Bill Marks, played by Liam Neeson, boards a transatlantic flight from New York to London as an air marshal. He is an alcoholic and a former NYPD officer, who is still grieving the loss of his daughter. During the flight, he receives a message on his secure phone, instructing him to transfer $150 million to a specific account or a passenger on-board the flight will be killed every 20 minutes.
Initially, he thinks it's a hoax or a prank, but when the first passenger is murdered and the money is transferred without his knowledge, he realizes that someone on the plane is orchestrating the plan. Marks informs the pilot and the ground authorities about the situation, but they are unable to identify the perpetrator as the contents of the messages keep changing.
Marks starts to investigate the passengers on-board and discovers that many of them have suspicious backgrounds, including a Muslim doctor, a wealthy businessman, a shady NYPD officer, and a young woman named Jen (played by Julianne Moore), who was seated next to him during takeoff. Marks finds evidence that implicates each of these passengers and even confronts them, but he is unable to determine the true identity of the culprit.
As the flight progresses, more passengers are killed, and Marks' credibility comes under scrutiny as the ground authorities suspect him of being the hijacker himself. When Jen comes to Marks' aid, the two of them work together to unravel the mystery and apprehend the real perpetrator. In the end, they discover that the hijacker is a man who had faked his own death and had disguised himself as an air marshal on the flight.
The movie ends with Marks successfully stopping the hijacker and saving the remaining passengers. We also discover that the $150 million transfer was part of a plot to frame Marks and Jen for the hijacking and that the real motive of the hijacker was revenge against Marks for killing his family during his time as an NYPD officer.