Samar Anand (Shahrukh Khan), a major in the Indian Army, defuses a bomb in Ladakh without fear or regard for his safety. Akira Rai (Anushka Sharma), a Discovery Channel filmmaker, dives into a river in Ladakh on a dare and soon regrets it after learning she has plunged into freezing water. Samar, who happens to be by the shore, rescues her. Samar gives her his jacket and leaves without retrieving it. Later, Akira finds his diary in the jacket pocket and begins reading.The diary recounts Samar's earlier years as a struggling immigrant in London, working as a busker who also performs other odd jobs to support himself and his roommate Zain (Sharib Hashmi). One day while cleaning snow outside a church, he sees Meera Thapar (Katrina Kaif) for the first time and instantly begins to like her. Samar is working part-time as a waiter when he meets Meera again at her and her fiance Roger's engagement party. Samar is a little bit disheartened seeing that. Meera grew up, motherless, in an affluent Indian family. Meera's mother (Neetu Singh) left her family for another man (Rishi Kapoor) when Meera was twelve. This leaves the dominant parental figure in her life to be her father (Anupam Kher), for whose company she works. Meera approached Samar to teach her a beautiful Punjabi song for her father's 50th birthday and in return Samar asks Meera to give him language tuitions for one month.Meanwhile, Samar founds out the real naughty Meera inside Meera. Samar notices that Meera often prays when he sees her at church, and after a night of wild street dancing, Samar and Meera begin to fall in love. Samar helps Meera face her past, taking her to visit her estranged mother and foster reconciliation. Some days later Meera decides to confess to her father about her relationship with Samar and break her engagement, but Samar has a serious accident on his motorbike. Meera, believing she is the cause of Samar's spate of bad luck, prays to God to save his life and promises never to see him again. Samar recovers and Meera admits her vow to him. Angry, he leaves her and even London. Samar challenges God to keep him alive while he risks his life every day, because he believes his death is the only way to make Meera lose her faith in God. He goes to India and enlists in the army, becoming a bomb-disposal expert.When Akira finishes reading the diary, she obtains permission to make a documentary about bomb-disposal squads. She asks Samar for help to make her film and develops an acquaintance with him and his team. Akira develops a crush on Samar; however, he does not reciprocate her feelings owing to his unresolved passion for Meera. Akira makes a successful film and prepares to leave for London. She wants Samar come along to help her publicise the documentary. After he reluctantly agrees to come to London, he is struck by a car.Samar is diagnosed with retrograde amnesia, and he remembers only the events before his first accident a decade ago. Concerned, Akira tracks Meera down and persuades her to aid in Samar's recovery. Meera agrees, pretending to be Samar's wife. In the meantime, Akira realises that Major Samar is only a fragment of the young Samar; he used to be happy and sociable, but is now bitter and lonely. One day Samar finds a bomb planted in the London Under ground, and helps defuse it. The event jogs his memory, and he realises that Meera is lying to him. Samar confronts Meera with a choice: to either be with him honestly, or to see him keep risking his life until he is dead. Meera is unable to give a definitive answer; heartbroken, Samar leaves for Kashmir, where he continues defusing bombs. During a conversation with Akira, Meera realises that her beliefs and prayers subjected Samar to a fate worse than death; realising her mistake, she goes to Kashmir and they reunite. Samar defuses his last bomb, and then proposes to her.