Chance enters the cockpit of a passenger airline as it starts to go down and the passengers panic. With him is Laura, a flight attendant, who asks who he really is. Chance explains that the plane is on fire and they've lost their captain and first officer to smoke inhalation. Chance does tell her that he's private security, not an insurance salesman, and protecting a client on the plane. He asks her to sit down and help him.
Six hours earlier
Winston meets with Phil Tennant, a computer CEO who has hired a hacker, Casper, to test his company's firewall defenses. Casper is paranoid and Tennant only knows Casper by his screen name. During his most recent hack, he discovered a fatal flaw in the internet: a skeleton key that could penetrate any data encryption. Casper is travelling to Tennant's headquarters in Seattle to help patch the flaw. Someone broke into Tennant's office and stole the information on the skeleton key. Now they know Casper's flight plan, and Tennant has no way to contact Casper and needs Chance to help.
Later, Winston explains to Guerrero and Chance how significant a threat the skeleton key could be. Casper will be protected once he gets to Seattle, so they figure the break-in artist will try to grab Casper on the plane. Winston sets Chance up as an insurance salesman, and Chance asks Guerrero to get a hypodermic disguised as a pen so they can inject the abductor with a knockout drug. However, Chance explains that they need two people on the plane: one to find Casper and one to find the abductor. He notes that a second insurance salesman would be to suspicious. Guerrero has set Winston up with a different cover identity: a flight attendant. They board and the plane takes off.
Winston wrestles with an armed man. In the cockpit, Laura tries to radio air control, but Chance warns that the fire has reached the radio antenna. He opens the landing wheel doors and depressurizes the cabin to try and put out the manual. When the doors don't open, he starts thumbing through the manual and finds a conduit that controls the wires. They open the air vents but the temperature continues to rise as the fire closes in.
Winston struggles with the man, who fires a shot and wounds a passenger. Winston finally takes the man, and then has a blonde woman help him.
As they try to deal with the fire, Laura admits that she's been trained to deal with all kinds of threats. She does admit she's been considering a career change, and wonders how Chance became a bodyguard. He simply says that he felt he had more potential than in his old job.
Chance hasn't spotted anyone yet. Winston has a list of everyone who boarded in England, where Casper came from. They've both spotted the air marshal, and Chance figures they won't spot either target from their list. He suggests they pull a Crazy Eddie and anger a passenger to see who does and doesn't react. Winston picks an annoying passenger, Larry, shakes up a soda can, and gives it to him. When the man gets angry, Chance notices that one passenger doesn't react.
Guerrero brings in an old "friend," Sergei, to have him identify which hacker Tennant used that would be looking for the skeleton keys. Sergei says he can't do anything with screen names and Guerrero gives him the phone numbers of the break-in crews that could have been hired to break into Tennant's office. He also gives Sergei a list of passengers. When Sergei hesitates, Guerrero just glares at him and Sergei agrees to help. Winston takes a call from Winston, who wants him to check the five names on the manifest that he believes are likely suspects.
Chance sits down with the passenger, Vincent, and drops some hints that he might be a killer. He prepares the hypodermic pen, but Vincent finally admits that he took some sedatives to cope with his fear of flying. Chance realizes that he's not Casper or the abductor.
Winston realizes that the plane is changing course and calls to tell Chance that the pilots had to reroute due to an electrical malfunction. They're diverting to Portland, and both men figure that the abductor will have kidnappers on the ground there. Chance goes to check on the electrical malfunction and confirms that it's sabotage. The saboteur attacks him, fires one stray shot, and after a brief struggle, Chance subdues the man and checks his phone log. He's unaware that the stray shot pierced an electrical line below the floor.
Chance can't open up the vents any further without endangering the passengers. He tells Laura that the strongest winds are above them, but the fire is in the cargo hold below them. Chance plans to flip the plane over to expose the flames to the high winds to put them out. He admits it's a risky but if they don't put out the fire, they're all dead. He calls Winston and has him move everyone to positions of safety.
Guerrero calls Winston that the list he provided turned up negative. When Guerrero wonders if Casper boarded in San Francisco as they believe, Winston figures that Casper forged a boarding pass.
The air marshal captures Chance at gunpoint. Chance claims he just stumbled across the saboteur by accident and shows his ID. The air marshal insists on handcuffing him as well until he straightens out everything later.
After checking the passenger lists, Winston approaches a blonde female passenger and sits down with her. He makes small talk and then notes that she forged a boarding pass showing she boarded in London to cover the fact she came on board at San Francisco. Winston notes she has no baggage claim checks, despite the fact she was in London for two weeks according to her pass. She realizes he's not a flight attendant and Winston explains that Tennant hired him and Chance. He warns her that there are people after her, and he and Chance have everything under control. Chance is taken by, handcuffed.
