Since the age of six, Ada has chosen not to speak, and the mysteries surrounding her silence and Flora's father's identity linger in the air. Instead of verbal communication, Ada expresses herself through the piano and sign language, with Flora acting as her devoted interpreter.
As fate would have it, Ada, Flora, and their beloved piano find themselves stranded on a New Zealand beach due to a ship's crew mishap. The following day, Alisdair arrives, accompanied by his Māori crew and his neighbor, George Baines—a retired sailor who has embraced Māori customs, evidenced by his facial tattoos. Alisdair informs Ada that the number of bearers is insufficient to carry the piano, insisting that sacrifices must be made. Unable to part with her piano, Ada turns to George for assistance. Initially unwilling, he ultimately relents, seemingly captivated by Ada's passion for her music and her vibrant spirit, which starkly contrasts with her more serious demeanor at the farm.
In a bold move, George proposes to Alisdair to exchange land for the piano and lessons from Ada. Alisdair, oblivious to George’s deepening feelings for Ada, agrees to the deal. Although Ada is furious at George's proposition, she cautiously consents to teach him to play the piano, limiting their lessons to the black keys only, while also resisting his deeper romantic advances. As she pushes away Alisdair's persistent advances, Ada explores her own sensuality with George. Realizing that a real emotional connection with Ada is unlikely, George eventually returns the piano to her, revealing that he desires her genuine affection.
Despite retrieving her piano, Ada finds herself yearning for George and decides to return to him. In a moment of betrayal, Alisdair overhears their intimate encounter and confronts Ada angrily, attempting to impose himself on her despite her determined resistance, and forcing her to vow that she will cease seeing George.
In a desperate act, Ada instructs Flora to deliver a package to George containing a piano key carved with a love note. However, Flora inadvertently gives it to Alisdair instead. Consumed by rage after reading the note, Alisdair retaliates violently, severing Ada's index finger, thus robbing her of the ability to play. He then sends Flora to deliver the gruesome message to George, warning him to stay away or face further consequences.
Later, as Alisdair touches a sleeping Ada, he imagines hearing her voice urging him to let George have her. Jealous and tormented, Alisdair questions George about whether Ada has ever spoken to him. When George denies this, he and Ada escape together, loading her belongings and piano onto a Māori canoe. As they navigate towards their ship, Ada suddenly asks George to throw the piano overboard. In a moment of despair, she allows the rope tethered to the piano to ensnare her leg, dragging her underwater in a desperate attempt to end her suffering. Just as she succumbs to the depths, a change of heart prompts her struggle for freedom as she is ultimately pulled to safety.
In the epilogue, Ada shares insights into her new life alongside George and Flora in Nelson, New Zealand. She teaches piano in their new home, where George has crafted a metal finger in place of her lost one. Amid her journey of healing, she practices and takes speech lessons, often haunted by dreams of her piano resting at the ocean's bottom, with her still bound to it.
