Saajan is a lonely accountant about to retire from his job. Ila is a young wife seeking her husband's attention, looking for ways to put romance back in her marriage, and tries to cook her way to her husband's heart. A neighbour who lives in the apartment above her gives her advice on cooking and marriage through shouted instructions. Through a rare mixup of the famous "dabbawalas" (a complicated system that picks up and delivers lunches from restaurants or homes to men at work) of Mumbai, the lunchbox with the delicious food Ila prepared for her husband instead gets delivered to widower Saajan. When the lunch containers are delivered back to Ila empty, she is happy, thinking her husband had enjoyed her cooking. When she questioned him, he did not seem to think much of it. Realizing the mistake of the delivery, she writes a note to the recipient of her lunch thanking him for enjoying the food and sends him her husband's favorite meal the following day.

Saajan replies saying that the food was salty. Upon advice from her neighbour, who believes the lunch went to Ila's husband, she responds by using hot chili in a lunch that turns out to be so spicy that Saajan resorts to eating bananas to help cool off his mouth. An exchange of the messages sent back and forth with the lunches ignites a friendship between Saajan and Ila who share memories and thoughts in notes passed back and forth in the lunch pails. Meanwhile Saajan deals with a young employee, Shaikh, he is supposed to train to replace him when he retires. Shaikh appears immature, annoying, lazy and irresponsible. On her end, Ila feels her husband is very remote. He not only complains about her food (which is delivered to him from the restaurant Saajan has ordered his lunches from), but she finds out he is having an affair. Also she supports her mother as her father suffers from lung cancer.

As Saajan and Shaikh get to know each other, Shaikh reveals that he is an orphan and has worked in several places, including Saudi Arabia. He is currently living with his girlfriend. As he rides the train home each day after work, he chops vegetables for their dinner on his portfolio. As Saajan trains Shaikh, he has told him what to do and left him on his own, but Shaikh had bluffed his way into the job and many mistakes come to the attention of the boss who calls both of them into his office, puzzled that, after decades of service with never a mistake, so much had gone wrong. He chastises Saajan but tells Shaikh to leave. Saajan takes the blame for all that has gone wrong and saves Shaikh's job. This act moves Saajan and Shaikh to have a true friendship prompting Shaikh to invite Saajan to dinner at his home with his girlfriend and asking him to be his best man in his wedding, since he has no one else to stand up for him and no family.

In their correspondence through the lunch containers, Ila mentions that Bhutan is a poor but happy and beautiful place where the cost of living is much less than India. Saajan contemplates moving there with her upon his retirement and writes back asking if she will be ready to move to Bhutan with him. Ila writes back saying they should meet and suggests a popular food joint. She is there at the appointed time but Saajan doesn't turn up. A dejected Ila sends an empty lunchbox the next day. Saajan writes to her saying that he did arrive, but did not meet her because he saw how young and beautiful she is and considered himself too old for her and advises her to move on.

Ila's father dies and as his body is taken away, Ila's mother confesses to her that she has been unhappy for most of her marriage. This helps Ila to feel released. She decides to search for Saajan by asking the address where the dabbawalas sent her food and goes to Saajan's office to meet him, but there she learns that he has retired and headed to Nashik. Ila writes a last message saying farewell to Saajan announcing that she will leave her husband and go to Bhutan.

The movie comes to an end with Saajan returning from Nashik and going in search of Ila with the Dabbawalas song being played in the background.

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.