The film starts with an unassumingly innocent girl Harleen Sahni (Katrina Kaif) who works as a bank receptionist. In a random turn of events (blind dating, to be precise), she accidentally happens to meet the extremely charming yet mysterious Rajveer Nanda (Hrithik Roshan), who is out on a secret mission. And anyone who comes in the line of his 'duty' simply gets eliminated by him... bang bang! There comes a situation when Harleen starts hating Rajveer, when she gets to know his identity. By then, the damage is already been done... sub-consciously she falls in love with him. The situation becomes such that, she cannot leave his side and the two engage in a series of action adventure across the world. Whenever she poses a question about Rajveer's background, either Rajveer ducks the question or the situation plays the villain. Amidst all this, enters the villainous Omar Zafar (Danny Denzongpa), who kills the nation loving army officer played by Jimmy Sheirgill, as he doesn't abide by his terms and conditions. The twists and turns of events lead Harleen to unravel Rajveer's biggest secret. 

Who is Rajveer exactly? Why is he dragging Harleen along with him? And what does he really want from Omar Zafar... forms the rest of the story.