Rahul Kapoor (Imran Khan) has always been on the road to perfection, as per the wishes of his domineering father (Boman Irani) and socialite mother (Ratna Pathak Shah). After suddenly losing his job as an architect in Vegas, he decides to hide the truth from his parents and find another job. Unable to cope with unemployment, he decides to go to a psychologist and runs into another patient, Riana Braganza (Kareena Kapoor). A series of events lead Riana to believe that he is sent by her ex-boyfriend to stalk her, and she ends up taking his file by mistake.
Riana, who later realizes her mistake, calls him and apologizes for her behaviour. They meet up during Christmas Eve and Riana invites Rahul to have a few drinks with her. A few drinks turn into a night of intoxicated revelry as they end up marrying each other. Horrified, they both decide to get it annulled as soon as possible. Meanwhile, Riana, who is also unemployed and has not paid her rent, loses her house and an initially hesitant Rahul finally lets her stay with him till her problem gets resolved. During these few days, they get to know each other better. Rahul is revealed to be interested in photography while Riana recounts her ambition to be a ballet dancer, which was destroyed after she broke her ankle in her childhood. Their friendship develops as Riana gives Rahul a long break he always wanted. As they grow closer, Rahul develops feelings for Riana, who fails to reciprocate them.
As New Year's Eve approaches, Riana plans to visit her family in India, and asks Rahul to accompany her. He disagrees initially, as his parents also live in Mumbai, but later succumbs to her argument. Upon arrival, she introduces him to her family, making his feelings towards her grow. The next day, Riana takes him to visit her school, and amidst reflecting over her past relationships, he unsuccessfully tries to kiss her. Realizing that she does not feel the way he does, he angrily leaves the school, only to almost run into a car revealed to be his mother's. He lies and tells his mother that he is in India for a research trip, and that Riana is a colleague. They proceed to have lunch with Rahul's parents, where he spins a web of intricate lies to avoid telling his father the truth. He leaves Riana's house and moves in with his parents, trying to tell them the truth, but once again is overpowered by his father.
Two days later, during a family dinner, Rahul angrily divulges the complete truth to his in-denial parents and their business partners (including Mr. Bulani), after realizing the meaninglessness of his pretense he indulged in showing all these years. As he rushes back to Riana to apologize for his behaviour, Riana apologises, she never intended to lead him on. After a night of reflection and deep conversation, both decide to maintain their friendship. The film ends with both returning to Las Vegas and finalizing their annulment with Rahul getting a new job while keeping time out for photography. He remains hopeful that one day he will convince Riana to go back to that chapel with him. Both remain, good friends, with an ending note from Rahul claiming that he is finally happy with his life.