US Federal Air Marshal Bill Marks (Liam Neeson) boards a busy international flight from New York to London.

As the aircraft reaches cruising altitude, Marks receives a series of bizarre and threatening text messages demanding that the airline transfers a ransom of 150 million dollars into an off-shore account. Failure to comply will result in one passenger on the flight being killed every 20 minutes.

The threats turn out to be genuine and everyone on board becomes a suspect.

While Captain David McMillan (Linus Roache) and his crew including a ballsy flight attendant named Nancy (Michelle Dockery) attempt to maintain calm in the claustrophobic aircraft, Marks searches for answers.

However, his gung-ho one-man crusade is hampered when authorities discover that the bank account is registered under his name and Marks is branded a terrorist.