Monk thinks Sharona is picking her aunt up at the airport, but instead she is going to New Jersey to see her aunt. She has waited until the last possible moment to tell Monk so that he cannot spend days obsessing over the situation and must make the decision to either go with her or stay on the spot. He decides to go with her, even though this will be his first (successful) ride on a plane. Of course his obsessions about clean bathrooms, people being too close, and people coughing play into all of this.
But on top of that, Monk becomes convinced that a man on the plane and his wife are in fact the man and an imposter, mainly based on him seeing in the airport that the wife had to stand on her toes to hug her husband, and the woman on the plane does not. Thus Monk thinks there is a murder mystery afoot, and he drives everyone crazy trying to solve the case, or even convince the flight crew that a murder has been committed in the first place, since the body is on the airport grounds somewhere.
Tim Daly, a costar of Shaloub's when they were both on Wings, has a guest appearance involving a matter separate from the murder mystery. And Brooke Adams, Shaloub's actual wife, plays a flight attendant who is driven off the wagon and to drink by Monk's annoying behavior.
