In the mid-1800s,[7] a Scotswoman named Ada McGrath who chooses not to speak is sold by her father into marriage with New Zealand frontiersman Alisdair Stewart, along with her daughter Flora. Ada has not spoken since the age of six, and the reason for this as well as the identity of Flora's father remain unknown. She communicates through playing the piano and sign language, with Flora acting as her interpreter.
Ada and Flora, along with their handcrafted piano, are stranded on a New Zealand beach by a ship's crew. The next day, Alisdair arrives with his Māori crew and neighbour George Baines, a retired sailor who's adapted to Maori customs, including facial tattoos. Alisdair initially tells Ada that they don't have enough bearers for the piano and then refuses to go back for it, claiming that they all need to make sacrifices. Ada is determined to be reunited with her piano, and coldly rejects Alisdair's advances. Desperate to retrieve her beloved piano, Ada seeks out George's help. Although he can't read her note, he is entranced by her music and agrees to help her. George offers Alisdair the land he's been coveting in exchange for the piano and Ada's lessons. Alisdair agrees, oblivious to George's attraction to Ada. Ada is enraged by George's proposition, but ultimately agrees to trade lessons for piano keys. She restricts the lessons to the black keys only and resists George's demands for more intimacy. Ada continues to rebuff Alisdair's advances while exploring her sensuality with George. George eventually realizes that Ada will never commit to him emotionally and returns the piano to her, acknowledging that their arrangement has made her a "whore" and him "wretched." George confesses that he wants Ada to care for him genuinely.
Although Ada has her piano back, she still longs for George and returns to him. Alisdair overhears them having sex and even watches them through a crack in the wall. Furious, he follows Ada the next day and confronts her in the forest, where he tries to force himself on her despite her strong resistance. Alisdair ultimately coerces Ada into promising she will no longer see George.
Shortly after, Ada instructs Flora to deliver a package to George, which contains a piano key with a love declaration engraved on it. Flora hesitates, but eventually delivers it to Alisdair instead. Enraged after reading the message, Alisdair cuts off Ada's index finger with an axe, depriving her of the ability to play the piano. He sends Flora to George with the severed finger, warning him to stay away from Ada or he will chop off more fingers. Later, while touching Ada as she sleeps, Alisdair hears what he thinks is her voice in his head, asking him to let George take her away. He goes to George's house and asks if Ada has ever spoken to him, but George assures him she has not. George and Ada leave together from the beach at which Ada first arrived in New Zealand, with her belongings and piano tied onto a Māori longboat. As they row to the ship, Ada asks George to throw the piano overboard. Ada allows her leg to be caught by the rope attached to the piano and is dragged underwater with it, in an attempt to drown herself. As she sinks, she appears to change her mind and struggles free before being pulled to safety.
In the epilogue, Ada describes her new life with George and Flora in Nelson, New Zealand, where she gives piano lessons in their new home. George has made her a metal finger to replace the one she lost, and Ada has been practicing and taking speech lessons. She sometimes dreams of the piano resting at the bottom of the ocean with her still tethered to it.
