A shy woman moves into her new apartment in Paris, helped by her sister Charlotte. She longs to be a professional pianist and has just moved out of the house of her piano teacher, Evguenie. On her first night, she hears strange noises and a picture on her wall begins to move. Fearing a supernatural presence, she flees to Evguenie's house. As a teacher, Evguenie is harsh and critical, encouraging poise and technical perfection over emotion. She returns to her flat the next day and realises that the noises are caused by her neighbour — a quiet and reclusive man, who barely ventures outside and whose only friend is the slovenly, kind Artus. He explains that the wall separating their apartments is hollow and that any noise made in either place can be heard. Not wanting to be disturbed, he moves the picture to scare new tenants away from the flat, thus leaving him in peace. The woman refuses to be driven away by his behaviour. The two live side-by-side, irritating one another with increasingly inventive noisy behaviour. Eventually, their mutual wind-up ends when the woman amplifies a metronome, and the man cannot bear it any more. When she is practising Chopin's Revolutionary Etude, the man finally speaks to her. He says that her playing lacks emotion, encourages her to let go of her self-consciousness, and to play the piece with real feeling. She follows his instructions, and the two become friends, talking to one another from either side of the wall. The man nicknames her 'Machine' and the woman nicknames him 'Machin'. They become fond of one another but decide that they should never meet face-to-face. However, they decide to 'date' — even 'introducing' each other to Artus and Charlotte from either side of the wall. In preparation for a competition, Machine invites Evguenie to her home for a final lesson. She explains to Machin that Evguenie can be unkind and asks him not to intervene. However, Machin, angered by the way that Evguenie bullies and belittles Machine, ejects Evguenie from the house. Machine is furious and decides that their relationship cannot continue. Machin asks to see her in person, but she refuses. Machin leaves the apartment and talks to Artus about his feelings for her. Artus encourages him to see her in person and suggests that he go to her audition to support her. The day of the audition arrives, and Charlotte and Artus — unbeknownst to one another — sit in the auditorium to listen. Machine begins to play a piece by Mendelssohn, but there is a disturbance from behind the wall at the back of the stage. It is revealed that Machin was listening from behind the wall, but he has been discovered by an employee, causing the commotion. The judges, unimpressed by her wooden performance, do not want to give her a chance to start again. Realising that Machin has ventured outside to hear her playing, Machine insists. She plays the Chopin instead, with great emotion. This performance impresses the judges. After the performance, Artus and Charlotte realise the other's identities and are immediately attracted to one another. Evguenie comments disdainfully on her performance. Angered by his continued negativity, Machine head-butts him. She returns to her apartment and confesses to Machin that she wished that she could have seen him after the performance — and that she wishes they could be together in reality. To her surprise, Machin knocks through the wall to her apartment, and they finally see each other for the first time.
