Castle and Alexis are on a trans-atlantic flight to London so that he can speak at The Sherlock Holmes Society. He’s excited but Alexis hope to ditch the event and meet up with some friends. 

Castle notices something is amiss with the flight crew. One of them knows who he is and that he consults with the NYPD and confides that their Air Marshal, Kyle Ford has gone missing. After checking the entire plane, they find the man’s body in the baggage area. 

Castle calls Beckett while Alexis shows pictures of the body to Lanie via her phone. They determine that the man was most likely killed via blunt force trauma to the head. 

From the ground, Beckett and team find out that a man on the plane is traveling with a stolen passport and may have ties to ISIS. Castle and the flight attendants detain him but he swears the terrorist ties are his cousins and he was only trying to get back to Syria to see his dying mother. 

When they realize the Air Marshal had a second cell phone, it links back to Marilyn Singer. Singer was recently fired from the airline and has the Air Marshal’s gun in her purse. She swears it’s all a set up and that she and Ford were dating. Castle believes her and keeps investigating. 

Eventually Beckett and team find out that someone deposited almost $10,000 in Ford’s account just before the flight. He was paid to keep an eye on a $13 million watch being transported by courier. When they find the courier on the plane, he’s asleep. He was drugged and the watch was swapped for a fake. 

Castle realizes that the flight attendant, Debbie drugged the courier’s drink. Debbie pulls out a gun. She was about to retire when the airline cut their pensions. She planned to steal the watch but then the Air Marshal got wind of the plan and she killed him. Alexis, who had gotten close to the woman earlier, manages to talk her into giving up the gun. 

As the plane lands, Alexis decides she’d rather spend her time with her Dad while Beckett breathes a sigh of relief back home.