Sharona blindsides Monk by announcing she is going to be flying to New Jersey to visit her Aunt Minn. The notion of going a whole week without her forces our obsessive compulsive hero to (gulp) swallow his paralyzing fear of airplanes and accompany her on the trip.
At the airport, Monk notices a handsome married couple, the Chabrols, tenderly kissing. But what he doesn't see is Mrs. Chabrol being murdered in a restroom a few minutes later, her suave French husband, Stephan, disposing of her body, and Stephan's mistress (dressed exactly like Mrs. Chabrol) assuming her identity on the plane.
Monk boards the aircraft, and quickly starts annoying every passenger and crewmember in sight. His anxiety is palpable, and it doesn't help that there is an extension-cord salesman in the next seat, Warren Beach (Garry Marshall!), who will not stop talking. Or that Sharona is too busy fawning over the TV celebrity in first class (Tim Daly as himself!) to give Monk the attention that he needs. Or that Monk soon becomes convinced that one of his fellow passengers - the Frenchman, Stephan Chabrol - has just killed his wife, after noticing that Stephan's "new" wife is at least two inches taller than the woman he saw Stephan kissing earlier.
Monk phones Lt. Disher with his theory, but the Lieutenant informs Monk that no corpses have turned up at San Francisco International - and that furthermore, in this age of heightened security, no civilian would ever get away with hiding a body there in the first place. Monk pleads with Lt. Disher to search the airport anyway, and Disher sighs but agrees to do it.
Meanwhile, Sharona and the extension cord salesman try to reassure Monk that there was no murder, and that his suspicions about Chabrol have been brought on by stress. But Monk's fears return the moment "Mrs. Chabrol" fails to recognize an old family friend, Bernard, who happens to be traveling on the same flight. When he comes over and addresses her in French, "Mrs. Chabrol" stares at him blankly.
Bernard, immediately suspicious, returns to his seat. Acting quickly, Stephan improvises an ingenious way of dispatching the meddling old Frenchman. Now, with Bernard dead, Monk has no case against Stephan unless he can locate his wife's body before he can make a connecting flight to Paris. Once overseas, Stephan and his mistress can stall extradition and disappear.
After Monk realizes that Stephan is a pilot (meaning he has a full knowledge of airplanes and the run of any airport) and also notices that Stephan appears to have tiny cement flakes on his shoes, he figures out where the missing body must be hidden. Monk alerts Lt. Disher to try searching an airport construction site back in San Francisco, and the real Mrs. Chabrol's body is located moments before Stephan's connecting flight to Paris leaves the runway.
It's another high crime dashed back to earth, courtesy of the detective who has all the Right Stuff - Adrian Monk!