Arun (Aamir Khan), a reclusive artist, moves into a run-down flat in the older part of Mumbai. After attending an exhibition of his works, he meets Shai (Monica Dogra), an American banker who has come to Mumbai to devote time to her hobby, photography, and takes her back to his flat, where they have a one-night stand. The next morning, Arun expresses his disinterest in continuing the relationship and Shai angrily storms out.
Shortly after, Munna (Prateik Babbar), a dhobi (washerman) who dreams about getting a break in Bollywood, arrives to deliver Arun's laundry. Arun moves into a new apartment the next day. While unpacking, he finds a small container that had been left by the previous tenant of the apartment. In it, he finds a silver chain, a ring, and three video tapes (Pehli chitti, Doosri chitti and Aakhri chitti) by Yasmin Noor. The tapes turn out to be video diaries in which Yasmin communicates with her brother.
In the tape, she is generally happy, and shows her apartment, which is the same one Arun is occupying now. The next morning, Arun is informed by his manager that art dealers, impressed with his art, have decided to open a gallery in Sydney featuring his works. Arun is pleased by this, as it would also give him a chance to visit his ex-wife and child, settled in Australia. He begins to work on a new painting.
When Munna delivers laundry to Shai's house, they meet and soon become friends. Munna agrees to show Shai around Mumbai and help her gather photographs about the occupations of the poor. In return, Shai agrees to shoot a portfolio of Munna to help him get into the film industry.
Arun continues watching Yasmin's tapes and learns more about her life, including that she has recently married a man who seems distant. Shai obtains Arun's new address from Munna. Shai's father's construction company is constructing a building opposite Arun's apartment. Shai starts shooting photos of Arun from the construction site without Arun being aware of it.
Munna continues struggling to find work in films. To earn a living, he washes clothes by day and kills rats by night. He disapproves of his brother's criminal activity but they are close, and Munna's brother uses his underworld contacts to try to get Munna a break in the industry.
In the meantime, Munna shows Shai the different facets of Mumbai, including the dhobi ghaat, where he works as a launderer. Munna develops feelings for Shai but is unable to express them, mostly because of his working-class status. One day, Arun spots Shai nearby and invites her to his house. He apologises about their first meeting. Munna sees them together and feels betrayed but Shai is able to convince him to keep helping her. One night, while Munna is at his rat-killing job, Shai sees him and takes pictures. Dismayed, Munna flees from her.
Arun watches the final video by Yasmin. Her tone has changed considerably since the previous tape, in which she revealed learning that her husband was in an extramarital affair. She strongly indicates that she intends to commit suicide. Shaken by the knowledge that Yasmin might have hanged herself in that very room, Arun flees the apartment and moves into a new one.
Munna arrives home to find that his brother has been killed. He and his family relocate to a new flat and Munna visits his brother's contact in the film industry to show him his portfolio. Arun studies the painting he has been working on, which is now complete. Yasmin is prominently featured along with other things she had left behind.
Shortly after, Shai finds Munna and Munna again tries to flee her, but she catches up with him. She insists upon their friendship and asks about Arun's whereabouts. Still hurt, Munna lies to her and they exchange an abrupt farewell before Shai leaves. Moments later, Munna changes his mind and runs after Shai's car, weaving through the traffic. The film ends when Munna catches up to the car and gives Shai Arun's new address.