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.

Rahul (Sharman Joshi) is a young man with dreams, working as a call center executive in a BPO in Mumbai. He silently loves Neha (Kangna Ranaut), a smart young woman who has made it up the ranks in a very short time, due to her relationship with her boss, Ranjeet (Kay Kay Menon).

In his 30s, Ranjeet is married to Shikha (Shilpa Shetty) and has a six-year-old daughter. Shikha is well educated, and a trained dancer. Ranjeet and Shikha decided that she should stay back at home and take care of the house and their child. Their eight-year marriage has turned loveless and bitter.

Neglected by an indifferent husband and bogged down by family chores, Shikha is attracted to maverick Akash (Shiney Ahuja). Akash is a struggling theater artist who is separated from his wife. Akash and Shikha's relationship starts as friendship and almost blossoms romantically, until Shikha feels guilty and cannot cross the limits that she has so fervently guarded all these years.

Amol (Dharmendra) is a 70-year-old man who has returned to India after 40 years to spend his last few years with his first love, Shivani (Nafisa Ali), who is also Shikha's former dance teacher.

Shikha's sister and Neha's roommate Shruti (Konkona Sen Sharma) works in at Radio Mirchi as a producer. She is 28 and desperate to get married. She meets Monty (Irrfan Khan, in an award-winning comedy role) through a matrimonial site, but she is unimpressed by the straightforward and socially inept Monty. While Monty likes her and is ready to marry her, she declines when she finds him ogling her. She is, however, dreamy-eyed about her RJ, Rishi K. Her boss hooks her up with Rishi K, and her relationship with him blossoms.

Rahul has a novel strategy to rise up the ladder of success. He provides his flat to his superiors at the office, where they can safely bring their mistresses, girlfriends or prostitutes, in return for recommendation for his promotion. Ranjeet also uses the flat to meet Neha.

Amidst one of their escapades in Rahul's flat, Neha asks Ranjeet about their future, which leads to an argument. He insults her and leaves. Indignant, she attempts suicide. Rahul finds her lying unconscious, and saves her. He brings her back to his house, which brings them closer.

Shruti is heartbroken when she discovers that Rishi K is gay. She leaves the job and gets a new one where she again meets Monty. Slowly, they become friends. Meanwhile, Shruti comes to know about Ranjeet and Neha. Ranjeet panics and confesses to Shikha of his two-year affair with Neha. An already guilt-stricken Shikha cannot hold it in any longer and tells him about her emotional relationship with Akash. Ranjeet is shocked and angry, and decides to leave his wife.

Shivani dies. Amol regrets the decision to leave Shivani 40 years back, stressing the importance of love to Shikha and Shruti. Shruti realizes that she loves Monty and tells him so just as he is about to go for his wedding. Meanwhile Akash decides to leave the country and asks Shikha to join him. On the other hand, Rahul is disillusioned with the way the metro has treated him and decides to resign and leave the city. Neha realizes that Rahul really loves her and follows him to the station. Ranjeet returns to Shikha, who is just about to leave for the station to see Akash.

Neha chases Rahul. Monty chases Shruti. Shikha comes to the station but not to accompany Akash. She wishes him good luck, tells him that she will miss him.

While the credits roll, we are shown Akash still roaming the streets of Mumbai, Shikha and Ranjeet are in a happier marriage, and Rahul is living happily with Neha. Shruti and Monty are also happily married and have a kid.

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