The film begins with Michelle McNally (Rani Mukerji), a blind and deaf woman, visiting her former teacher Debraj Sahai (Amitabh Bachchan), who now has Alzheimer's disease, in a hospital. The film then flashbacks to the past to her childhood.
Michelle was a girl who lost her eyesight and hearing after recovering from an illness at the age of two and exists in a black world where she is isolated in the darkness of her own existence, trapped by her inability to see, hear and express. She grows up becoming more and more frustrated by the black and dark world around her, making her a violent, uncontrollable eight-year-old child. Her parents, Paul and Catherine, are at their wits' end trying to control her, until one day the light shines through the end of the tunnel.
Debraj Sahai enters their lives, an elderly alcoholic teacher for the deaf and blind, Debraj sees himself as a magician and is a disillusioned eccentric man. He takes it upon himself to bring young Michelle into the light. He uses harsh methods, although always for Michelle's long-term benefit. Initially, his methods are met with disapproval by Michelle's father, who asks Debraj to leave. Although her father thinks he is gone, Debraj stays as the teacher while her father is away on business for 20 days. Her mother grudgingly approves of his continuation given her fear of Michelle being sent to an asylum. By the 20th day, Debraj manages to teach Michelle some words and better manners, but he has difficulty teaching her the meaning of words. When Michelle's father returns, Debraj packs his bags. At the very last moment, as he is walking towards the door suitcase in hand, Debraj gets frustrated with Michelle's continuing insolent behaviour and he throws her into a fountain full of water. Michelle suddenly takes to Debraj's lessons at that moment and begins to understand meaning. She is able to recognize her mother and father, and can vocalize the first syllables of some words small enough for her to understand. The McNallys decide to keep Debraj as Michelle's teacher.
Many years later, Michelle has learned a great deal, becoming relaxed and expressive, and she is even able to dance and expertly sign. Debraj convinces the Principal of a university to grant her an interview which she passes and gets admission to pursue a Bachelor of Arts degree, the first deaf-blind person to do so at that university. Michelle moves away from home and lives with Debraj and one of her servants. Over the next 2 years, she struggles to gain her Bachelor of Arts degree, failing year after year, but she still maintains her spirit. One reason is that she must rely on Debraj completely for interpretation of the material and studying. This problem is overcome by the Principal of the school preparing the whole first-year Arts course of study in Braille. Another reason for her continuing failure is that her typing skills are lacking and typing is her only way to write down what she knows during exams. However, she also soon overcomes this deficiency through another flash of increased competence when she almost quits the university and gets into an argument with Debraj.
At the same time as Michelle suffers through and triumphs over her challenges, we also witness numerous other changes. Debraj begins to succumb to Alzheimer's, first forgetting the way out of the Principal's office and then forgetting Michelle and leaving her stranded during an ice cream celebration for her improved typing. We also see Michelle reconcile with her sister Sara, who we find out was jealous of her parents' affection for Michelle throughout her life. After attending Sara's wedding, Michelle begins to wonder about love, which she has not experienced, and she even asks Debraj to kiss her on the lips. Debraj reluctantly does so but decides to leave Michelle on her own because of this demand and the position she has put him in. Twelve years after enrolment, Michelle does manage to gain her BA, and with her proud parents looking on her, she even gives a speech to the graduating class. Wearing no black graduation robe, she thanks her parents and her teacher and she announces that she will only wear the robe so that her teacher may see her first.
Debraj is in a mental hospital due to his almost complete inability to remember his past and even how to speak. Michelle visits him wearing her robe and we see glimmers of memory return as Debraj realises that she has graduated and even does a victory dance. As the window opens to the rain outside, we see Debraj's hand in Michelle's reaching into the rain, and we hear the teacher-student pair say the first syllable of the word "water", with echoes of the scene in which Michelle first begins understanding the meaning of words earlier in the movie. However this time, it is Debraj who is beginning to learn to speak and understand.
The movie ends with a scene of Michelle amongst a crowd of people all dressed in black carrying candles walking towards a church. There is a voiceover of a letter Michelle has written to Debraj's friend Mrs Nair explaining that today was her teacher's first day of school, and that like hers, his alphabet began with "B L A C K".