Siddharth "Sid" Mehra (Ranbir Kapoor) is the spoiled, carefree son of wealthy businessman Ram Mehra (Anupam Kher). A resident of Bombay, his only interest is to have fun with friends and to spend his father's money. Thus, he spends little time at college and is not prepared for the exit exams. His father however, not being aware of this fact, tells Sid it is time to join his company, which Sid reluctantly does only after his father tempts him with the promise of a new car.
Meanwhile, Sid meets Aisha (Konkona Sen Sharma) a woman from Calcutta, who is an aspiring writer. On their first meeting, they go out on a walk along the streets of Bombay. Sid shows her the city and takes her to Marine Drive by the sea. Sid tells her that one of the fascinating things of being in Bombay is its luscious monsoon. They seem to strike a chord instantly and a friendship ensues. Sid helps her to find (and furnish) her own apartment. She also lands a job with Mumbai Beat, (based on Time Out! Mumbai) a hip magazine run by the handsome, sophisticated editor-in-chief Kabir (Rahul Khanna). On the eve of her 27th birthday Aisha invites Sid to her place because she does not want to be alone. When Sid tentatively explores moving their relationship beyond friendship, he finds that Aisha perceives him as boyish and immature. Despite feeling hurt, he maintains his friendship with her.
Their relationship intensifies when the exam results come out and Sid finds that he failed. His failure leads to a family fight that ends with him moving out of the house. With nowhere to go, Sid (who has never been on his own) asks Aisha if he can stay with her. Aisha is initially happy to have the company (although she clarifies that his stay is purely platonic). Eventually, however, Sid's bad habits revive themselves as he leaves her place a mess and makes the kind of demands on her that he is used to making on everyone. When she finds he has not eaten all day, Aisha is amused by the fact that Sid cannot cook and does not know how to feed himself. Over time, Sid learns that to survive in the world he has to begin cooking and cleaning. He has also realized he must work and Aisha helps him become a photography intern at the magazine she works for.
Aisha who had a crush on her boss, editor in chief Kabir, is ecstatic when he selects her article for the magazine and also asks her out on a live jazz performance. However, as she goes out with him, she realizes she does not much in common with Kabir. Meanwhile, she slowly realizes her feelings toward Sid and understands she is in love with him.
As Sid begins working, he finds meaning in life and realizes that there are more options than working in a standard corporate environment. He also begins to see for the first time how his behavior has hurt everyone he loves and that he needs to change his ways. After he is hired as full-time staff and receives his first pay check, he confronts his father. They reconcile and Sid's father asks him to come home. Sid, who often feels a burden to Aisha, tells her that he won't be dependent on her charity anymore and expects her to be thrilled. He doesn't realize that Aisha has fallen in love with him and thus is furious to hear that he is leaving. Their departure is a hostile one and Sid leaves without resolution.
When Sid returns home from Aisha's place, he thinks constantly of her. However, he remembers that she had told him they can only be friends and so he does not consider that she might share his feelings. When the latest issue of Mumbai Beat arrives, Sid reads Aisha's column and is startled to discover that it is about her feelings for him. As rain starts pouring, he rushes to meet her at the same beach where they had sat the very first day they met. Sid expresses his love for her and the two embrace