Un travail minutieux de récolte d'images a permis de reconstituer, minute par minute, ce qu'ont vécu les New-Yorkais lors du 11 septembre 2001, entre l'impact du premier avion et l'effondrement de la seconde tour. De leurs appartements, de Times Square aux quais du New Jersey, les New-Yorkais commentent, observent, et surtout filment la catastrophe. Si certains restent incrédules, d'autres sont pris de panique. Les rumeurs circulent, les transports s'arrêtent et les lignes téléphoniques sont mises hors service. L'île de Manhattan se retrouve coupée du monde. Ses habitants cherchent à fuir en marchant le long des routes et prennent d'assaut les ferries.
Vikram travaille à l'aéroport de Chandigarh avec son ami Rajeev, chef de la sécurité. Sa fille voyage sur un vol qui se fait détourner par 6 terroristes, qui travaillent pour un dénommé Rasheed, capturé par la police indienne, et atterri de force à l'aéroport de Chandigarh. Les terroristes demandent la libération de Rasheed. Vikram va s'infiltrer dans l'avion pour tenter de sauver sa fille...
New York begins in the United States in 2008, with the arrest by the FBI of Omar Aijaz (Neil Nitin Mukesh) after guns were found in the trunk of a taxi cab he owned. Omar, a young Muslim man originally from Delhi, is then taken into custody and interrogated by FBI Agent Roshan (Irrfan Khan) (also a Muslim man originally from South Asia who has been living in the United States for twenty years). Omar then discovers that he was set up by the FBI in order to force him to spy on a former college friend, Samir Shaikh (John Abraham), whom he hasn't seen in seven years and who the FBI believes is a terrorist. In the process, Omar discovers that Sam has married Maya (Katrina Kaif) (a mutual friend whom Omar had a crush on in university) and finds out that Samir and Maya have a young son, Danyal (Aidan Wagner).
Montine McLeod (Melissa Leo) is a flight attendant who gets stuck with Omar Hassan (Anthony Keyvan) after their flight gets grounded in Longview, Texas, the morning of 11 September 2001. Omar is an unaccompanied minor; in the chaos that ensues, Montine becomes his temporary guardian. Montine later discovers that Omar's father works in the World Trade Center. She decides to drive the boy home after he informs her his father is at home waiting for him in New York. On the journey, they begin to bond and learn more about each other. Montine makes a stop on the way to New York as she receives a call that her mother is dying. Upon arriving at her mother's house, she finds her mother already dead. After dealing with her mother's death, she continues on the road to take Omar home. When they get there, they learn Omar's father has not been home in the two days since the destruction of the Twin Towers. Later that night, Montine receives a call from her employer and is forced to bring Omar with her. At the ensuing meeting, Montine is terminated; Omar is taken from her and given to his school head master for the night before leaving for a new school the following day (the reason he was on the original flight). The day Omar is set to leave for Los Angeles, he locks himself in the bathroom and refuses to come out. Montine is asked to come; she coaxes Omar out of the bathroom and she walks him to his flight to Los Angeles.
Following a Miss Universe style contest in Miami, Florida, the five semi-finalists are flown to Nassau, Bahamas in a flying boat along with the American contestant's stage mother, the group's escort Kate Malloy, the Miss Beautiful Master of Ceremonies Marv Barker, and a deadheading pilot, Paul Fabiani.
On September 11, 2001, as information seeps out about the 9/11 attacks on America, the FAA begins directing all remaining aircraft in the air to nearby airports, completely clearing the skies. A total of 39 transatlantic flights at or near their "point of no return" (AKA point of safe return) are diverted to the nearest airport in Canada, Gander, Newfoundland. The anxious passengers leaving on business and vacation trips have no idea why their flights are being sent to a remote town in Canada. With wild rumours spreading, one British airliner's crew tells the passengers what is known, that a terrorist attack has taken place in the United States.
À l'origine, les frères Naudet projetaient de produire un petit film documentaire retraçant le parcours d'une nouvelle recrue chez les pompiers de New York que l'on surnomme habituellement le « probie » (jeune pompier en période probatoire). Après quelques mois de tournage, lorsqu'un des frères est en train de filmer les pompiers, intervenant sur une fuite de gaz non loin du World Trade Center, le 11 septembre 2001, il pointe sa caméra en direction du premier Boeing 767, au moment où celui-ci percute l'une des deux tours. Les deux frères suivent alors, chacun de leur côté, l'équipe des pompiers dans les tout premiers moments de leur intervention. Jules est ainsi le seul journaliste qui a pu filmer de l'intérieur et en continu l'organisation des opérations de sauvetage dans la tour 1 (tour Nord) ; étant au moment du crash avec le commandant Joseph Pfeiffer, chef de bataillon des pompiers responsables du WTC, il a été autorisé à le suivre durant toutes les opérations. Au moment où la tour 2 (tour Sud) s'effondre, la décision est prise d'évacuer d'urgence la tour Nord. Le journaliste et une partie des pompiers parviennent finalement à s'échapper de cette tour peu avant que celle-ci ne s'effondre également.