Jay Trivedi (played by Ranbir Kapoor) is a young, honest lawyer who has just started his civil practice. Son of an eminent, highly accomplished criminal lawyer Mukesh Trivedi (played by Anupam Kher), he has been taught by his father to follow the ethics of law while adhered to the terms of general morality. Being from a well-off family that's largely a result of Mukesh's four decade long practice as a successful lawyer, Jay is not compelled to take up lucrative cases. In fact, Mukesh advises Jay to initially work on cases that would help the poor and the underprivileged before moving on to bigger profiles in his career. Jay, with assistance from Mukesh on the criminal aspects of the cases, efficiently handles a couple of such cases, one for his neighbor's troubled maid who was constantly pestered by her deceased husband's creditors to exchange sex for the owed money and another for a top notch surgeon who had got fabricated in a case where his employer, a private hospital had accused him of running a private clinic and diverting his patients whereas the real reason had been his stand against the outrageously expensive, unnecessary medical tests the hospital endorsed.
While Jay is busy in his own practice, a lady in her late twenties, Kalpana (played by Deepika Padukone) visits his father Mukesh for a mysterious case that Jay gets curious about. Jay, after a couple of confrontations with Kalpana falls in love with her. Kalpana however, offers no such indication. Mukesh doesn't let Jay know what the case was about, as he believes in work ethics of keeping his cases and clients' details confidential.
After a week though, Kalpana stops visiting their house. Jay, who was already deeply in love with her now gets baffled because of her absence and asks his father again about her whereabouts who denies revealing it yet again. Jay breaks into his father's room in their mansion that night and manages to get Kalpana's address.
The next day, he visits Kalpana and tries to get more details about the case. When she just says his father did not agree to take up her case, he promises to take it himself without even knowing what the case was. A surprised Kalpana hates that idea, chides him for his immaturity for he isn't even a criminal lawyer, doesn't know a bit about the case and yet assures to take it for her. But an adamant Jay who's madly in love with Kalpana doesn't give up, follows her everywhere and explains his capability in handling a wide array of cases and situations. With no other way in front of her eyes, and also with the days swiftly passing towards the court date, Kalpana agrees for him to take up the case.
She then reveals that she has been framed in a false case of murder. Jay is rattled as he had never taken up a case that involved a murder directly. She elaborates that she has been accused of murdering her boss Prem, who caught her leaking away the project secrets of their pharmaceutical firm to their competitors. Kalpana however, maintains that he was murdered by his wife who was fed up of his extra marital affairs, womanizer habits, and who also had an eye on his financial assets.
When Mukesh learns that Jay has taken up Kalpana's case, he is infuriated. He reprimands Jay for being so immature to have taken up a case that he rejected, only because Jay was in love with the accused. But Jay holds to what he had done and sternly tells his father that he had taken up the case and that he would see it to its completion. This creates a rift in the Trivedi family and Mukesh disowns his son and throws him out of his house.
Jay and Kalpana start living together in their small rented house. With Kalpana having got suspended from her job after the case started and Jay having very little savings, they find it hard to take the case forward. But the hardship they endure only makes their love for each other stronger. Kalpana sells her only ancestral house to get some more money needed for the case to sustain. Jay is utterly surprised to see his father Mukesh having taken up the case against him. Jay and Mukesh argue against each other in the court of law. Mukesh realizes the difficulty in proving the motive as he couldn't find substantial evidence to claim their accusation of Kalpana trading the company secrets to their competitors. Over time, he comes to the conclusion that Kalpana is in fact innocent. Eventually, due to the lack of evidence, the court acquits Kalpana of both the charge of murder and of leaking company secrets.
After Jay's grand success in his first criminal case, Mukesh, who respects courts' decisions to their core, is softened towards his son and Kalpana and urges Jay to come back home. Jay is equally happy with the things going on and makes a plan to formally propose and marry Kalpana and return home.
He's devastated when he finds Kalpana missing leaving behind a letter addressed to him. The letter reveals that Kalpana is in fact the perpetrator of the murder, along with her friend who worked for the competitor pharmaceutical firm where he held a top position. They made huge profits because of the trading secrets leaked by Kalpana and when Prem, her boss got suspicious, they hatched a plan to kill him. The letter also reveals that Kalpana had no ancestral property at all and the timely money that used to arrive was indeed from her boyfriend, the money made from leaked trade secrets. The letter ends by saying that Kalpana is both sorry and extremely grateful to Jay for everything he did, that she felt ashamed of what she did to him and that she's going to a remote island to live with her friend.
Jay is emotionally shattered and returns home alone. He confesses everything to his father Mukesh who appears equally perplexed and disturbed. That moment, the door bell rings and they find Kalpana at the doorstep. Kalpana admits her mistakes and tells Jay that on her way to the island, she thought about all the things that had happened over the course of the case and that the only thing she found "real" and "genuine" was their love for each other. That the purity and genuineness of Jay's love had brought her back to him and that she wouldn't mind going to jail but cannot bear the guilt of hurting true love.
Jay accepts her back into his life as they hug each other. Whether Mukesh succumbs to the innocence of love or pursues a case against Kalpana again is best left to the imagination of the viewer.