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 t 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".

Late at night on a deserted road, well-known actor Armaan Kapoor (Vivan Bhatena) loses control of his car for no evident reason, apparently trying to avoid something, and drives into the sea. Inspector Surjan Shekhawat (Aamir Khan) with his assistant Devrath Kulkarni (Rajkummar Rao) begins an investigation into Armaan's death. Surjan's personal life is in turmoil since his 8-year-old son, Karan, drowned recently during a family outing. His wife, Roshni (Rani Mukerji), is still grieving and Surjan has developed a guilt complex.

Surjan learns that on the night of Armaan's accident, Armaan was travelling with 2 million rupees in cash, which is missing from the car. Armaan had given the money to a blackmailer, a pimp named Shashi (Subrat Dutta). Three years back, Shashi had performed a cover-up for Armaan and his friend Sanjay Kejriwal (Suhas Ahuja). Once he learns of Armaan's death, Shashi takes off with the cash, leaving behind a mistress, Mallika (Aditi Vasudev). Shashi's friend, Tehmur (Nawazuddin Siddiqui), steals Shashi's discarded sim card. Shashi begins blackmailing Sanjay. In panic, Sanjay employs some thugs to murder Shashi. Tehmur begins to use Shashi's old sim card to blackmail Sanjay with Shashi's murder.

Meanwhile, Roshni seeks solace from a medium (Shernaz Patel) who is in touch with Karan's soul. One night, Surjan is propositioned by an escort, Rosie (Kareena Kapoor). He declines Rosie's advances but asks her if she can become an informer. Through Rosie, Surjan finds out about Shashi's connection with the case. The police find Shashi's body, along with the blackmail money and a DVD. The DVD contains embarrassing CCTV footage of Armaan and his friends leaving a hotel with a girl. Convinced of Armaan's link with prostitutes, Surjan focuses his search on the slums near the Hotel Lido. To speak unobserved, Rosie takes Surjan to a quiet place by the riverside and tells him that three years ago, she was picked up by Armaan and his friends, Sanjay and Nikhil (Prashant Prakash). However, she doesn't reveal anything more. Surjan tries to approach Nikhil for more information but discovers that Nikhil has been brain-dead after the incident three years ago. After a heated argument with Roshni, Surjan confides in Rosie, who comforts him by taking him to the Lido.

Tehmur is killed by the two thugs in Sanjay's employ. One of the thugs is captured by the police and reveals that Sanjay has been behind the murders of both Shashi and Tehmur. Surjan arrests Sanjay, who confesses that three years ago, Sanjay, Armaan and Nikhil had picked up an escort to celebrate Nikhil's birthday. While driving, Nikhil began making out with the girl in the backseat. The car door opened by accident, throwing both Nikhil and the girl from the moving car to the road. Both were badly injured. Armaan and Sanjay rushed Nikhil to the hospital but fearing scandal, left the girl unattended on the road. Sanjay had called Shashi to ask him to take care of her. However, Shashi informed them that the girl had died and that he had footage of the three leaving with her. Since then, Shashi had continued to extort money from them by blackmail.

Surjan re-watches the footage and realizes that the girl is Rosie. As he drives Sanjay to the police station, they approach the same road where Armaan had his car accident. Both Surjan and Sanjay see Rosie standing ahead. In a re-enactment of Armaan's accident, Sanjay swerves the car into the river. With Sanjay already dead, Surjan struggles to get himself out of the car. Rosie appears underwater and saves him. At the police station, Surjan's superior advises him against reporting his incredible story. Surjan agrees to write in the official report that Armaan Kapoor's death was an accident. He returns to the riverside place where he used to meet Rosie and uncovers her remains where Shashi had hidden her body. Remembering that he had been told that "the dead come to those who are troubled," he gives her a proper funeral.

Now more accepting of the supernatural, Surjan finds a letter from Karan, penned by the medium. Through the letter, Karan tells Surjan not to blame himself for his death and that he wants both his parents to be happy. Surjan cries and Roshni consoles him. The two are reconciled.