Karthik (Farhan Akhtar) is an introvert who lacks confidence in dealing with everyday life situations and feels trapped in his average job at a construction company, that produces even lesser than average results as he is continuously troubled by an incident of his childhood. His elder brother, Kumar, used to torture him, but whenever he complained to his parents about him, they did not believe him. One day, Kumar took Karthik to show him some beautiful tourist spot, and took him to a well, trying to throw him in it, but Karthik escaped. Kumar next tried to tie him with rope but accidentally fell inside the well and died. Karthik thought himself to be responsible for his brother's death. Shonali Mukherjee (Deepika Padukone) is a co-worker at Karthik's company.
Karthik’s boss Kamath (Ram Kapoor) and landlord treat him badly, often humiliating and disrespecting him, and Karthik has to put up with these insults on a daily basis purely on account of his inability to counter them strongly. Shonali, on the other hand, portrays for Karthik, an independent, classy and confident woman, who remains unaware of his secret love for her, much less his existence even though he's worked at the company for close to 4 years. Karthik attempts to email her his feelings very regularly, but on each occasion, stops himself and saves the mails in his Drafts folder, as he concludes that Shonali is dating his co-worker Ashish.
After being derided by his boss yet again, he figures that his life can’t get any better and decides to commit suicide by consuming too many sleeping pills. At the exact moment that he is about to consume the sleeping pills, the phone rings and a stranger, having the same voice as him, tells him that he is also Karthik, and that he is his well wisher. By and by, Karthik finds that this stranger can exactly locate his psyche, state of mind and emotional levels and come up with solutions that can make things better. The caller convinces him that he has the ability to change him, if he strictly follows his guidelines. The caller expressly tells Karthik that he should not mention his phone conversations to anyone, lest he will be perceived as mentally ill. Karthik agrees to keep the phone conversations to himself.
Gradually these phone conversations become his life guide. His chats take place every morning at 5:00 am and the caller provides advice on every problem that arises in Karthik's life. The caller keeps a close eye on his life, guides him to become a successful man and win his secret love, Shonali (by making Ashish reveal in front of Shonali that he had a wife and a 6 year old son in Delhi) and eventually enables him to bring color to his otherwise monotonous and unremarkable existence.
One day, when Karthik and Shonali were together, Shonali tells him that he should tell her everything, and not hide anything from her, be it costing anything. He tells Shonali about the mysterious caller, but Shonali does not believe him and asks him to consult a doctor.
The night when he receives the call, the caller asked him to lie to Shonali and told him not to consult a doctor. The next day he lies to Shonali, but she catches it. Karthik said he would definitely go to consult a doctor. When he goes to his psychiatrist, she asks him if she could come to his house and check the call at 5 am. When she arrives at Karthik's house, the phone rang and the same voice told her that it existed whether she believed it or not and when she heard it, the cup from whch she was drinking coffee, fell down. Karthik got up and when to get a wipe to clean it, and when he came back in the room, he saw that there was nobody and the door was open.
The next day he tells Shonali that he consulted a doctor, but she ran away. Shonali does not believe him and says that she will herself stay with him and see the calls arriving. The phone again rings in front of her also but she stops Karthik from receiving it. The phone eventually stops ringing.
Karthik receives the calls again the next morning, and the caller warns him that if he could bring him up, then he could throw him down also, and says that he would destroy everything. When he goes to the office, his boss throws him out of the office telling that he called him last night and black mailed him that he would tell his wife about his affair if he sold illegal land. While he was moving out, he meets Shonali who tells him that he called her last night and made fun of her. He tries to tell her that he did not do anything but she does not listen. Karthik thought that he should go somewhere which even he would not know, and so he gets a pair of opaque black glasses and asked an old man at the railway station to get him a railway ticket but not tell him the place the train was going. He wore his glasses and sat in the train pretending to be blind. He thinks that he himself won't know where he is, then how would the caller know? He goes down the train on the station and asks the driver of his cab to take him to any small hotel of the area. When he reaches there, he asks the receptionist to remove the telephone, room number plate.
After the passing of a few months, Karthik is comfortably living in a small town in Kerala. His life is back to normal with respect to work and other elements except for the fact that he refuses to have a phone line. However, upon his boss's request, he is forced to purchase a land line but goes to great lengths to ensure that he himself is unaware of the number. but purchases the same model that he had earlier.
After a day of restless sleep, he is glad that no calls come through and takes this as a sign that those events will not occur again. But the next day, at exactly 5am he receives a call from Karthik, who tells him that he has found him and will now find a way to finish him. In the meantime, he has contacted Shonali and even intends to meet up with her one day. She is contacted by Dr. Kapadia who explains that Karthik is Schizophrenic. He has an alter ego that is more assertive and advises him on what to do and how to live life. And he has been dealing with this condition from a young age when he created a fake brother. Turns out that the Phone Model that he purchased has the capacity to record messages and act as a playback feature at a certain time. And these functions work regardless of whether a phone is plugged in and the messages can also be sent to other numbers. This explains the phone "calls". He would wake up in the middle of the night, leave himself messages in the alter ego state and then go to sleep.