This entry in the TV series of Agatha Christie's Hercule Poirot is about espionage, with much intrigue. This plot also has some extras. One is the visit of an FBI agent from America who's coming to help Scotland Yard catch a thief who has some stolen secret U.S. Navy documents. The American agent is a belligerent jerk. But, as with all such doubting characters in the beginning, he comes around to Poirot's superior sleuthing. In the end he sees Poirot as a capable and valuable ally for the formal law enforcement bodies.
This plot has two separate stories that eventually converge. Poirot's faithful and able secretary, Miss Lemon (Pauline Moran) gets to do some front-line work for the boss. When the FBI agent takes over Japp's office at Scotland Yard, Japp doesn't hesitate to give Poirot information and contacts so that he can pursue the thief and stolen documents.
One of the funniest things in "The Adventure of the Cheap Flat" is FBI agent Burt repeatedly and loudly insisting that the Brits and others are wrong. He says that there's no Mafia or organized crime in America. The vehicles in this film place the story in the mid-1930s. That's when the newly created FBI replaced the previous Justice Department's Bureau of Investigation. The purpose of both was especially to fight organized crime.
Here are a couple humorous lines from the film - both by Poirot to Hastings. "Really, Hastings, you must try to keep up with the newer idioms." "Hastings - it is always wise to make a thorough inspection before arriving at a final decision." This is another very enjoyable Agatha Christie entry in the life of the greatest detective of all time.