Plot

Saajan Fernandez has been grieving since his wife died. A woman named Ila (Nimrat Kaur) wants to seek her husband Rajeev's (Nakul Vaid) attention to rekindle their love and the only way she'll do it is cooking. This woman wants to cook her way through her husband's heart. Saajan and Ila live in the bustling city of Mumbai.

Through a rare mix-up of the Dabbawallas (complicated food delivery system in Mumbai that picks up and delivers lunches from restaurants or homes to people at work), the Tiffin carrier (lunchbox) Ila prepares for her husband gets accidentally delivered, instead, to Saajan Fernandes (Irrfan Khan), a middle-aged widower who is about to retire from his job of an accountant, within a month. His company has already assigned him his replacement to train. Saajan worked at the office for 35 years. Saajan realizes at lunchtime that the lunchbox is not his, and yet proceeds to relish the delicious food inside. Saajan finishes the contents of the box and post lunch the box is picked up by Dabbawallas and returned to Ila.

Ila gets the box and finds it empty. Ila is very happy that the box has been licked clean, indicating that Rajeev loved the food. After expecting some praise from her husband, Ila realizes that he didn't eat her lunch at all, & probably has not been doing so for a while. Rajeev says that he loved the Cauliflower, but Ila never served that. She had made lady finger.

Ila eventually realizes the mistake and with the advice of her neighbor aunt, Mrs. Deshpande (Bharti Achrekar), living in the apartment above her, writes a letter to Saajan about the mix-up and places it in the lunchbox (along with her husband's favorite meal) the next day. An exchange of the messages sent back and forth in the letters with the lunches ignites a friendship between the two, as they share memories and events of their own individual lives.

The next day Ila sends Saajan a note thanking him for the empty lunchbox & letting him know that the box was intended for her unappreciative husband. Saajan responds to her letter, thanks her for the food and even offers her tips on how to balance the salts and chilies. Mrs. Deshpande is offended that the person did not even have the courtesy to thank Ila for the food and was instead complaining. Ila continues to send lunch to Saajan & doesn't tell the Dabbawallas that they are delivering the box to the wrong address. Meanwhile Ila's husband is actually receiving Saajan's lunchbox from his caterer. The next day, Ila sends food that is laced with the spiciest chilies. Saajan writes back that the chilly was on the higher side, but he ate 2 bananas to balance the fire in his mouth.

At work, Saajan is tasked with training his replacement, Aslam Shaikh (Nawazuddin Siddiqui). Shaikh used to head accounting for a firm in Saudi Arabia and comes with very good references. Socially distant after his wife's death, Saajan is initially reluctant to interact with Shaikh and train him. Shaikh is very persistent and continues to pester Saajan to start his training. After Shaikh reveals that he is an orphan who taught himself accounting, Saajan gradually warms up to him, and eventually the duo strike a close friendship. At one point, Saajan saves Shaikh's job by covering for his blatant mistakes and becomes the best man at his marriage with Mehrunissa (Shruti Bapna).

Ila tells Saajan about Mrs. Deshpande and says that her husband was in coma for 15 years, but then one day he came out of it, and now only stares at the ceiling fan. She says that it seems that uncle's life is linked to the fan. His heartbeat almost stopped one day when the power was cut, but before he died, the power came back on, and he was revived. Since then, Mrs. Deshpande has a generator at home. Saajan says that uncle is better off in bed as the world has changed. His wife got a horizontal burial plot, but when he went to buy a burial plot for himself, he was offered only a vertical one. Saajan is distraught that he spent all his life standing in trains and buses and now would have to do that after his death too.

Ila shares her marital woes with Saajan via letters and says that her husband is not interested in her. Saajan advises her to have another kid. Ila's husband completely ignores her when she propositions him. Ila and Saajan continue to correspond & share personal details with each other. Saajan asks Ila not to be suicidal & in turn Ila asks Saajan to quit smoking. Ila discovers that Rajeev is having an extramarital affair and gives up hope of rekindling her marriage.

In one of the lunchbox letters, she suggests moving to Bhutan where the cost of living is much cheaper than in India. Saajan writes back with the suggestion that the two move there together. Ila then offers to meet in person at a popular restaurant but at the appointed time, Saajan does not show up.

Upon receiving an empty lunchbox in disappointment the next day, Saajan writes back to the dejected Ila and apologizes to her, stating that he did arrive and watched her from a distance, but could not approach her. He explains how young and beautiful she looked, while surmising that he is too old for her and advising her to move on.

Sometime later, Ila's father, battling with lung cancer, dies in the care of her mother (Lillete Dubey), who confesses how unhappy her marriage was. She gives Ila the advice "Sometimes, the wrong train takes us to the right station". Ila is angry & figures out Saajan's office address & comes to meet him. She finds that Saajan has finally taken his early retirement & relocated to Nasik. Ila is devastated. She decides to relocate to Bhutan by selling off her jewels with her young daughter, Yashvi.

Meanwhile Saajan decides to come back to Mumbai & decides to meet Ila. He traces her address through the Dabbawallas & is headed to her home the same afternoon when Ila is heading out the door to her Bhutan. The film ends with Ila waiting for Yashvi to return from school and Saajan heading to her house with the Dabbawallas who regularly picked up and delivered the eponymous lunchbox.