Avantika Ahuja (Kareena Kapoor) who lives in the United States returns to Delhi after her father has a cardiac arrest. While in Delhi she meets Ehsaan Khan (Saif Ali Khan), a University professor from Mumbai. Being in the same career field at the same place, love blossoms between the two. Meanwhile Avantika gets a call from New York asking her to rejoin her job. She breaks this news to Ehsaan and expresses her desire to work in the US, Ehsaan agrees and joins her but before that Avantika's father has some reservations about his daughter marrying a Muslim. However he gives in as the couple has decided to get married. Eventually they get married and move to the USA.
Avantika helps Ehsaan in getting a job at the same university where she teaches and Ehsaan is appointed as a professor to teach Islam and the Modern World. Avantika's life changes when her neighbor Salma (Nauheed Cyrusi) informs her about her house arrest and requests Avantika to help her by contacting her friend Rihana (Dia Mirza), a TV reporter. Avantika meets Rihana and tells her about Salma's problem and meanwhile she meets Riyaz (Vivek Oberoi), Rihana's boyfriend and colleague who takes Salma's problem as a case of domestic violence. Unfortunately, Rihana has to fly out of the city before meeting Salma and she gets her flight information when Avantika is still at her office.
Avantika becomes pregnant and waits at her house to share this with Ehsaan but when she hears voices coming from their neighbor's house, she investigates and finds out that her neighbors Bhaijaan (Om Puri) and the others are planning to send Tahir and blast the flight in which Rihana is traveling, and expose them. Her neighbors see her and chase her to her house where she meets Ehsaan who consoles her but soon reveals that he is also a part of the terrorist group. Bhaijaan ask Ehsaan to kill her in order to make their operation successful, however Ehsaan denies after learning that Avantika is pregnant. Ehsaan keeps Avantika in house arrest and threatens to kill her father in India if she tries to inform anyone. Avantika realizes that she is a pawn in Ehsaan’s game. It turns out he married her to legitimately obtain Social Security Cards and a new identity in the USA. Avantika calls Rihana and leaves a voice message on her office number to stop her from boarding the flight. Unfortunately, the plane explodes killing Rihana.
Riyaz, heartbroken and revengeful finds Avantika's voice message for Rihana and begins a mission to uncover the real culprits behind the flight bombings. Riyaz also joins Ehsaan's lectures and soon befriends him. Ehsaan finds him to be a perfect replacement for one of his group member Aqeel (Salma's husband) who was killed by Ehsaan while they were taking out Salma's body to bury it. Ehsaan introduces Riyaz to Bhaijaan who becomes suspicious of Riyaz's thoughts to fight against the Americans. Ehsaan makes Riyaz a part of their next mission to bomb several subway stations in New York.
Riyaz tries to spoil the plan with the help of Avantika. Time starts running out for Riyaz & Avantika and they become helpless when Bhaijaan advances the mission by 3 days. While carrying bombs in the subway Riyaz's colleague bumps into him and tells Ehsaan & Bhaijaan about his real identity. Riyaz tries to escape and in the chaos, shoots Bhaijaan. While taking his last breath, Bhaijaan informs Ehsaan that there are 3 more bombs placed in the bags of 2 wives of the terrorists involved and one of it placed in Avantika's bag which is being escorted by Aapa (Kirron Kher). He has a change of heart to save his love and child and doesn't kill Riyaz when he corners him and instead they both make a plan to stop all the other terrorists. Riyaz tries to save the people at the train stations but 2 bombs explode killing Hamid and another terrorist. Ehsaan kills Aapa while saving Avantika. Ultimately he sacrifices himself at the end to save the innocent Avantika. Before leaving, Avantika asks Ehsaan his real name, which he reveals to be Khalid. As she leaves the train, she hears the gunshot of Ehsaan killing himself.