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 $150m 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.