Cassie, Tim, Emily, Brandon, and Jay are on their way back from a camping trip, flying from Idaho to New York City on Cassie's family's private plane. This might be a usual thing for the others, but for Jay, it's only his second time ever on a plane. And what starts as a normal flight soon veers desperately out of control.
One of the regular pilots is sick, so there's a replacement. Cassie has suddenly fallen ill for no reason. And Jay notices the plane is flying west instead of east.
As the military works feverishly on the ground to find out what's going on, the friends are trapped thirty-seven thousand feet in the air on a plane that's clearly been hijacked. Only no one knows who's in control or why it's happening. Their only chance to survive is by working together, but when everyone is a suspect, trusting the wrong person is a deadly mistake.
With each passing minute, the gas gauge drops, alliances shift, and danger rises. Will anyone make it off Flight 21 alive?