The air marshal talks to the flight attendances, including Laura. He goes to get the saboteur and has Winston watch Chance. Winston slips him a paper clip to use as a picklock and tells him that he's found Casper. Chance tells him that he checked the saboteur's pocket and discover that the man's partner on the plane sent him a warning. Winston brings Casper forward and starts looking for the second bad guy, while Chance smells smoke.
A fire starts below decks and the smoke gets into the oxygen supply, knocking out the pilot and co-pilot. Chance frees himself and offers his help. The air marshal doesn't trust him and draws his gun, and insists that air traffic control can help him land the plane. Chance warns that there isn't enough time. When the lights flicker, Chance grabs the air marshal and disarms him. Laura grabs the gun and insists that Chance try to land the plane. The air marshal goes for a second gun. While Winston and Chance struggle with him, Laura looks significantly at the captive saboteur. She then goes into the cockpit with Chance while Winston subdues the air marshal. As Laura sits down, she makes sure the gun she has hidden is secure.
Winston warns the passengers and has them move to the left side of the cabin. Chance flips the plane and holds it upside down until the fire goes out. However, Chance discovers that the controls have locked and he can't flip them back. He realizes that the flight computer has locked them on course and locked them out. As he goes back to talk to Casper, he warns Laura that they're heading for a storm and will arrive in 10 minutes.
In the cabin, the pilot confirms that the computer crashed due to the maneuver. He explains they can't roll the plane back without the computer, but Casper suggests that she download the flight management software to her laptop and use it to override the flight computer. She uses the skeleton key to download the software via wireless, but she won't be able to finish the download until they hit the storm. They have to patch the computer in manually, and the avionics are near the wheel well. Laura suggests a panel that she learned about from when she dated a ground crew guy.
Winston frees the air marshal and explains what's going on. The air marshal wonders why the second abductor didn't free his buddy during the confusion.
As Laura and Chance bundle up to go up into the wheel well, she asks what his name is. He tells her Christopher Chance, but she wonders if that's what it really is. As they go, Laura secretly slips a paper clip to the first abductor.
Sergei tells Guerrero that he struck out. Guerrero clearly doesn't believe him and asks what he would do if he had a skeleton key that could open any safe, anywhere. Sergei admits that it would be tempting, and Guerrero asks if he would take it. Sergei says that he wouldn't, and Guerrero agrees, pointing out that it would make the person a target. Guerrero suggests that Sergei might have tried to make a deal with the hacker, and if he did, Guerrero would have a real problem with him. Sergei considers briefly and then tells Guerrero where he can find the hacker.
Winston gets the passengers ready and then braces himself with Casper. Above, Chance and Laura enter the wheel well. As he hooks up the laptop to the avionics computer, Winston warns him to be back inside the plane before they flip. Chance completes the hookup and the pilot evens off the plane.
The first abductor frees himself and goes for Winston's gun... and the air marshal knocks him out.
Laura reaches for her gun, and Chance reveals that he already took it. Laura realizes that he know she was a fake all along. She refuses to go to prison, but Chance suggests that she try a new career like she suggested earlier. Laura admits that she killed the flight attendant to get on the plane, and doesn't believe Chance's insistence that people can change. As the plane rolls, Laura kicks the gun out of Chance's hand. The two of them fight in the wheel well as the plane rotates 180 degrees. Chance falls down and manages to grab onto the landing wheel. Laura tries to knock him loose, but he injects her with the knockout drug from the hypodermic pen. He pulls himself up as she falls out, and quickly grabs her hand. As she passes out, she asks Chance what his name is but he doesn't say anything. He loses his grip and she falls to her death.
The hacker who arranged Casper's abduction, Nick Chepekian, is buying a pizza when a thug grabs him at gunpoint and tells him to take a phone call. It's Guerrero, who asks him what he would do with a skeleton key.
The plane lands safely at Portland and Winston and Chance remain behind with Casper. They wonder if the abduction team is waiting for them. Instead, a man disguised as a TSA guard arrives and tells them that Guerrero sent him and he's there to get Casper to safety. When Chance asks about the abduction team, the man assures him that they've been taken care of.
Chance and Winston deliver Casper to Bennett. He prepares to leave with Chance and informs him that Tennant has given them first-class tickets to find home. Winston suggests they might be better off taking a train. Chance asks if he remembers what Winston told him when they started their current operation. Winston says that he told him that he wasn't as stuck as he thought he was. When Chance wonders how he knew he'd go for it, Winston admits that he didn't. As they drive away, Winston insists this is the last time he's going out in the field with him.